Re: [PATCH v2] mm: fix mlock incorrent event account

[Vlastimil Babka <vbabka@suse.cz>]

On 05/26/2017 05:54 AM, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
> 
> Recently, when I address in the issue, Subject "mlock: fix mlock count
> can not decrease in race condition" had been take over, I review
> the code and find the potential issue. it will result in the incorrect
> account, it will make us misunderstand straightforward.
> 
> The following testcase can prove the issue.
> 
> int main(void)
> {
>     char *map;
>     int fd;
> 
>     fd = open("test", O_CREAT|O_RDWR);
>     unlink("test");
>     ftruncate(fd, 4096);
>     map = mmap(NULL, 4096, PROT_WRITE, MAP_PRIVATE, fd, 0);
>     map[0] = 11;
>     mlock(map, 4096);
>     ftruncate(fd, 0);
>     close(fd);
>     munlock(map, 4096);
>     munmap(map, 4096);
> 
>     return 0;
> }
> 
> before:
> unevictable_pgs_mlocked 10589
> unevictable_pgs_munlocked 10588
> unevictable_pgs_cleared 1
> 
> apply the patch;
> after:
> unevictable_pgs_mlocked 9497
> unevictable_pgs_munlocked 9497
> unevictable_pgs_cleared 1
> 
> unmap_mapping_range unmap them,  page_remove_rmap will deal with
> clear_page_mlock situation.  we clear page Mlock flag and successful
> isolate the page,  the page will putback the evictable list. but it is not
> record the munlock event.
> 
> The patch add the event account when successful page isolation.
> 
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>

Hi,

I think this is by design. UNEVICTABLE_PGMUNLOCKED is supposed for explicit
munlock() actions from userspace. Truncation etc is counted by
UNEVICTABLE_PGCLEARED.

Vlastimil

> ---
>  mm/mlock.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index c483c5c..941930b 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -64,6 +64,7 @@ void clear_page_mlock(struct page *page)
>  			    -hpage_nr_pages(page));
>  	count_vm_event(UNEVICTABLE_PGCLEARED);
>  	if (!isolate_lru_page(page)) {
> +		count_vm_event(UNEVICTABLE_PGMUNLOCKED);
>  		putback_lru_page(page);
>  	} else {
>  		/*
> 

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