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

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  [PATCH 1/2 v2] mm: vmscan: do not pass reclaimed slab to vmpressure Vinayak Menon <vinmenon@codeaurora.org> - 2017-01-27 09:20 +0100
    [PATCH 2/2] mm: vmpressure: fix sending wrong events on underflow Vinayak Menon <vinmenon@codeaurora.org> - 2017-01-27 09:20 +0100

#1567933 — [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@codeaurora.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;
 
-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

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#1567941 — [PATCH 2/2] mm: vmpressure: fix sending wrong events on underflow

At the end of a window period, if the reclaimed pages
is greater than scanned, an unsigned underflow can
result in a huge pressure value and thus a critical event.
Reclaimed pages is found to go higher than scanned because
of the addition of reclaimed slab pages to reclaimed in
shrink_node without a corresponding increment to scanned
pages. Minchan Kim mentioned that this can also happen in
the case of a THP page where the scanned is 1 and reclaimed
could be 512.

Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
---
 mm/vmpressure.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 149fdf6..3281b34 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -112,8 +112,10 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 						    unsigned long reclaimed)
 {
 	unsigned long scale = scanned + reclaimed;
-	unsigned long pressure;
+	unsigned long pressure = 0;
 
+	if (reclaimed >= scanned)
+		goto out;
 	/*
 	 * We calculate the ratio (in percents) of how many pages were
 	 * scanned vs. reclaimed in a given time frame (window). Note that
@@ -124,6 +126,7 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 	pressure = scale - (reclaimed * scale / scanned);
 	pressure = pressure * 100 / scale;
 
+out:
 	pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
 		 scanned, reclaimed);
 
-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

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