fsdax: Fix infinite loop in dax_iomap_rw()

[linux-stable] / fs / dax.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * fs/dax.c - Direct Access filesystem code
 * Copyright (c) 2013-2014 Intel Corporation
 * Author: Matthew Wilcox <matthew.r.wilcox@intel.com>
 * Author: Ross Zwisler <ross.zwisler@linux.intel.com>
 */

#include <linux/atomic.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/dax.h>
#include <linux/fs.h>
#include <linux/highmem.h>
#include <linux/memcontrol.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/pagevec.h>
#include <linux/sched.h>
#include <linux/sched/signal.h>
#include <linux/uio.h>
#include <linux/vmstat.h>
#include <linux/pfn_t.h>
#include <linux/sizes.h>
#include <linux/mmu_notifier.h>
#include <linux/iomap.h>
#include <linux/rmap.h>
#include <asm/pgalloc.h>

#define CREATE_TRACE_POINTS
#include <trace/events/fs_dax.h>

static inline unsigned int pe_order(enum page_entry_size pe_size)
{
        if (pe_size == PE_SIZE_PTE)
                return PAGE_SHIFT - PAGE_SHIFT;
        if (pe_size == PE_SIZE_PMD)
                return PMD_SHIFT - PAGE_SHIFT;
        if (pe_size == PE_SIZE_PUD)
                return PUD_SHIFT - PAGE_SHIFT;
        return ~0;
}

/* We choose 4096 entries - same as per-zone page wait tables */
#define DAX_WAIT_TABLE_BITS 12
#define DAX_WAIT_TABLE_ENTRIES (1 << DAX_WAIT_TABLE_BITS)

/* The 'colour' (ie low bits) within a PMD of a page offset.  */
#define PG_PMD_COLOUR   ((PMD_SIZE >> PAGE_SHIFT) - 1)
#define PG_PMD_NR       (PMD_SIZE >> PAGE_SHIFT)

/* The order of a PMD entry */
#define PMD_ORDER       (PMD_SHIFT - PAGE_SHIFT)

static wait_queue_head_t wait_table[DAX_WAIT_TABLE_ENTRIES];

static int __init init_dax_wait_table(void)
{
        int i;

        for (i = 0; i < DAX_WAIT_TABLE_ENTRIES; i++)
                init_waitqueue_head(wait_table + i);
        return 0;
}
fs_initcall(init_dax_wait_table);

/*
 * DAX pagecache entries use XArray value entries so they can't be mistaken
 * for pages.  We use one bit for locking, one bit for the entry size (PMD)
 * and two more to tell us if the entry is a zero page or an empty entry that
 * is just used for locking.  In total four special bits.
 *
 * If the PMD bit isn't set the entry has size PAGE_SIZE, and if the ZERO_PAGE
 * and EMPTY bits aren't set the entry is a normal DAX entry with a filesystem
 * block allocation.
 */
#define DAX_SHIFT       (4)
#define DAX_LOCKED      (1UL << 0)
#define DAX_PMD         (1UL << 1)
#define DAX_ZERO_PAGE   (1UL << 2)
#define DAX_EMPTY       (1UL << 3)

static unsigned long dax_to_pfn(void *entry)
{
        return xa_to_value(entry) >> DAX_SHIFT;
}

static void *dax_make_entry(pfn_t pfn, unsigned long flags)
{
        return xa_mk_value(flags | (pfn_t_to_pfn(pfn) << DAX_SHIFT));
}

static bool dax_is_locked(void *entry)
{
        return xa_to_value(entry) & DAX_LOCKED;
}

static unsigned int dax_entry_order(void *entry)
{
        if (xa_to_value(entry) & DAX_PMD)
                return PMD_ORDER;
        return 0;
}

static unsigned long dax_is_pmd_entry(void *entry)
{
        return xa_to_value(entry) & DAX_PMD;
}

static bool dax_is_pte_entry(void *entry)
{
        return !(xa_to_value(entry) & DAX_PMD);
}

static int dax_is_zero_entry(void *entry)
{
        return xa_to_value(entry) & DAX_ZERO_PAGE;
}

static int dax_is_empty_entry(void *entry)
{
        return xa_to_value(entry) & DAX_EMPTY;
}

/*
 * true if the entry that was found is of a smaller order than the entry
 * we were looking for
 */
static bool dax_is_conflict(void *entry)
{
        return entry == XA_RETRY_ENTRY;
}

/*
 * DAX page cache entry locking
 */
struct exceptional_entry_key {
        struct xarray *xa;
        pgoff_t entry_start;
};

struct wait_exceptional_entry_queue {
        wait_queue_entry_t wait;
        struct exceptional_entry_key key;
};

/**
 * enum dax_wake_mode: waitqueue wakeup behaviour
 * @WAKE_ALL: wake all waiters in the waitqueue
 * @WAKE_NEXT: wake only the first waiter in the waitqueue
 */
enum dax_wake_mode {
        WAKE_ALL,
        WAKE_NEXT,
};

static wait_queue_head_t *dax_entry_waitqueue(struct xa_state *xas,
                void *entry, struct exceptional_entry_key *key)
{
        unsigned long hash;
        unsigned long index = xas->xa_index;

        /*
         * If 'entry' is a PMD, align the 'index' that we use for the wait
         * queue to the start of that PMD.  This ensures that all offsets in
         * the range covered by the PMD map to the same bit lock.
         */
        if (dax_is_pmd_entry(entry))
                index &= ~PG_PMD_COLOUR;
        key->xa = xas->xa;
        key->entry_start = index;

        hash = hash_long((unsigned long)xas->xa ^ index, DAX_WAIT_TABLE_BITS);
        return wait_table + hash;
}

static int wake_exceptional_entry_func(wait_queue_entry_t *wait,
                unsigned int mode, int sync, void *keyp)
{
        struct exceptional_entry_key *key = keyp;
        struct wait_exceptional_entry_queue *ewait =
                container_of(wait, struct wait_exceptional_entry_queue, wait);

        if (key->xa != ewait->key.xa ||
            key->entry_start != ewait->key.entry_start)
                return 0;
        return autoremove_wake_function(wait, mode, sync, NULL);
}

/*
 * @entry may no longer be the entry at the index in the mapping.
 * The important information it's conveying is whether the entry at
 * this index used to be a PMD entry.
 */
static void dax_wake_entry(struct xa_state *xas, void *entry,
                           enum dax_wake_mode mode)
{
        struct exceptional_entry_key key;
        wait_queue_head_t *wq;

        wq = dax_entry_waitqueue(xas, entry, &key);

        /*
         * Checking for locked entry and prepare_to_wait_exclusive() happens
         * under the i_pages lock, ditto for entry handling in our callers.
         * So at this point all tasks that could have seen our entry locked
         * must be in the waitqueue and the following check will see them.
         */
        if (waitqueue_active(wq))
                __wake_up(wq, TASK_NORMAL, mode == WAKE_ALL ? 0 : 1, &key);
}

