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    Subject[PATCH 4.14 023/117] mm, slub: restore the original intention of prefetch_freepointer()
    Date
    From: Vlastimil Babka <vbabka@suse.cz>

    commit 0882ff9190e3bc51e2d78c3aadd7c690eeaa91d5 upstream.

    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.

    Link: http://lkml.kernel.org/r/20180809085245.22448-1-vbabka@suse.cz
    Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    Acked-by: Kees Cook <keescook@chromium.org>
    Cc: Daniel Micay <danielmicay@gmail.com>
    Cc: Eric Dumazet <edumazet@google.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Matthias Schiffer <mschiffer@universe-factory.net>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sven Eckelmann <sven@narfation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/slub.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    diff --git a/mm/slub.c b/mm/slub.c
    index 3c1a16f03b2bd..481518c3f61a9 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -269,8 +269,7 @@ 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)
    --
    2.20.1


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