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    Subject[patch 2/2] slab: always consider arch mandated alignment
    From: Heiko Carstens <heiko.carstens@de.ibm.com>

    Since ARCH_KMALLOC_MINALIGN didn't work on s390 I tried ARCH_SLAB_MINALIGN
    instead, just to find out that it didn't work too.
    In case of CONFIG_DEBUG_SLAB kmem_cache_create() creates caches with an
    alignment lesser than ARCH_SLAB_MINALIGN, which it shouldn't according to
    this comment in mm/slab.c :

    * Enforce a minimum alignment for all caches.
    * Intended for archs that get misalignment faults even for BYTES_PER_WORD
    * aligned buffers. Includes ARCH_KMALLOC_MINALIGN.
    * If possible: Do not enable this flag for CONFIG_DEBUG_SLAB, it disables
    * some debug features.

    For example the following might happen if kmem_cache_create() gets called
    with -- size: 64; align: 0; flags with SLAB_HWCACHE_ALIGN, SLAB_RED_ZONE and
    SLAB_STORE_USER set. ARCH_SLAB_MINALIGN is 8.
    These are the steps as numbered in kmem_cache_create() where 5) is after the
    "if (flags & SLAB_RED_ZONE)" statement.

    1) align: 0 ralign 64
    2) align: 0 ralign 64
    3) align: 0 ralign 64
    4) align: 64 ralign 64
    5) align: 4 ralign 64

    Note that in this case in step 2) the flags SLAB_RED_ZONE and SLAB_STORE_USER
    don't get masked out and that this causes an BYTES_PER_WORD alignment in
    step 5) which is lesser than ARCH_SLAB_MINALIGN.

    Cc: Christoph Lameter <clameter@sgi.com>
    Cc: Pekka Enberg <penberg@cs.helsinki.fi>
    Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
    ---

    mm/slab.c | 3 +++
    1 files changed, 3 insertions(+)

    Index: linux-2.6/mm/slab.c
    ===================================================================
    --- linux-2.6.orig/mm/slab.c 2006-07-26 09:55:54.000000000 +0200
    +++ linux-2.6/mm/slab.c 2006-07-26 09:57:07.000000000 +0200
    @@ -2103,6 +2103,9 @@
    if (ralign > BYTES_PER_WORD)
    flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
    }
    + if (BYTES_PER_WORD < ARCH_SLAB_MINALIGN)
    + flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
    +
    /* 3) caller mandated alignment: disables debug if necessary */
    if (ralign < align) {
    ralign = align;
    -
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