/*
 * Look up entry in page cache, wait for it to become unlocked if it
 * is a DAX entry and return it.  The caller must subsequently call
 * put_unlocked_entry() if it did not lock the entry or dax_unlock_entry()
 * if it did.  The entry returned may have a larger order than @order.
 * If @order is larger than the order of the entry found in i_pages, this
 * function returns a dax_is_conflict entry.
 *
 * Must be called with the i_pages lock held.
 */
static void *get_unlocked_entry(struct xa_state *xas, unsigned int order)
{
        void *entry;
        struct wait_exceptional_entry_queue ewait;
        wait_queue_head_t *wq;

        init_wait(&ewait.wait);
        ewait.wait.func = wake_exceptional_entry_func;

        for (;;) {
                entry = xas_find_conflict(xas);
                if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
                        return entry;
                if (dax_entry_order(entry) < order)
                        return XA_RETRY_ENTRY;
                if (!dax_is_locked(entry))
                        return entry;

                wq = dax_entry_waitqueue(xas, entry, &ewait.key);
                prepare_to_wait_exclusive(wq, &ewait.wait,
                                          TASK_UNINTERRUPTIBLE);
                xas_unlock_irq(xas);
                xas_reset(xas);
                schedule();
                finish_wait(wq, &ewait.wait);
                xas_lock_irq(xas);
        }
}

/*
 * The only thing keeping the address space around is the i_pages lock
 * (it's cycled in clear_inode() after removing the entries from i_pages)
 * After we call xas_unlock_irq(), we cannot touch xas->xa.
 */
static void wait_entry_unlocked(struct xa_state *xas, void *entry)
{
        struct wait_exceptional_entry_queue ewait;
        wait_queue_head_t *wq;

        init_wait(&ewait.wait);
        ewait.wait.func = wake_exceptional_entry_func;

        wq = dax_entry_waitqueue(xas, entry, &ewait.key);
        /*
         * Unlike get_unlocked_entry() there is no guarantee that this
         * path ever successfully retrieves an unlocked entry before an
         * inode dies. Perform a non-exclusive wait in case this path
         * never successfully performs its own wake up.
         */
        prepare_to_wait(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
        xas_unlock_irq(xas);
        schedule();
        finish_wait(wq, &ewait.wait);
}

static void put_unlocked_entry(struct xa_state *xas, void *entry,
                               enum dax_wake_mode mode)
{
        if (entry && !dax_is_conflict(entry))
                dax_wake_entry(xas, entry, mode);
}

/*
 * We used the xa_state to get the entry, but then we locked the entry and
 * dropped the xa_lock, so we know the xa_state is stale and must be reset
 * before use.
 */
static void dax_unlock_entry(struct xa_state *xas, void *entry)
{
        void *old;

        BUG_ON(dax_is_locked(entry));
        xas_reset(xas);
        xas_lock_irq(xas);
        old = xas_store(xas, entry);
        xas_unlock_irq(xas);
        BUG_ON(!dax_is_locked(old));
        dax_wake_entry(xas, entry, WAKE_NEXT);
}

/*
 * Return: The entry stored at this location before it was locked.
 */
static void *dax_lock_entry(struct xa_state *xas, void *entry)
{
        unsigned long v = xa_to_value(entry);
        return xas_store(xas, xa_mk_value(v | DAX_LOCKED));
}

static unsigned long dax_entry_size(void *entry)
{
        if (dax_is_zero_entry(entry))
                return 0;
        else if (dax_is_empty_entry(entry))
                return 0;
        else if (dax_is_pmd_entry(entry))
                return PMD_SIZE;
        else
                return PAGE_SIZE;
}

static unsigned long dax_end_pfn(void *entry)
{
        return dax_to_pfn(entry) + dax_entry_size(entry) / PAGE_SIZE;
}

/*
 * Iterate through all mapped pfns represented by an entry, i.e. skip
 * 'empty' and 'zero' entries.
 */
#define for_each_mapped_pfn(entry, pfn) \
        for (pfn = dax_to_pfn(entry); \
                        pfn < dax_end_pfn(entry); pfn++)

/*
 * TODO: for reflink+dax we need a way to associate a single page with
 * multiple address_space instances at different linear_page_index()
 * offsets.
 */
static void dax_associate_entry(void *entry, struct address_space *mapping,
                struct vm_area_struct *vma, unsigned long address)
{
        unsigned long size = dax_entry_size(entry), pfn, index;
        int i = 0;

        if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
                return;

        index = linear_page_index(vma, address & ~(size - 1));
        for_each_mapped_pfn(entry, pfn) {
                struct page *page = pfn_to_page(pfn);

                WARN_ON_ONCE(page->mapping);
                page->mapping = mapping;
                page->index = index + i++;
        }
}

static void dax_disassociate_entry(void *entry, struct address_space *mapping,
                bool trunc)
{
        unsigned long pfn;

        if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
                return;

        for_each_mapped_pfn(entry, pfn) {
                struct page *page = pfn_to_page(pfn);

                WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
                WARN_ON_ONCE(page->mapping && page->mapping != mapping);
                page->mapping = NULL;
                page->index = 0;
        }
}

static struct page *dax_busy_page(void *entry)
{
        unsigned long pfn;

        for_each_mapped_pfn(entry, pfn) {
                struct page *page = pfn_to_page(pfn);

                if (page_ref_count(page) > 1)
                        return page;
        }
        return NULL;
}

/*
 * dax_lock_page - Lock the DAX entry corresponding to a page
 * @page: The page whose entry we want to lock
 *
 * Context: Process context.
 * Return: A cookie to pass to dax_unlock_page() or 0 if the entry could
 * not be locked.
 */
dax_entry_t dax_lock_page(struct page *page)
{
        XA_STATE(xas, NULL, 0);
        void *entry;

        /* Ensure page->mapping isn't freed while we look at it */
        rcu_read_lock();
        for (;;) {
                struct address_space *mapping = READ_ONCE(page->mapping);

                entry = NULL;
                if (!mapping || !dax_mapping(mapping))
                        break;

