[PATCH] slab: respect architecture and caller mandated alignment

As explained by Heiko, on s390 (32-bit) ARCH_KMALLOC_MINALIGN is set to
eight because their common I/O layer allocates data structures that need to
have an eight byte alignment.  This does not work when CONFIG_SLAB_DEBUG is
enabled because kmem_cache_create will override alignment to BYTES_PER_WORD
which is four.

So change kmem_cache_create to ensure cache alignment is always at minimum
what the architecture or caller mandates even if slab debugging is enabled.

Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 16 insertions, 9 deletions

diff --git a/mm/slab.c b/mm/slab.c
index 00584dbbec03..d47d0e186973 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2096,6 +2096,15 @@ kmem_cache_create (const char *name, size_t size, size_t align,
         } else {
                 ralign = BYTES_PER_WORD;
         }
+
+        /*
+         * Redzoning and user store require word alignment. Note this will be
+         * overridden by architecture or caller mandated alignment if either
+         * is greater than BYTES_PER_WORD.
+         */
+        if (flags & SLAB_RED_ZONE || flags & SLAB_STORE_USER)
+                ralign = BYTES_PER_WORD;
+
         /* 2) arch mandated alignment: disables debug if necessary */
         if (ralign < ARCH_SLAB_MINALIGN) {
                 ralign = ARCH_SLAB_MINALIGN;
@@ -2109,8 +2118,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
                         flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
         }
         /*
-         * 4) Store it. Note that the debug code below can reduce
-         *    the alignment to BYTES_PER_WORD.
+         * 4) Store it.
          */
         align = ralign;
 
@@ -2122,20 +2130,19 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 #if DEBUG
         cachep->obj_size = size;
 
+        /*
+         * Both debugging options require word-alignment which is calculated
+         * into align above.
+         */
         if (flags & SLAB_RED_ZONE) {
-                /* redzoning only works with word aligned caches */
-                align = BYTES_PER_WORD;
-
                 /* add space for red zone words */
                 cachep->obj_offset += BYTES_PER_WORD;
                 size += 2 * BYTES_PER_WORD;
         }
         if (flags & SLAB_STORE_USER) {
-                /* user store requires word alignment and
-                 * one word storage behind the end of the real
-                 * object.
+                /* user store requires one word storage behind the end of
+                 * the real object.
                  */
-                align = BYTES_PER_WORD;
                 size += BYTES_PER_WORD;
         }
 #if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)