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    Subject[PATCH 3/7] mempool: fix first round failure behavior
    Date
    mempool modifies gfp_mask so that the backing allocator doesn't try
    too hard or trigger warning message when there's pool to fall back on.
    In addition, for the first try, it removes __GFP_WAIT and IO, so that
    it doesn't trigger reclaim or wait when allocation can be fulfilled
    from pool; however, when that allocation fails and pool is empty too,
    it waits for the pool to be replenished before retrying.

    Allocation which could have succeeded after a bit of reclaim has to
    wait on the reserved items and it's not like mempool doesn't retry
    with __GFP_WAIT and IO. It just does that *after* someone returns an
    element, pointlessly delaying things.

    Fix it by retrying immediately if the first round of allocation
    attempts w/o __GFP_WAIT and IO fails.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    ---
    mm/mempool.c | 13 +++++++++++--
    1 files changed, 11 insertions(+), 2 deletions(-)

    diff --git a/mm/mempool.c b/mm/mempool.c
    index e3a802a..6326cc6 100644
    --- a/mm/mempool.c
    +++ b/mm/mempool.c
    @@ -221,14 +221,23 @@ repeat_alloc:
    return element;
    }

    - /* We must not sleep in the GFP_ATOMIC case */
    + /*
    + * We use gfp mask w/o __GFP_WAIT or IO for the first round. If
    + * alloc failed with that and @pool was empty, retry immediately.
    + */
    + if (gfp_temp != gfp_mask) {
    + gfp_temp = gfp_mask;
    + spin_unlock_irqrestore(&pool->lock, flags);
    + goto repeat_alloc;
    + }
    +
    + /* We must not sleep if !__GFP_WAIT */
    if (!(gfp_mask & __GFP_WAIT)) {
    spin_unlock_irqrestore(&pool->lock, flags);
    return NULL;
    }

    /* Let's wait for someone else to return an element to @pool */
    - gfp_temp = gfp_mask;
    init_wait(&wait);
    prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);

    --
    1.7.3.1


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