When something registers and unregisters many shrinkers, such as:
for x in $(seq 10000); do unshare -Ui true; done
Sometimes the following error is printed to the kernel log:
debugfs: Directory '...' with parent 'shrinker' already present!
This occurs since commit
badc28d4924b ("mm: shrinkers: fix deadlock in
shrinker debugfs") / v6.2: Since the call to `debugfs_remove_recursive`
was moved outside the `shrinker_rwsem`/`shrinker_mutex` lock, but the call
to `ida_free` stayed inside, a newly registered shrinker can be
re-assigned that ID and attempt to create the debugfs directory before the
directory from the previous shrinker has been removed.
The locking changes in commit
f95bdb700bc6 ("mm: vmscan: make global slab
shrink lockless") made the race condition more likely, though it existed
before then.
Commit
badc28d4924b ("mm: shrinkers: fix deadlock in shrinker debugfs")
could be reverted since the issue is addressed should no longer occur
since the count and scan operations are lockless since commit
20cd1892fcc3
("mm: shrinkers: make count and scan in shrinker debugfs lockless").
However, since this is a contended lock, prefer instead moving `ida_free`
outside the lock to avoid the race.
Link: https://lkml.kernel.org/r/20230503013232.299211-1-joanbrugueram@gmail.com
Fixes: badc28d4924b ("mm: shrinkers: fix deadlock in shrinker debugfs")
Signed-off-by: Joan Bruguera Micó <joanbrugueram@gmail.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
#ifdef CONFIG_SHRINKER_DEBUG
extern int shrinker_debugfs_add(struct shrinker *shrinker);
-extern struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker);
+extern struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
+ int *debugfs_id);
+extern void shrinker_debugfs_remove(struct dentry *debugfs_entry,
+ int debugfs_id);
extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
const char *fmt, ...);
#else /* CONFIG_SHRINKER_DEBUG */
{
return 0;
}
-static inline struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
+static inline struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
+ int *debugfs_id)
{
+ *debugfs_id = -1;
return NULL;
}
+static inline void shrinker_debugfs_remove(struct dentry *debugfs_entry,
+ int debugfs_id)
+{
+}
static inline __printf(2, 3)
int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
{
}
EXPORT_SYMBOL(shrinker_debugfs_rename);
-struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
+struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
+ int *debugfs_id)
{
struct dentry *entry = shrinker->debugfs_entry;
kfree_const(shrinker->name);
shrinker->name = NULL;
- if (entry) {
- ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id);
- shrinker->debugfs_entry = NULL;
- }
+ *debugfs_id = entry ? shrinker->debugfs_id : -1;
+ shrinker->debugfs_entry = NULL;
return entry;
}
+void shrinker_debugfs_remove(struct dentry *debugfs_entry, int debugfs_id)
+{
+ debugfs_remove_recursive(debugfs_entry);
+ ida_free(&shrinker_debugfs_ida, debugfs_id);
+}
+
static int __init shrinker_debugfs_init(void)
{
struct shrinker *shrinker;
void unregister_shrinker(struct shrinker *shrinker)
{
struct dentry *debugfs_entry;
+ int debugfs_id;
if (!(shrinker->flags & SHRINKER_REGISTERED))
return;
shrinker->flags &= ~SHRINKER_REGISTERED;
if (shrinker->flags & SHRINKER_MEMCG_AWARE)
unregister_memcg_shrinker(shrinker);
- debugfs_entry = shrinker_debugfs_remove(shrinker);
+ debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
mutex_unlock(&shrinker_mutex);
atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
- debugfs_remove_recursive(debugfs_entry);
+ shrinker_debugfs_remove(debugfs_entry, debugfs_id);
kfree(shrinker->nr_deferred);
shrinker->nr_deferred = NULL;