slab: fix generic PAGE_POISONING conflict with SLAB_RED_ZONE

A generic page poisoning mechanism was added with commit:
 6a11f75b6a17b5d9ac5025f8d048382fd1f47377
which destructively poisons full pages with a bitpattern.

On arches where PAGE_POISONING is used, this conflicts with the slab
redzone checking enabled by DEBUG_SLAB, scribbling bits all over its
magic words and making it complain about that quite emphatically.

On x86 (and I presume at present all the other arches which set
ARCH_SUPPORTS_DEBUG_PAGEALLOC too), the kernel_map_pages() operation
is non destructive so it can coexist with the other DEBUG_SLAB
mechanisms just fine.

This patch favours the expensive full page destruction test for
cases where there is a collision and it is explicitly selected.

Signed-off-by: Ron Lee <ron@debian.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

1 files changed, 9 insertions, 0 deletions

diff --git a/mm/slab.c b/mm/slab.c
index 9a90b00d2f91..1a6040d0f41d 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2353,6 +2353,15 @@ kmem_cache_create (const char *name, size_t size, size_t align,
                 /* really off slab. No need for manual alignment */
                 slab_size =
                     cachep->num * sizeof(kmem_bufctl_t) + sizeof(struct slab);
+
+#ifdef CONFIG_PAGE_POISONING
+                /* If we're going to use the generic kernel_map_pages()
+                 * poisoning, then it's going to smash the contents of
+                 * the redzone and userword anyhow, so switch them off.
+                 */
+                if (size % PAGE_SIZE == 0 && flags & SLAB_POISON)
+                        flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
+#endif
         }
 
         cachep->colour_off = cache_line_size();