Patch "mm: vmpressure: fix sending wrong events on underflow" has been added to the 4.10-stable tree

This is a note to let you know that I've just added the patch titled

mm: vmpressure: fix sending wrong events on underflow

to the 4.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-vmpressure-fix-sending-wrong-events-on-underflow.patch
and it can be found in the queue-4.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vinayak Menon <[email protected]>
Date: Fri, 24 Feb 2017 14:59:39 -0800
Subject: mm: vmpressure: fix sending wrong events on underflow

From: Vinayak Menon <[email protected]>

commit e1587a4945408faa58d0485002c110eb2454740c upstream.

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.

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

@@ -112,9 +112,16 @@ static enum vmpressure_levels vmpressure
 						    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
 	pressure = scale - (reclaimed * scale / scanned);
 	pressure = pressure * 100 / scale;
 
+out:
 	pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
 		 scanned, reclaimed);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.10/mm-vmpressure-fix-sending-wrong-events-on-underflow.patch