                /*
                 * In the device-dax case there's no need to lock, a
                 * struct dev_pagemap pin is sufficient to keep the
                 * inode alive, and we assume we have dev_pagemap pin
                 * otherwise we would not have a valid pfn_to_page()
                 * translation.
                 */
                entry = (void *)~0UL;
                if (S_ISCHR(mapping->host->i_mode))
                        break;

                xas.xa = &mapping->i_pages;
                xas_lock_irq(&xas);
                if (mapping != page->mapping) {
                        xas_unlock_irq(&xas);
                        continue;
                }
                xas_set(&xas, page->index);
                entry = xas_load(&xas);
                if (dax_is_locked(entry)) {
                        rcu_read_unlock();
                        wait_entry_unlocked(&xas, entry);
                        rcu_read_lock();
                        continue;
                }
                dax_lock_entry(&xas, entry);
                xas_unlock_irq(&xas);
                break;
        }
        rcu_read_unlock();
        return (dax_entry_t)entry;
}

void dax_unlock_page(struct page *page, dax_entry_t cookie)
{
        struct address_space *mapping = page->mapping;
        XA_STATE(xas, &mapping->i_pages, page->index);

        if (S_ISCHR(mapping->host->i_mode))
                return;

        dax_unlock_entry(&xas, (void *)cookie);
}

/*
 * Find page cache entry at given index. If it is a DAX entry, return it
 * with the entry locked. If the page cache doesn't contain an entry at
 * that index, add a locked empty entry.
 *
 * When requesting an entry with size DAX_PMD, grab_mapping_entry() will
 * either return that locked entry or will return VM_FAULT_FALLBACK.
 * This will happen if there are any PTE entries within the PMD range
 * that we are requesting.
 *
 * We always favor PTE entries over PMD entries. There isn't a flow where we
 * evict PTE entries in order to 'upgrade' them to a PMD entry.  A PMD
 * insertion will fail if it finds any PTE entries already in the tree, and a
 * PTE insertion will cause an existing PMD entry to be unmapped and
 * downgraded to PTE entries.  This happens for both PMD zero pages as
 * well as PMD empty entries.
 *
 * The exception to this downgrade path is for PMD entries that have
 * real storage backing them.  We will leave these real PMD entries in
 * the tree, and PTE writes will simply dirty the entire PMD entry.
 *
 * Note: Unlike filemap_fault() we don't honor FAULT_FLAG_RETRY flags. For
 * persistent memory the benefit is doubtful. We can add that later if we can
 * show it helps.
 *
 * On error, this function does not return an ERR_PTR.  Instead it returns
 * a VM_FAULT code, encoded as an xarray internal entry.  The ERR_PTR values
 * overlap with xarray value entries.
 */
static void *grab_mapping_entry(struct xa_state *xas,
                struct address_space *mapping, unsigned int order)
{
        unsigned long index = xas->xa_index;
        bool pmd_downgrade;     /* splitting PMD entry into PTE entries? */
        void *entry;

retry:
        pmd_downgrade = false;
        xas_lock_irq(xas);
        entry = get_unlocked_entry(xas, order);

        if (entry) {
                if (dax_is_conflict(entry))
                        goto fallback;
                if (!xa_is_value(entry)) {
                        xas_set_err(xas, -EIO);
                        goto out_unlock;
                }

                if (order == 0) {
                        if (dax_is_pmd_entry(entry) &&
                            (dax_is_zero_entry(entry) ||
                             dax_is_empty_entry(entry))) {
                                pmd_downgrade = true;
                        }
                }
        }

        if (pmd_downgrade) {
                /*
                 * Make sure 'entry' remains valid while we drop
                 * the i_pages lock.
                 */
                dax_lock_entry(xas, entry);

                /*
                 * Besides huge zero pages the only other thing that gets
                 * downgraded are empty entries which don't need to be
                 * unmapped.
                 */
                if (dax_is_zero_entry(entry)) {
                        xas_unlock_irq(xas);
                        unmap_mapping_pages(mapping,
                                        xas->xa_index & ~PG_PMD_COLOUR,
                                        PG_PMD_NR, false);
                        xas_reset(xas);
                        xas_lock_irq(xas);
                }

                dax_disassociate_entry(entry, mapping, false);
                xas_store(xas, NULL);   /* undo the PMD join */
                dax_wake_entry(xas, entry, WAKE_ALL);
                mapping->nrpages -= PG_PMD_NR;
                entry = NULL;
                xas_set(xas, index);
        }

        if (entry) {
                dax_lock_entry(xas, entry);
        } else {
                unsigned long flags = DAX_EMPTY;

                if (order > 0)
                        flags |= DAX_PMD;
                entry = dax_make_entry(pfn_to_pfn_t(0), flags);
                dax_lock_entry(xas, entry);
                if (xas_error(xas))
                        goto out_unlock;
                mapping->nrpages += 1UL << order;
        }

out_unlock:
        xas_unlock_irq(xas);
        if (xas_nomem(xas, mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM))
                goto retry;
        if (xas->xa_node == XA_ERROR(-ENOMEM))
                return xa_mk_internal(VM_FAULT_OOM);
        if (xas_error(xas))
                return xa_mk_internal(VM_FAULT_SIGBUS);
        return entry;
fallback:
        xas_unlock_irq(xas);
        return xa_mk_internal(VM_FAULT_FALLBACK);
}

/**
 * dax_layout_busy_page_range - find first pinned page in @mapping
 * @mapping: address space to scan for a page with ref count > 1
 * @start: Starting offset. Page containing 'start' is included.
 * @end: End offset. Page containing 'end' is included. If 'end' is LLONG_MAX,
 *       pages from 'start' till the end of file are included.
 *
 * DAX requires ZONE_DEVICE mapped pages. These pages are never
 * 'onlined' to the page allocator so they are considered idle when
 * page->count == 1. A filesystem uses this interface to determine if
 * any page in the mapping is busy, i.e. for DMA, or other
 * get_user_pages() usages.
 *
 * It is expected that the filesystem is holding locks to block the
 * establishment of new mappings in this address_space. I.e. it expects
 * to be able to run unmap_mapping_range() and subsequently not race
 * mapping_mapped() becoming true.
 */
struct page *dax_layout_busy_page_range(struct address_space *mapping,
                                        loff_t start, loff_t end)
{
        void *entry;
        unsigned int scanned = 0;
        struct page *page = NULL;
        pgoff_t start_idx = start >> PAGE_SHIFT;
        pgoff_t end_idx;
        XA_STATE(xas, &mapping->i_pages, start_idx);

        /*
         * In the 'limited' case get_user_pages() for dax is disabled.
         */
        if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
                return NULL;

        if (!dax_mapping(mapping) || !mapping_mapped(mapping))
                return NULL;

