Commit e1587a49 authored Feb 24, 2017 by Vinayak Menon Committed by Linus Torvalds Feb 24, 2017

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 3a4f8a0b

mm/vmpressure.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -112,8 +112,15 @@ 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
		@@ -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);

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