mempool: fix first round failure behavior

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.

[akpm@linux-foundation.org: shorten the lock hold time]
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/mm/mempool.c b/mm/mempool.c
index e3a802a0cea0..d9049811f352 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) {
+                spin_unlock_irqrestore(&pool->lock, flags);
+                gfp_temp = gfp_mask;
+                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);