linux-kernel - [PATCH 1/2 v2] mm: vmscan: do not pass reclaimed slab to vmpressure

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Date:   Fri, 27 Jan 2017 13:43:36 +0530
From:   Vinayak Menon <vinmenon@...eaurora.org>
To:     akpm@...ux-foundation.org, hannes@...xchg.org,
        mgorman@...hsingularity.net, vbabka@...e.cz, mhocko@...e.com,
        riel@...hat.com, vdavydov.dev@...il.com,
        anton.vorontsov@...aro.org, minchan@...nel.org,
        shashim@...eaurora.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vinayak Menon <vinmenon@...eaurora.org>
Subject: [PATCH 1/2 v2] mm: vmscan: do not pass reclaimed slab to vmpressure

It is noticed that during a global reclaim the memory
reclaimed via shrinking the slabs can sometimes result
in reclaimed pages being greater than the scanned pages
in shrink_node. When this is passed to vmpressure, the
unsigned arithmetic results in the pressure value to be
huge, thus resulting in a critical event being sent to
root cgroup. While this can be fixed by underflow checks
in vmpressure, adding reclaimed slab without a corresponding
increment of nr_scanned results in incorrect vmpressure
reporting. So do not consider reclaimed slab pages in
vmpressure calculation.

Signed-off-by: Vinayak Menon <vinmenon@...eaurora.org>
---
 mm/vmscan.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 947ab6f..37c4486 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2594,16 +2594,16 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 				    sc->nr_scanned - nr_scanned,
 				    node_lru_pages);
 
-		if (reclaim_state) {
-			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
-			reclaim_state->reclaimed_slab = 0;
-		}
-
 		/* Record the subtree's reclaim efficiency */
 		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
 			   sc->nr_scanned - nr_scanned,
 			   sc->nr_reclaimed - nr_reclaimed);
 
+		if (reclaim_state) {
+			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
+			reclaim_state->reclaimed_slab = 0;
+		}
+
 		if (sc->nr_reclaimed - nr_reclaimed)
 			reclaimable = true;
 
-- 
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