+ mm-vmpressure-fix-sending-wrong-events-on-underflow.patch added to -mm tree

+ mm-vmpressure-fix-sending-wrong-events-on-underflow.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm: vmpressure: fix sending wrong events on underflow
has been added to the -mm tree.  Its filename is
     mm-vmpressure-fix-sending-wrong-events-on-underflow.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-vmpressure-fix-sending-wrong-events-on-underflow.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-vmpressure-fix-sending-wrong-events-on-underflow.patch

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------------------------------------------------------
From: Vinayak Menon <vinmenon@codeaurora.org>
Subject: 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.

Link: http://lkml.kernel.org/r/1486641577-11685-1-git-send-email-vinmenon@codeaurora.org
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Shiraz Hashim <shashim@codeaurora.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

diff -puN mm/vmpressure.c~mm-vmpressure-fix-sending-wrong-events-on-underflow mm/vmpressure.c
--- a/mm/vmpressure.c~mm-vmpressure-fix-sending-wrong-events-on-underflow
+++ a/mm/vmpressure.c
@@ -112,9 +112,16 @@ static enum vmpressure_levels vmpressure
 						    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
 	pressure = scale - (reclaimed * scale / scanned);
 	pressure = pressure * 100 / scale;
 
+out:
 	pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
 		 scanned, reclaimed);
 
_

Patches currently in -mm which might be from vinmenon@codeaurora.org are

mm-vmpressure-fix-sending-wrong-events-on-underflow.patch
mm-vmscan-do-not-pass-reclaimed-slab-to-vmpressure.patch