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* [PATCH 1/2 v2] mm: vmpressure: fix sending wrong events on underflow
@ 2017-02-09 11:59 ` Vinayak Menon
  0 siblings, 0 replies; 14+ messages in thread
From: Vinayak Menon @ 2017-02-09 11:59 UTC (permalink / raw)
  To: akpm, hannes, mgorman, vbabka, mhocko, riel, vdavydov.dev,
	anton.vorontsov, minchan, shashim
  Cc: linux-mm, linux-kernel, Vinayak Menon

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.

Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
---
v2: Adding a comment and reordering the patches
    as per Michal's suggestion

 mm/vmpressure.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 149fdf6..6063581 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -112,9 +112,16 @@ 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;
 
 	/*
+	 * reclaimed can be greater than scanned in cases
+	 * like THP, where the scanned is 1 and reclaimed
+	 * could be 512
+	 */
+	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
 	 * time is in VM reclaimer's "ticks", i.e. number of pages
@@ -124,6 +131,7 @@ 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

^ permalink raw reply	[flat|nested] 14+ messages in thread

end of thread, other threads:[~2017-02-10  9:25 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-02-09 11:59 [PATCH 1/2 v2] mm: vmpressure: fix sending wrong events on underflow Vinayak Menon
2017-02-09 11:59 ` Vinayak Menon
2017-02-09 11:59 ` [PATCH 2/2 v5] mm: vmscan: do not pass reclaimed slab to vmpressure Vinayak Menon
2017-02-09 11:59   ` Vinayak Menon
2017-02-09 12:20   ` Michal Hocko
2017-02-09 12:20     ` Michal Hocko
2017-02-10  8:45     ` vinayak menon
2017-02-10  8:45       ` vinayak menon
2017-02-10  9:05       ` Michal Hocko
2017-02-10  9:05         ` Michal Hocko
2017-02-09 12:10 ` [PATCH 1/2 v2] mm: vmpressure: fix sending wrong events on underflow Michal Hocko
2017-02-09 12:10   ` Michal Hocko
2017-02-09 12:22   ` Michal Hocko
2017-02-09 12:22     ` Michal Hocko

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