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
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>
Change-Id: Ic1221fa129c9b3512127537d62df7bd4a7606d24
Git-commit: 746d8d8f3ebb49a62ffcffb4b1f41cc27d020a2e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index f514dc4..f5383e4 100644 (file)
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -149,9 +149,16 @@ static unsigned long vmpressure_calc_pressure(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
@@ -161,6 +168,7 @@ static unsigned long vmpressure_calc_pressure(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);