[PATCH UPDATED 2/2] mempool: fix first round failure behavior

[Tejun Heo]

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@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
The code hasn't changed.  Only the description and comment are
updated.  It doesn't affect anything regarding emergency pool.  It
just changes when the first retry happens.

That said, I still find it a bit unsettling that a GFP_ATOMIC
allocation which would otherwise succeed may fail when issued through
mempool.  Maybe the RTTD is clearing __GFP_NOMEMALLOC on retry if the
gfp requsted by the caller is !__GFP_WAIT && !__GFP_NOMEMALLOC?

Thanks.

 mm/mempool.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

Index: work/mm/mempool.c
===================================================================
--- work.orig/mm/mempool.c
+++ work/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);
 
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