        /* If end == LLONG_MAX, all pages from start to till end of file */
        if (end == LLONG_MAX)
                end_idx = ULONG_MAX;
        else
                end_idx = end >> PAGE_SHIFT;
        /*
         * If we race get_user_pages_fast() here either we'll see the
         * elevated page count in the iteration and wait, or
         * get_user_pages_fast() will see that the page it took a reference
         * against is no longer mapped in the page tables and bail to the
         * get_user_pages() slow path.  The slow path is protected by
         * pte_lock() and pmd_lock(). New references are not taken without
         * holding those locks, and unmap_mapping_pages() will not zero the
         * pte or pmd without holding the respective lock, so we are
         * guaranteed to either see new references or prevent new
         * references from being established.
         */
        unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1, 0);

        xas_lock_irq(&xas);
        xas_for_each(&xas, entry, end_idx) {
                if (WARN_ON_ONCE(!xa_is_value(entry)))
                        continue;
                if (unlikely(dax_is_locked(entry)))
                        entry = get_unlocked_entry(&xas, 0);
                if (entry)
                        page = dax_busy_page(entry);
                put_unlocked_entry(&xas, entry, WAKE_NEXT);
                if (page)
                        break;
                if (++scanned % XA_CHECK_SCHED)
                        continue;

                xas_pause(&xas);
                xas_unlock_irq(&xas);
                cond_resched();
                xas_lock_irq(&xas);
        }
        xas_unlock_irq(&xas);
        return page;
}
EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);

struct page *dax_layout_busy_page(struct address_space *mapping)
{
        return dax_layout_busy_page_range(mapping, 0, LLONG_MAX);
}
EXPORT_SYMBOL_GPL(dax_layout_busy_page);

static int __dax_invalidate_entry(struct address_space *mapping,
                                          pgoff_t index, bool trunc)
{
        XA_STATE(xas, &mapping->i_pages, index);
        int ret = 0;
        void *entry;

        xas_lock_irq(&xas);
        entry = get_unlocked_entry(&xas, 0);
        if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
                goto out;
        if (!trunc &&
            (xas_get_mark(&xas, PAGECACHE_TAG_DIRTY) ||
             xas_get_mark(&xas, PAGECACHE_TAG_TOWRITE)))
                goto out;
        dax_disassociate_entry(entry, mapping, trunc);
        xas_store(&xas, NULL);
        mapping->nrpages -= 1UL << dax_entry_order(entry);
        ret = 1;
out:
        put_unlocked_entry(&xas, entry, WAKE_ALL);
        xas_unlock_irq(&xas);
        return ret;
}

/*
 * Delete DAX entry at @index from @mapping.  Wait for it
 * to be unlocked before deleting it.
 */
int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index)
{
        int ret = __dax_invalidate_entry(mapping, index, true);

        /*
         * This gets called from truncate / punch_hole path. As such, the caller
         * must hold locks protecting against concurrent modifications of the
         * page cache (usually fs-private i_mmap_sem for writing). Since the
         * caller has seen a DAX entry for this index, we better find it
         * at that index as well...
         */
        WARN_ON_ONCE(!ret);
        return ret;
}

/*
 * Invalidate DAX entry if it is clean.
 */
int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
                                      pgoff_t index)
{
        return __dax_invalidate_entry(mapping, index, false);
}

static pgoff_t dax_iomap_pgoff(const struct iomap *iomap, loff_t pos)
{
        return PHYS_PFN(iomap->addr + (pos & PAGE_MASK) - iomap->offset);
}

static int copy_cow_page_dax(struct vm_fault *vmf, const struct iomap_iter *iter)
{
        pgoff_t pgoff = dax_iomap_pgoff(&iter->iomap, iter->pos);
        void *vto, *kaddr;
        long rc;
        int id;

        id = dax_read_lock();
        rc = dax_direct_access(iter->iomap.dax_dev, pgoff, 1, DAX_ACCESS,
                                &kaddr, NULL);
        if (rc < 0) {
                dax_read_unlock(id);
                return rc;
        }
        vto = kmap_atomic(vmf->cow_page);
        copy_user_page(vto, kaddr, vmf->address, vmf->cow_page);
        kunmap_atomic(vto);
        dax_read_unlock(id);
        return 0;
}

/*
 * By this point grab_mapping_entry() has ensured that we have a locked entry
 * of the appropriate size so we don't have to worry about downgrading PMDs to
 * PTEs.  If we happen to be trying to insert a PTE and there is a PMD
 * already in the tree, we will skip the insertion and just dirty the PMD as
 * appropriate.
 */
static void *dax_insert_entry(struct xa_state *xas,
                struct address_space *mapping, struct vm_fault *vmf,
                void *entry, pfn_t pfn, unsigned long flags, bool dirty)
{
        void *new_entry = dax_make_entry(pfn, flags);

        if (dirty)
                __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);

        if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
                unsigned long index = xas->xa_index;
                /* we are replacing a zero page with block mapping */
                if (dax_is_pmd_entry(entry))
                        unmap_mapping_pages(mapping, index & ~PG_PMD_COLOUR,
                                        PG_PMD_NR, false);
                else /* pte entry */
                        unmap_mapping_pages(mapping, index, 1, false);
        }

        xas_reset(xas);
        xas_lock_irq(xas);
        if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
                void *old;

                dax_disassociate_entry(entry, mapping, false);
                dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
                /*
                 * Only swap our new entry into the page cache if the current
                 * entry is a zero page or an empty entry.  If a normal PTE or
                 * PMD entry is already in the cache, we leave it alone.  This
                 * means that if we are trying to insert a PTE and the
                 * existing entry is a PMD, we will just leave the PMD in the
                 * tree and dirty it if necessary.
                 */
                old = dax_lock_entry(xas, new_entry);
                WARN_ON_ONCE(old != xa_mk_value(xa_to_value(entry) |
                                        DAX_LOCKED));
                entry = new_entry;
        } else {
                xas_load(xas);  /* Walk the xa_state */
        }

        if (dirty)
                xas_set_mark(xas, PAGECACHE_TAG_DIRTY);

        xas_unlock_irq(xas);
        return entry;
}

static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
                struct address_space *mapping, void *entry)
{
        unsigned long pfn, index, count, end;
        long ret = 0;
        struct vm_area_struct *vma;

        /*
         * A page got tagged dirty in DAX mapping? Something is seriously
         * wrong.
         */
        if (WARN_ON(!xa_is_value(entry)))
                return -EIO;

        if (unlikely(dax_is_locked(entry))) {
                void *old_entry = entry;

                entry = get_unlocked_entry(xas, 0);

                /* Entry got punched out / reallocated? */
                if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
                        goto put_unlocked;
                /*
                 * Entry got reallocated elsewhere? No need to writeback.
                 * We have to compare pfns as we must not bail out due to
                 * difference in lockbit or entry type.
                 */
                if (dax_to_pfn(old_entry) != dax_to_pfn(entry))
                        goto put_unlocked;
                if (WARN_ON_ONCE(dax_is_empty_entry(entry) ||
                                        dax_is_zero_entry(entry))) {
                        ret = -EIO;
                        goto put_unlocked;
                }

                /* Another fsync thread may have already done this entry */
                if (!xas_get_mark(xas, PAGECACHE_TAG_TOWRITE))
                        goto put_unlocked;
        }

        /* Lock the entry to serialize with page faults */
        dax_lock_entry(xas, entry);

        /*
         * We can clear the tag now but we have to be careful so that concurrent
         * dax_writeback_one() calls for the same index cannot finish before we
         * actually flush the caches. This is achieved as the calls will look
         * at the entry only under the i_pages lock and once they do that
         * they will see the entry locked and wait for it to unlock.
         */
        xas_clear_mark(xas, PAGECACHE_TAG_TOWRITE);
        xas_unlock_irq(xas);

