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    Subject[PATCH 019 of 29] md: md: kill STRIPE_OP_MOD_DMA in raid5 offload

    From: Dan Williams <dan.j.williams@intel.com>

    This micro-optimization allowed the raid code to skip a re-read of the
    parity block after checking parity. It took advantage of the fact that
    xor-offload-engines have their own internal result buffer and can check
    parity without writing to memory. Remove it for the following reasons:

    1/ It is a layering violation for MD to need to manage the DMA and
    non-DMA paths within async_xor_zero_sum
    2/ Bad precedent to toggle the 'ops' flags outside the lock
    3/ Hard to realize a performance gain as reads will not need an updated
    parity block and writes will dirty it anyways.

    Signed-off-by: Dan Williams <dan.j.williams@intel.com>
    Signed-off-by: Neil Brown <neilb@suse.de>

    ### Diffstat output
    ./drivers/md/raid5.c | 10 ----------
    ./include/linux/raid/raid5.h | 2 --
    2 files changed, 12 deletions(-)

    diff .prev/drivers/md/raid5.c ./drivers/md/raid5.c
    --- .prev/drivers/md/raid5.c 2008-06-27 16:20:58.000000000 +1000
    +++ ./drivers/md/raid5.c 2008-06-27 16:22:03.000000000 +1000
    @@ -837,15 +837,10 @@ ops_run_postxor(struct stripe_head *sh,
    static void ops_complete_check(void *stripe_head_ref)
    {
    struct stripe_head *sh = stripe_head_ref;
    - int pd_idx = sh->pd_idx;

    pr_debug("%s: stripe %llu\n", __func__,
    (unsigned long long)sh->sector);

    - if (test_and_clear_bit(STRIPE_OP_MOD_DMA_CHECK, &sh->ops.pending) &&
    - sh->ops.zero_sum_result == 0)
    - set_bit(R5_UPTODATE, &sh->dev[pd_idx].flags);
    -
    set_bit(STRIPE_OP_CHECK, &sh->ops.complete);
    set_bit(STRIPE_HANDLE, &sh->state);
    release_stripe(sh);
    @@ -873,11 +868,6 @@ static void ops_run_check(struct stripe_
    tx = async_xor_zero_sum(xor_dest, xor_srcs, 0, count, STRIPE_SIZE,
    &sh->ops.zero_sum_result, 0, NULL, NULL, NULL);

    - if (tx)
    - set_bit(STRIPE_OP_MOD_DMA_CHECK, &sh->ops.pending);
    - else
    - clear_bit(STRIPE_OP_MOD_DMA_CHECK, &sh->ops.pending);
    -
    atomic_inc(&sh->count);
    tx = async_trigger_callback(ASYNC_TX_DEP_ACK | ASYNC_TX_ACK, tx,
    ops_complete_check, sh);
    diff .prev/include/linux/raid/raid5.h ./include/linux/raid/raid5.h
    --- .prev/include/linux/raid/raid5.h 2008-06-27 16:20:33.000000000 +1000
    +++ ./include/linux/raid/raid5.h 2008-06-27 16:22:03.000000000 +1000
    @@ -267,10 +267,8 @@ struct r6_state {

    /* modifiers to the base operations
    * STRIPE_OP_MOD_REPAIR_PD - compute the parity block and write it back
    - * STRIPE_OP_MOD_DMA_CHECK - parity is not corrupted by the check
    */
    #define STRIPE_OP_MOD_REPAIR_PD 7
    -#define STRIPE_OP_MOD_DMA_CHECK 8

    /*
    * Plugging:

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