linux-kernel - [PATCH] mm, slub: restore the original intention of prefetch

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Date:   Thu,  9 Aug 2018 10:52:45 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Paul Menzel <pmenzel+linux-mm@...gen.mpg.de>,
        Alex Deucher <alexander.deucher@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        Daniel Micay <danielmicay@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH] mm, slub: restore the original intention of prefetch_freepointer()

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@...e.cz>
Cc: Kees Cook <keescook@...omium.org>
Cc: Daniel Micay <danielmicay@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
---
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