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>

1 files changed, 9 insertions, 1 deletions

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 149fdf6c5c56..6063581f705c 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);