        /*
         * If dax_writeback_mapping_range() was given a wbc->range_start
         * in the middle of a PMD, the 'index' we use needs to be
         * aligned to the start of the PMD.
         * This allows us to flush for PMD_SIZE and not have to worry about
         * partial PMD writebacks.
         */
        pfn = dax_to_pfn(entry);
        count = 1UL << dax_entry_order(entry);
        index = xas->xa_index & ~(count - 1);
        end = index + count - 1;

        /* Walk all mappings of a given index of a file and writeprotect them */
        i_mmap_lock_read(mapping);
        vma_interval_tree_foreach(vma, &mapping->i_mmap, index, end) {
                pfn_mkclean_range(pfn, count, index, vma);
                cond_resched();
        }
        i_mmap_unlock_read(mapping);

        dax_flush(dax_dev, page_address(pfn_to_page(pfn)), count * PAGE_SIZE);
        /*
         * After we have flushed the cache, we can clear the dirty tag. There
         * cannot be new dirty data in the pfn after the flush has completed as
         * the pfn mappings are writeprotected and fault waits for mapping
         * entry lock.
         */
        xas_reset(xas);
        xas_lock_irq(xas);
        xas_store(xas, entry);
        xas_clear_mark(xas, PAGECACHE_TAG_DIRTY);
        dax_wake_entry(xas, entry, WAKE_NEXT);

        trace_dax_writeback_one(mapping->host, index, count);
        return ret;

 put_unlocked:
        put_unlocked_entry(xas, entry, WAKE_NEXT);
        return ret;
}

/*
 * Flush the mapping to the persistent domain within the byte range of [start,
 * end]. This is required by data integrity operations to ensure file data is
 * on persistent storage prior to completion of the operation.
 */
int dax_writeback_mapping_range(struct address_space *mapping,
                struct dax_device *dax_dev, struct writeback_control *wbc)
{
        XA_STATE(xas, &mapping->i_pages, wbc->range_start >> PAGE_SHIFT);
        struct inode *inode = mapping->host;
        pgoff_t end_index = wbc->range_end >> PAGE_SHIFT;
        void *entry;
        int ret = 0;
        unsigned int scanned = 0;

        if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
                return -EIO;

        if (mapping_empty(mapping) || wbc->sync_mode != WB_SYNC_ALL)
                return 0;

        trace_dax_writeback_range(inode, xas.xa_index, end_index);

        tag_pages_for_writeback(mapping, xas.xa_index, end_index);

        xas_lock_irq(&xas);
        xas_for_each_marked(&xas, entry, end_index, PAGECACHE_TAG_TOWRITE) {
                ret = dax_writeback_one(&xas, dax_dev, mapping, entry);
                if (ret < 0) {
                        mapping_set_error(mapping, ret);
                        break;
                }
                if (++scanned % XA_CHECK_SCHED)
                        continue;

                xas_pause(&xas);
                xas_unlock_irq(&xas);
                cond_resched();
                xas_lock_irq(&xas);
        }
        xas_unlock_irq(&xas);
        trace_dax_writeback_range_done(inode, xas.xa_index, end_index);
        return ret;
}
EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);

static int dax_iomap_pfn(const struct iomap *iomap, loff_t pos, size_t size,
                         pfn_t *pfnp)
{
        pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
        int id, rc;
        long length;

        id = dax_read_lock();
        length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
                                   DAX_ACCESS, NULL, pfnp);
        if (length < 0) {
                rc = length;
                goto out;
        }
        rc = -EINVAL;
        if (PFN_PHYS(length) < size)
                goto out;
        if (pfn_t_to_pfn(*pfnp) & (PHYS_PFN(size)-1))
                goto out;
        /* For larger pages we need devmap */
        if (length > 1 && !pfn_t_devmap(*pfnp))
                goto out;
        rc = 0;
out:
        dax_read_unlock(id);
        return rc;
}

/*
 * The user has performed a load from a hole in the file.  Allocating a new
 * page in the file would cause excessive storage usage for workloads with
 * sparse files.  Instead we insert a read-only mapping of the 4k zero page.
 * If this page is ever written to we will re-fault and change the mapping to
 * point to real DAX storage instead.
 */
static vm_fault_t dax_load_hole(struct xa_state *xas,
                struct address_space *mapping, void **entry,
                struct vm_fault *vmf)
{
        struct inode *inode = mapping->host;
        unsigned long vaddr = vmf->address;
        pfn_t pfn = pfn_to_pfn_t(my_zero_pfn(vaddr));
        vm_fault_t ret;

        *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
                        DAX_ZERO_PAGE, false);

        ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
        trace_dax_load_hole(inode, vmf, ret);
        return ret;
}

#ifdef CONFIG_FS_DAX_PMD
static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
                const struct iomap *iomap, void **entry)
{
        struct address_space *mapping = vmf->vma->vm_file->f_mapping;
        unsigned long pmd_addr = vmf->address & PMD_MASK;
        struct vm_area_struct *vma = vmf->vma;
        struct inode *inode = mapping->host;
        pgtable_t pgtable = NULL;
        struct page *zero_page;
        spinlock_t *ptl;
        pmd_t pmd_entry;
        pfn_t pfn;

        zero_page = mm_get_huge_zero_page(vmf->vma->vm_mm);

        if (unlikely(!zero_page))
                goto fallback;

        pfn = page_to_pfn_t(zero_page);
        *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
                        DAX_PMD | DAX_ZERO_PAGE, false);

        if (arch_needs_pgtable_deposit()) {
                pgtable = pte_alloc_one(vma->vm_mm);
                if (!pgtable)
                        return VM_FAULT_OOM;
        }

        ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
        if (!pmd_none(*(vmf->pmd))) {
                spin_unlock(ptl);
                goto fallback;
        }

        if (pgtable) {
                pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
                mm_inc_nr_ptes(vma->vm_mm);
        }
        pmd_entry = mk_pmd(zero_page, vmf->vma->vm_page_prot);
        pmd_entry = pmd_mkhuge(pmd_entry);
        set_pmd_at(vmf->vma->vm_mm, pmd_addr, vmf->pmd, pmd_entry);
        spin_unlock(ptl);
        trace_dax_pmd_load_hole(inode, vmf, zero_page, *entry);
        return VM_FAULT_NOPAGE;

fallback:
        if (pgtable)
                pte_free(vma->vm_mm, pgtable);
        trace_dax_pmd_load_hole_fallback(inode, vmf, zero_page, *entry);
        return VM_FAULT_FALLBACK;
}
#else
static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
                const struct iomap *iomap, void **entry)
{
        return VM_FAULT_FALLBACK;
}
#endif /* CONFIG_FS_DAX_PMD */

static int dax_memzero(struct dax_device *dax_dev, pgoff_t pgoff,
                unsigned int offset, size_t size)
{
        void *kaddr;
        long ret;

        ret = dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
        if (ret > 0) {
                memset(kaddr + offset, 0, size);
                dax_flush(dax_dev, kaddr + offset, size);
        }
        return ret;
}

static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
{
        const struct iomap *iomap = &iter->iomap;
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        loff_t pos = iter->pos;
        u64 length = iomap_length(iter);
        s64 written = 0;

