Re: [PATCH v2] mlock: fix mlock count can not decrease in race condition

[Vlastimil Babka]

On 05/25/2017 04:13 AM, Yisheng Xie wrote:
> Kefeng reported that when run the follow test the mlock count

> in meminfo
> cannot be decreased:

"increases 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;
>  }
> 
> When __munlock_pagevec, we ClearPageMlock but isolation_failed in race
> condition, and we do not count these page into delta_munlocked, which cause
> mlock counter incorrect for we had Clear the PageMlock and cannot count down
> the number in the feture.

Can I suggest the following instead:

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 count the number of page whoes PageMlock flag is cleared.
> 
> Fixes: 1ebb7cc6a583 (" mm: munlock: batch NR_MLOCK zone state updates")
> Signed-off-by: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
> Reported-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> Tested-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks!

> Cc: Joern Engel <joern@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Xishi Qiu <qiuxishi@xxxxxxxxxx>
> CC: zhongjiang <zhongjiang@xxxxxxxxxx>
> Cc: Hanjun Guo <guohanjun@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
> v2:
>  - use delta_munlocked for it doesn't do the increment in fastpath - Vlastimil
> 
> Hi Andrew:
> Could you please help to fold this?
> 
> Thanks
> Yisheng Xie
> 
>  mm/mlock.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index c483c5c..b562b55 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -284,7 +284,7 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>  {
>  	int i;
>  	int nr = pagevec_count(pvec);
> -	int delta_munlocked;
> +	int delta_munlocked = -nr;
>  	struct pagevec pvec_putback;
>  	int pgrescued = 0;
>  
> @@ -304,6 +304,8 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>  				continue;
>  			else
>  				__munlock_isolation_failed(page);
> +		} else {
> +			delta_munlocked++;
>  		}
>  
>  		/*
> @@ -315,7 +317,6 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>  		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));
>  
>