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

[zhong jiang <zhongjiang@huawei.com>]

On 2017/5/26 17:06, Vlastimil Babka wrote:
> 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
 it seems make sense. but the we just mmap the specified ragne and mlock it.
 when the process drop out, exit_mmap will call munlock_vma_pages_all to
 unmap the mlock range and make the event normal.
 
 Do you think?

 Thanks
zhongjinag
>> ---
>>  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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