[PATCH V3 3/3] mm: page_alloc: drain pcp lists before oom kill

[Charan Teja Kalla <quic_charante@quicinc.com>]

pcp lists are drained from __alloc_pages_direct_reclaim(), only if some
progress is made in the attempt.

struct page *__alloc_pages_direct_reclaim() {
    .....
   *did_some_progress = __perform_reclaim(gfp_mask, order, ac);
   if (unlikely(!(*did_some_progress)))
      goto out;
retry:
    page = get_page_from_freelist();
    if (!page && !drained) {
        drain_all_pages(NULL);
        drained = true;
        goto retry;
    }
out:
}

After the above, allocation attempt can fallback to
should_reclaim_retry() to decide reclaim retries. If it too return
false, allocation request will simply fallback to oom kill path without
even attempting the draining of the pcp pages that might help the
allocation attempt to succeed.

VM system running with ~50MB of memory shown the below stats during OOM
kill:
Normal free:760kB boost:0kB min:768kB low:960kB high:1152kB
reserved_highatomic:0KB managed:49152kB free_pcp:460kB

Though in such system state OOM kill is imminent, but the current kill
could have been delayed if the pcp is drained as pcp + free is even
above the high watermark.

Fix this missing drain of pcp list in should_reclaim_retry() along with
unreserving the high atomic page blocks, like it is done in
__alloc_pages_direct_reclaim().

Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
---
 mm/page_alloc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b91c99e..8eee292 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3857,8 +3857,10 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 		cond_resched();
 out:
 	/* Before OOM, exhaust highatomic_reserve */
-	if (!ret)
-		return unreserve_highatomic_pageblock(ac, true);
+	if (!ret) {
+		ret =  unreserve_highatomic_pageblock(ac, true);
+		drain_all_pages(NULL);
+	}
 
 	return ret;
 }
-- 
2.7.4