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@xxxxxxx>
---
mm/slub.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..d86be1b0d09f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2036,14 +2036,6 @@ 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;
@@ -2062,9 +2054,17 @@ 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);
}
--
2.37.2