        /* already zeroed?  we're done. */
        if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
                return length;

        do {
                unsigned offset = offset_in_page(pos);
                unsigned size = min_t(u64, PAGE_SIZE - offset, length);
                pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
                long rc;
                int id;

                id = dax_read_lock();
                if (IS_ALIGNED(pos, PAGE_SIZE) && size == PAGE_SIZE)
                        rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
                else
                        rc = dax_memzero(iomap->dax_dev, pgoff, offset, size);
                dax_read_unlock(id);

                if (rc < 0)
                        return rc;
                pos += size;
                length -= size;
                written += size;
                if (did_zero)
                        *did_zero = true;
        } while (length > 0);

        return written;
}

int dax_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
                const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = inode,
                .pos            = pos,
                .len            = len,
                .flags          = IOMAP_DAX | IOMAP_ZERO,
        };
        int ret;

        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = dax_zero_iter(&iter, did_zero);
        return ret;
}
EXPORT_SYMBOL_GPL(dax_zero_range);

int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
                const struct iomap_ops *ops)
{
        unsigned int blocksize = i_blocksize(inode);
        unsigned int off = pos & (blocksize - 1);

        /* Block boundary? Nothing to do */
        if (!off)
                return 0;
        return dax_zero_range(inode, pos, blocksize - off, did_zero, ops);
}
EXPORT_SYMBOL_GPL(dax_truncate_page);

static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
                struct iov_iter *iter)
{
        const struct iomap *iomap = &iomi->iomap;
        loff_t length = iomap_length(iomi);
        loff_t pos = iomi->pos;
        struct dax_device *dax_dev = iomap->dax_dev;
        loff_t end = pos + length, done = 0;
        ssize_t ret = 0;
        size_t xfer;
        int id;

        if (iov_iter_rw(iter) == READ) {
                end = min(end, i_size_read(iomi->inode));
                if (pos >= end)
                        return 0;

                if (iomap->type == IOMAP_HOLE || iomap->type == IOMAP_UNWRITTEN)
                        return iov_iter_zero(min(length, end - pos), iter);
        }

        if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
                return -EIO;

        /*
         * Write can allocate block for an area which has a hole page mapped
         * into page tables. We have to tear down these mappings so that data
         * written by write(2) is visible in mmap.
         */
        if (iomap->flags & IOMAP_F_NEW) {
                invalidate_inode_pages2_range(iomi->inode->i_mapping,
                                              pos >> PAGE_SHIFT,
                                              (end - 1) >> PAGE_SHIFT);
        }

        id = dax_read_lock();
        while (pos < end) {
                unsigned offset = pos & (PAGE_SIZE - 1);
                const size_t size = ALIGN(length + offset, PAGE_SIZE);
                pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
                ssize_t map_len;
                bool recovery = false;
                void *kaddr;

                if (fatal_signal_pending(current)) {
                        ret = -EINTR;
                        break;
                }

                map_len = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size),
                                DAX_ACCESS, &kaddr, NULL);
                if (map_len == -EIO && iov_iter_rw(iter) == WRITE) {
                        map_len = dax_direct_access(dax_dev, pgoff,
                                        PHYS_PFN(size), DAX_RECOVERY_WRITE,
                                        &kaddr, NULL);
                        if (map_len > 0)
                                recovery = true;
                }
                if (map_len < 0) {
                        ret = map_len;
                        break;
                }

                map_len = PFN_PHYS(map_len);
                kaddr += offset;
                map_len -= offset;
                if (map_len > end - pos)
                        map_len = end - pos;

                if (recovery)
                        xfer = dax_recovery_write(dax_dev, pgoff, kaddr,
                                        map_len, iter);
                else if (iov_iter_rw(iter) == WRITE)
                        xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
                                        map_len, iter);
                else
                        xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr,
                                        map_len, iter);

                pos += xfer;
                length -= xfer;
                done += xfer;

                if (xfer == 0)
                        ret = -EFAULT;
                if (xfer < map_len)
                        break;
        }
        dax_read_unlock(id);

        return done ? done : ret;
}

/**
 * dax_iomap_rw - Perform I/O to a DAX file
 * @iocb:       The control block for this I/O
 * @iter:       The addresses to do I/O from or to
 * @ops:        iomap ops passed from the file system
 *
 * This function performs read and write operations to directly mapped
 * persistent memory.  The callers needs to take care of read/write exclusion
 * and evicting any page cache pages in the region under I/O.
 */
ssize_t
dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
                const struct iomap_ops *ops)
{
        struct iomap_iter iomi = {
                .inode          = iocb->ki_filp->f_mapping->host,
                .pos            = iocb->ki_pos,
                .len            = iov_iter_count(iter),
                .flags          = IOMAP_DAX,
        };
        loff_t done = 0;
        int ret;

        if (!iomi.len)
                return 0;

        if (iov_iter_rw(iter) == WRITE) {
                lockdep_assert_held_write(&iomi.inode->i_rwsem);
                iomi.flags |= IOMAP_WRITE;
        } else {
                lockdep_assert_held(&iomi.inode->i_rwsem);
        }

        if (iocb->ki_flags & IOCB_NOWAIT)
                iomi.flags |= IOMAP_NOWAIT;

        while ((ret = iomap_iter(&iomi, ops)) > 0)
                iomi.processed = dax_iomap_iter(&iomi, iter);

        done = iomi.pos - iocb->ki_pos;
        iocb->ki_pos = iomi.pos;
        return done ? done : ret;
}
EXPORT_SYMBOL_GPL(dax_iomap_rw);

static vm_fault_t dax_fault_return(int error)
{
        if (error == 0)
                return VM_FAULT_NOPAGE;
        return vmf_error(error);
}

/*
 * MAP_SYNC on a dax mapping guarantees dirty metadata is
 * flushed on write-faults (non-cow), but not read-faults.
 */
static bool dax_fault_is_synchronous(unsigned long flags,
                struct vm_area_struct *vma, const struct iomap *iomap)
{
        return (flags & IOMAP_WRITE) && (vma->vm_flags & VM_SYNC)
                && (iomap->flags & IOMAP_F_DIRTY);
}

