In SLUB, prefetch_freepointer() is used when allocating an object from cache's
freelist, to make sure the next object in the list is cache-hot, since it's
probable it will be allocated soon.
Commit 2482ddec670f ("mm: add SLUB free list pointer obfuscation") has
unintentionally changed the prefetch in a way where the prefetch is turned to a
real fetch, and only the next->next pointer is prefetched. In case there is not
a stream of allocations that would benefit from prefetching, the extra real
fetch might add a useless cache miss to the allocation. Restore the previous
behavior.
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
While I don't expect this to be causing the bug at hand, it's worth fixing.
For the bug it might mean that the page fault moves elsewhere.
mm/slub.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 51258eff4178..ce2b9e5cea77 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -271,8 +271,7 @@ static inline void *get_freepointer(struct kmem_cache *s, void *object)
static void prefetch_freepointer(const struct kmem_cache *s, void *object)
{
- if (object)
- prefetch(freelist_dereference(s, object + s->offset));
+ prefetch(object + s->offset);
}
static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
--
2.18.0
