[PATCH 2/2] mm: vmpressure: fix sending wrong events on underflow

From: Vinayak Menon
Date: Fri Jan 27 2017 - 03:15:27 EST

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.

Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
mm/vmpressure.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 149fdf6..3281b34 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -112,8 +112,10 @@ 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;

+ 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 +126,7 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
pressure = scale - (reclaimed * scale / scanned);
pressure = pressure * 100 / scale;

pr_debug("%s: %3lu (s: %lu r: %lu)\n", __func__, pressure,
scanned, reclaimed);

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