/*
 * When handling a synchronous page fault and the inode need a fsync, we can
 * insert the PTE/PMD into page tables only after that fsync happened. Skip
 * insertion for now and return the pfn so that caller can insert it after the
 * fsync is done.
 */
static vm_fault_t dax_fault_synchronous_pfnp(pfn_t *pfnp, pfn_t pfn)
{
        if (WARN_ON_ONCE(!pfnp))
                return VM_FAULT_SIGBUS;
        *pfnp = pfn;
        return VM_FAULT_NEEDDSYNC;
}

static vm_fault_t dax_fault_cow_page(struct vm_fault *vmf,
                const struct iomap_iter *iter)
{
        vm_fault_t ret;
        int error = 0;

        switch (iter->iomap.type) {
        case IOMAP_HOLE:
        case IOMAP_UNWRITTEN:
                clear_user_highpage(vmf->cow_page, vmf->address);
                break;
        case IOMAP_MAPPED:
                error = copy_cow_page_dax(vmf, iter);
                break;
        default:
                WARN_ON_ONCE(1);
                error = -EIO;
                break;
        }

        if (error)
                return dax_fault_return(error);

        __SetPageUptodate(vmf->cow_page);
        ret = finish_fault(vmf);
        if (!ret)
                return VM_FAULT_DONE_COW;
        return ret;
}

/**
 * dax_fault_iter - Common actor to handle pfn insertion in PTE/PMD fault.
 * @vmf:        vm fault instance
 * @iter:       iomap iter
 * @pfnp:       pfn to be returned
 * @xas:        the dax mapping tree of a file
 * @entry:      an unlocked dax entry to be inserted
 * @pmd:        distinguish whether it is a pmd fault
 */
static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
                const struct iomap_iter *iter, pfn_t *pfnp,
                struct xa_state *xas, void **entry, bool pmd)
{
        struct address_space *mapping = vmf->vma->vm_file->f_mapping;
        const struct iomap *iomap = &iter->iomap;
        size_t size = pmd ? PMD_SIZE : PAGE_SIZE;
        loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT;
        bool write = vmf->flags & FAULT_FLAG_WRITE;
        bool sync = dax_fault_is_synchronous(iter->flags, vmf->vma, iomap);
        unsigned long entry_flags = pmd ? DAX_PMD : 0;
        int err = 0;
        pfn_t pfn;

        if (!pmd && vmf->cow_page)
                return dax_fault_cow_page(vmf, iter);

        /* if we are reading UNWRITTEN and HOLE, return a hole. */
        if (!write &&
            (iomap->type == IOMAP_UNWRITTEN || iomap->type == IOMAP_HOLE)) {
                if (!pmd)
                        return dax_load_hole(xas, mapping, entry, vmf);
                return dax_pmd_load_hole(xas, vmf, iomap, entry);
        }

        if (iomap->type != IOMAP_MAPPED) {
                WARN_ON_ONCE(1);
                return pmd ? VM_FAULT_FALLBACK : VM_FAULT_SIGBUS;
        }

        err = dax_iomap_pfn(&iter->iomap, pos, size, &pfn);
        if (err)
                return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err);

        *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn, entry_flags,
                                  write && !sync);

        if (sync)
                return dax_fault_synchronous_pfnp(pfnp, pfn);

        /* insert PMD pfn */
        if (pmd)
                return vmf_insert_pfn_pmd(vmf, pfn, write);

        /* insert PTE pfn */
        if (write)
                return vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
        return vmf_insert_mixed(vmf->vma, vmf->address, pfn);
}

static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
                               int *iomap_errp, const struct iomap_ops *ops)
{
        struct address_space *mapping = vmf->vma->vm_file->f_mapping;
        XA_STATE(xas, &mapping->i_pages, vmf->pgoff);
        struct iomap_iter iter = {
                .inode          = mapping->host,
                .pos            = (loff_t)vmf->pgoff << PAGE_SHIFT,
                .len            = PAGE_SIZE,
                .flags          = IOMAP_DAX | IOMAP_FAULT,
        };
        vm_fault_t ret = 0;
        void *entry;
        int error;

        trace_dax_pte_fault(iter.inode, vmf, ret);
        /*
         * Check whether offset isn't beyond end of file now. Caller is supposed
         * to hold locks serializing us with truncate / punch hole so this is
         * a reliable test.
         */
        if (iter.pos >= i_size_read(iter.inode)) {
                ret = VM_FAULT_SIGBUS;
                goto out;
        }

        if ((vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page)
                iter.flags |= IOMAP_WRITE;

        entry = grab_mapping_entry(&xas, mapping, 0);
        if (xa_is_internal(entry)) {
                ret = xa_to_internal(entry);
                goto out;
        }

        /*
         * It is possible, particularly with mixed reads & writes to private
         * mappings, that we have raced with a PMD fault that overlaps with
         * the PTE we need to set up.  If so just return and the fault will be
         * retried.
         */
        if (pmd_trans_huge(*vmf->pmd) || pmd_devmap(*vmf->pmd)) {
                ret = VM_FAULT_NOPAGE;
                goto unlock_entry;
        }

        while ((error = iomap_iter(&iter, ops)) > 0) {
                if (WARN_ON_ONCE(iomap_length(&iter) < PAGE_SIZE)) {
                        iter.processed = -EIO;  /* fs corruption? */
                        continue;
                }

                ret = dax_fault_iter(vmf, &iter, pfnp, &xas, &entry, false);
                if (ret != VM_FAULT_SIGBUS &&
                    (iter.iomap.flags & IOMAP_F_NEW)) {
                        count_vm_event(PGMAJFAULT);
                        count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
                        ret |= VM_FAULT_MAJOR;
                }

                if (!(ret & VM_FAULT_ERROR))
                        iter.processed = PAGE_SIZE;
        }

        if (iomap_errp)
                *iomap_errp = error;
        if (!ret && error)
                ret = dax_fault_return(error);

unlock_entry:
        dax_unlock_entry(&xas, entry);
out:
        trace_dax_pte_fault_done(iter.inode, vmf, ret);
        return ret;
}

#ifdef CONFIG_FS_DAX_PMD
static bool dax_fault_check_fallback(struct vm_fault *vmf, struct xa_state *xas,
                pgoff_t max_pgoff)
{
        unsigned long pmd_addr = vmf->address & PMD_MASK;
        bool write = vmf->flags & FAULT_FLAG_WRITE;

        /*
         * Make sure that the faulting address's PMD offset (color) matches
         * the PMD offset from the start of the file.  This is necessary so
         * that a PMD range in the page table overlaps exactly with a PMD
         * range in the page cache.
         */
        if ((vmf->pgoff & PG_PMD_COLOUR) !=
            ((vmf->address >> PAGE_SHIFT) & PG_PMD_COLOUR))
                return true;

        /* Fall back to PTEs if we're going to COW */
        if (write && !(vmf->vma->vm_flags & VM_SHARED))
                return true;

        /* If the PMD would extend outside the VMA */
        if (pmd_addr < vmf->vma->vm_start)
                return true;
        if ((pmd_addr + PMD_SIZE) > vmf->vma->vm_end)
                return true;

