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mm/slub: perform free consistency checks before call_rcu 

For SLAB_TYPESAFE_BY_RCU caches we use call_rcu to perform empty slab
freeing. The rcu callback rcu_free_slab() calls __free_slab() that
currently includes checking the slab consistency for caches with
SLAB_CONSISTENCY_CHECKS flags. This check needs the slab->objects field
to be intact.

Because in the next patch we want to allow rcu_head in struct slab to
become larger in debug configurations and thus potentially overwrite
more fields through a union than slab_list, we want to limit the fields
used in rcu_free_slab().  Thus move the consistency checks to
free_slab() before call_rcu(). This can be done safely even for
SLAB_TYPESAFE_BY_RCU caches where accesses to the objects can still
occur after freeing them.

As a result, only the slab->slab_cache field has to be physically
separate from rcu_head for the freeing callback to work. We also save
some cycles in the rcu callback for caches with consistency checks
enabled.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
This commit is contained in:
Vlastimil Babka 2022-08-26 11:09:11 +02:00
parent 247f34f7b8
commit bc29d5bd2b
1 changed files with 10 additions and 10 deletions
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20
mm/slub.c
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@ -1999,14 +1999,6 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
int order = folio_order(folio);
int pages = 1 << order;
if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
void *p;
slab_pad_check(s, slab);
for_each_object(p, s, slab_address(slab), slab->objects)
check_object(s, slab, p, SLUB_RED_INACTIVE);
}
__slab_clear_pfmemalloc(slab);
__folio_clear_slab(folio);
folio->mapping = NULL;
@ -2025,9 +2017,17 @@ static void rcu_free_slab(struct rcu_head *h)
static void free_slab(struct kmem_cache *s, struct slab *slab)
{
if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU)) {
if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
void *p;
slab_pad_check(s, slab);
for_each_object(p, s, slab_address(slab), slab->objects)
check_object(s, slab, p, SLUB_RED_INACTIVE);
}
if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
call_rcu(&slab->rcu_head, rcu_free_slab);
} else
else
__free_slab(s, slab);
}