Patch "mlock: fix mlock count can not decrease in race condition" has been added to the 4.9-stable tree

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

mlock: fix mlock count can not decrease in race condition

to the 4.9-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:
mlock-fix-mlock-count-can-not-decrease-in-race-condition.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Yisheng Xie <[email protected]>
Date: Fri, 2 Jun 2017 14:46:43 -0700
Subject: mlock: fix mlock count can not decrease in race condition

From: Yisheng Xie <[email protected]>

commit 70feee0e1ef331b22cc51f383d532a0d043fbdcc upstream.

Kefeng reported that when running the follow test, the mlock count in meminfo will increase permanently:

[1] testcase
linux:~ # cat test_mlockal
grep Mlocked /proc/meminfo
for j in seq 0 10
do
for i in seq 4 15
do
./p_mlockall >> log &
done
sleep 0.2
done

 # wait some time to let mlock counter decrease and 5s may not enough
 sleep 5
 grep Mlocked /proc/meminfo

linux:~ # cat p_mlockall.c
#include <sys/mman.h>
#include <stdlib.h>
#include <stdio.h>

#define SPACE_LEN 4096

int main(int argc, char ** argv)

 {
	 	int ret;
	 	void *adr = malloc(SPACE_LEN);
	 	if (!adr)
	 		return -1;

	 	ret = mlockall(MCL_CURRENT | MCL_FUTURE);
	 	printf("mlcokall ret = %d\n", ret);

	 	ret = munlockall();
	 	printf("munlcokall ret = %d\n", ret);

	 	free(adr);
	 	return 0;
	 }

In __munlock_pagevec() we should decrement NR_MLOCK for each page where we clear the PageMlocked flag. Commit 1ebb7cc6a583 ("mm: munlock: batch NR_MLOCK zone state updates") has introduced a bug where we don't decrement NR_MLOCK for pages where we clear the flag, but fail to isolate them from the lru list (e.g. when the pages are on some other cpu's percpu pagevec). Since PageMlocked stays cleared, the NR_MLOCK accounting gets permanently disrupted by this.

Fix it by counting the number of page whose PageMlock flag is cleared.

---
 mm/mlock.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

@@ -285,7 +285,7 @@ static void __munlock_pagevec(struct pag
 {
 	int i;
 	int nr = pagevec_count(pvec);
-	int delta_munlocked;
+	int delta_munlocked = -nr;
 	struct pagevec pvec_putback;
 	int pgrescued = 0;
 

@@ -305,6 +305,8 @@ static void __munlock_pagevec(struct pag
 				continue;
 			else
 				__munlock_isolation_failed(page);
+		} else {
+			delta_munlocked++;
 		}
 
 		/*

@@ -316,7 +318,6 @@ static void __munlock_pagevec(struct pag
 		pagevec_add(&pvec_putback, pvec->pages[i]);
 		pvec->pages[i] = NULL;
 	}
-	delta_munlocked = -nr + pagevec_count(&pvec_putback);
 	__mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
 	spin_unlock_irq(zone_lru_lock(zone));
 

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

queue-4.9/mlock-fix-mlock-count-can-not-decrease-in-race-condition.patch