        /* If the PMD would extend beyond the file size */
        if ((xas->xa_index | PG_PMD_COLOUR) >= max_pgoff)
                return true;

        return false;
}

static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
                               const struct iomap_ops *ops)
{
        struct address_space *mapping = vmf->vma->vm_file->f_mapping;
        XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, PMD_ORDER);
        struct iomap_iter iter = {
                .inode          = mapping->host,
                .len            = PMD_SIZE,
                .flags          = IOMAP_DAX | IOMAP_FAULT,
        };
        vm_fault_t ret = VM_FAULT_FALLBACK;
        pgoff_t max_pgoff;
        void *entry;
        int error;

        if (vmf->flags & FAULT_FLAG_WRITE)
                iter.flags |= IOMAP_WRITE;

        /*
         * Check whether offset isn't beyond end of file now. Caller is
         * supposed to hold locks serializing us with truncate / punch hole so
         * this is a reliable test.
         */
        max_pgoff = DIV_ROUND_UP(i_size_read(iter.inode), PAGE_SIZE);

        trace_dax_pmd_fault(iter.inode, vmf, max_pgoff, 0);

        if (xas.xa_index >= max_pgoff) {
                ret = VM_FAULT_SIGBUS;
                goto out;
        }

        if (dax_fault_check_fallback(vmf, &xas, max_pgoff))
                goto fallback;

        /*
         * grab_mapping_entry() will make sure we get an empty PMD entry,
         * a zero PMD entry or a DAX PMD.  If it can't (because a PTE
         * entry is already in the array, for instance), it will return
         * VM_FAULT_FALLBACK.
         */
        entry = grab_mapping_entry(&xas, mapping, PMD_ORDER);
        if (xa_is_internal(entry)) {
                ret = xa_to_internal(entry);
                goto fallback;
        }

        /*
         * It is possible, particularly with mixed reads & writes to private
         * mappings, that we have raced with a PTE fault that overlaps with
         * the PMD we need to set up.  If so just return and the fault will be
         * retried.
         */
        if (!pmd_none(*vmf->pmd) && !pmd_trans_huge(*vmf->pmd) &&
                        !pmd_devmap(*vmf->pmd)) {
                ret = 0;
                goto unlock_entry;
        }

        iter.pos = (loff_t)xas.xa_index << PAGE_SHIFT;
        while ((error = iomap_iter(&iter, ops)) > 0) {
                if (iomap_length(&iter) < PMD_SIZE)
                        continue; /* actually breaks out of the loop */

                ret = dax_fault_iter(vmf, &iter, pfnp, &xas, &entry, true);
                if (ret != VM_FAULT_FALLBACK)
                        iter.processed = PMD_SIZE;
        }

unlock_entry:
        dax_unlock_entry(&xas, entry);
fallback:
        if (ret == VM_FAULT_FALLBACK) {
                split_huge_pmd(vmf->vma, vmf->pmd, vmf->address);
                count_vm_event(THP_FAULT_FALLBACK);
        }
out:
        trace_dax_pmd_fault_done(iter.inode, vmf, max_pgoff, ret);
        return ret;
}
#else
static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
                               const struct iomap_ops *ops)
{
        return VM_FAULT_FALLBACK;
}
#endif /* CONFIG_FS_DAX_PMD */

/**
 * dax_iomap_fault - handle a page fault on a DAX file
 * @vmf: The description of the fault
 * @pe_size: Size of the page to fault in
 * @pfnp: PFN to insert for synchronous faults if fsync is required
 * @iomap_errp: Storage for detailed error code in case of error
 * @ops: Iomap ops passed from the file system
 *
 * When a page fault occurs, filesystems may call this helper in
 * their fault handler for DAX files. dax_iomap_fault() assumes the caller
 * has done all the necessary locking for page fault to proceed
 * successfully.
 */
vm_fault_t dax_iomap_fault(struct vm_fault *vmf, enum page_entry_size pe_size,
                    pfn_t *pfnp, int *iomap_errp, const struct iomap_ops *ops)
{
        switch (pe_size) {
        case PE_SIZE_PTE:
                return dax_iomap_pte_fault(vmf, pfnp, iomap_errp, ops);
        case PE_SIZE_PMD:
                return dax_iomap_pmd_fault(vmf, pfnp, ops);
        default:
                return VM_FAULT_FALLBACK;
        }
}
EXPORT_SYMBOL_GPL(dax_iomap_fault);

/*
 * dax_insert_pfn_mkwrite - insert PTE or PMD entry into page tables
 * @vmf: The description of the fault
 * @pfn: PFN to insert
 * @order: Order of entry to insert.
 *
 * This function inserts a writeable PTE or PMD entry into the page tables
 * for an mmaped DAX file.  It also marks the page cache entry as dirty.
 */
static vm_fault_t
dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
{
        struct address_space *mapping = vmf->vma->vm_file->f_mapping;
        XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, order);
        void *entry;
        vm_fault_t ret;

        xas_lock_irq(&xas);
        entry = get_unlocked_entry(&xas, order);
        /* Did we race with someone splitting entry or so? */
        if (!entry || dax_is_conflict(entry) ||
            (order == 0 && !dax_is_pte_entry(entry))) {
                put_unlocked_entry(&xas, entry, WAKE_NEXT);
                xas_unlock_irq(&xas);
                trace_dax_insert_pfn_mkwrite_no_entry(mapping->host, vmf,
                                                      VM_FAULT_NOPAGE);
                return VM_FAULT_NOPAGE;
        }
        xas_set_mark(&xas, PAGECACHE_TAG_DIRTY);
        dax_lock_entry(&xas, entry);
        xas_unlock_irq(&xas);
        if (order == 0)
                ret = vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
#ifdef CONFIG_FS_DAX_PMD
        else if (order == PMD_ORDER)
                ret = vmf_insert_pfn_pmd(vmf, pfn, FAULT_FLAG_WRITE);
#endif
        else
                ret = VM_FAULT_FALLBACK;
        dax_unlock_entry(&xas, entry);
        trace_dax_insert_pfn_mkwrite(mapping->host, vmf, ret);
        return ret;
}

/**
 * dax_finish_sync_fault - finish synchronous page fault
 * @vmf: The description of the fault
 * @pe_size: Size of entry to be inserted
 * @pfn: PFN to insert
 *
 * This function ensures that the file range touched by the page fault is
 * stored persistently on the media and handles inserting of appropriate page
 * table entry.
 */
vm_fault_t dax_finish_sync_fault(struct vm_fault *vmf,
                enum page_entry_size pe_size, pfn_t pfn)
{
        int err;
        loff_t start = ((loff_t)vmf->pgoff) << PAGE_SHIFT;
        unsigned int order = pe_order(pe_size);
        size_t len = PAGE_SIZE << order;

        err = vfs_fsync_range(vmf->vma->vm_file, start, start + len - 1, 1);
        if (err)
                return VM_FAULT_SIGBUS;
        return dax_insert_pfn_mkwrite(vmf, pfn, order);
}
EXPORT_SYMBOL_GPL(dax_finish_sync_fault);