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slab: distinguish lock and trylock for sheaf_flush_main()

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sheaf_flush_main() can be called from __pcs_replace_full_main() where
it's fine if the trylock fails, and pcs_flush_all() where it's not
expected to and for some flush callers (when destroying the cache or
memory hotremove) it would be actually a problem if it failed and left
the main sheaf not flushed. The flush callers can however safely use
local_lock() instead of trylock.

The trylock failure should not happen in practice on !PREEMPT_RT, but
can happen on PREEMPT_RT. The impact is limited in practice because when
a trylock fails in the kmem_cache_destroy() path, it means someone is
using the cache while destroying it, which is a bug on its own. The memory
hotremove path is unlikely to be employed in a production RT config, but
it's possible.

To fix this, split the function into sheaf_flush_main() (using
local_lock()) and sheaf_try_flush_main() (using local_trylock()) where
both call __sheaf_flush_main_batch() to flush a single batch of objects.
This will also allow lockdep to verify our context assumptions.

The problem was raised in an off-list question by Marcelo.

Fixes: 2d517aa09b ("slab: add opt-in caching layer of percpu sheaves")
Cc: stable@vger.kernel.org
Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Hao Li <hao.li@linux.dev>
Link: https://patch.msgid.link/20260211-b4-sheaf-flush-v1-1-4e7f492f0055@suse.cz
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
This commit is contained in:
Vlastimil Babka
2026-02-11 10:42:30 +01:00
committed by Vlastimil Babka (SUSE)
parent e9217ca77d
commit 48647d3f9a
1 changed files with 37 additions and 10 deletions
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47
mm/slub.c
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@@ -2858,19 +2858,19 @@ static void __kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);
* object pointers are moved to a on-stack array under the lock. To bound the
* stack usage, limit each batch to PCS_BATCH_MAX.
*
* returns true if at least partially flushed
* Must be called with s->cpu_sheaves->lock locked, returns with the lock
* unlocked.
*
* Returns how many objects are remaining to be flushed
*/
static bool sheaf_flush_main(struct kmem_cache *s)
static unsigned int __sheaf_flush_main_batch(struct kmem_cache *s)
{
struct slub_percpu_sheaves *pcs;
unsigned int batch, remaining;
void *objects[PCS_BATCH_MAX];
struct slab_sheaf *sheaf;
bool ret = false;
next_batch:
if (!local_trylock(&s->cpu_sheaves->lock))
return ret;
lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
pcs = this_cpu_ptr(s->cpu_sheaves);
sheaf = pcs->main;
@@ -2888,10 +2888,37 @@ next_batch:
stat_add(s, SHEAF_FLUSH, batch);
ret = true;
return remaining;
}
if (remaining)
goto next_batch;
static void sheaf_flush_main(struct kmem_cache *s)
{
unsigned int remaining;
do {
local_lock(&s->cpu_sheaves->lock);
remaining = __sheaf_flush_main_batch(s);
} while (remaining);
}
/*
* Returns true if the main sheaf was at least partially flushed.
*/
static bool sheaf_try_flush_main(struct kmem_cache *s)
{
unsigned int remaining;
bool ret = false;
do {
if (!local_trylock(&s->cpu_sheaves->lock))
return ret;
ret = true;
remaining = __sheaf_flush_main_batch(s);
} while (remaining);
return ret;
}
@@ -5704,7 +5731,7 @@ alloc_empty:
if (put_fail)
stat(s, BARN_PUT_FAIL);
if (!sheaf_flush_main(s))
if (!sheaf_try_flush_main(s))
return NULL;
if (!local_trylock(&s->cpu_sheaves->lock))