[PATCH] mm, oom: protect !costly allocations some more

[Michal Hocko]

should_reclaim_retry will give up retries for higher order allocations
if none of the eligible zones has any requested or higher order pages
available even if we pass the watermak check for order-0. This is done
because there is no guarantee that the reclaimable and currently free
pages will form the required order.

This can, however, lead to situations were the high-order request (e.g.
order-2 required for the stack allocation during fork) will trigger
OOM too early - e.g. after the first reclaim/compaction round. Such a
system would have to be highly fragmented and the OOM killer is just a
matter of time but let's stick to our MAX_RECLAIM_RETRIES for the high
order and not costly requests to make sure we do not fail prematurely.

This also means that we do not reset no_progress_loops at the
__alloc_pages_slowpath for high order allocations to guarantee a bounded
number of retries.

Longterm it would be much better to communicate with the compaction
and retry only if the compaction considers it meaningfull.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/page_alloc.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 269a04f20927..f05aca36469b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3106,6 +3106,18 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 		}
 	}
 
+	/*
+	 * OK, so the watermak check has failed. Make sure we do all the
+	 * retries for !costly high order requests and hope that multiple
+	 * runs of compaction will generate some high order ones for us.
+	 *
+	 * XXX: ideally we should teach the compaction to try _really_ hard
+	 * if we are in the retry path - something like priority 0 for the
+	 * reclaim
+	 */
+	if (order && order <= PAGE_ALLOC_COSTLY_ORDER)
+		return true;
+
 	return false;
 }
 
@@ -3281,11 +3293,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		goto noretry;
 
 	/*
-	 * Costly allocations might have made a progress but this doesn't mean
-	 * their order will become available due to high fragmentation so do
-	 * not reset the no progress counter for them
+	 * High order allocations might have made a progress but this doesn't
+	 * mean their order will become available due to high fragmentation so
+	 * do not reset the no progress counter for them
 	 */
-	if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER)
+	if (did_some_progress && !order)
 		no_progress_loops = 0;
 	else
 		no_progress_loops++;
-- 
2.7.0

-- 
Michal Hocko
SUSE Labs