[PATCH 1/2 v2] mm: vmpressure: fix sending wrong events on underflow

[Vinayak Menon]

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.

Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
---
v2: Adding a comment and reordering the patches
    as per Michal's suggestion

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

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 149fdf6..6063581 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);
 
-- 
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