Re: [PATCH] mm/vmscan: skip layzfree page on reclaim_clean_pages_from_list

[Jaewon Kim]

On 2020ë 04ì 18ì 08:45, Jaewon Kim wrote:
>
> On 2020ë 04ì 18ì 00:13, Minchan Kim wrote:
>> On Thu, Apr 16, 2020 at 05:38:37PM -0700, Minchan Kim wrote:
>>> Hi Jaewon,
>>>
>>> On Thu, Apr 16, 2020 at 12:35:14PM +0900, Jaewon Kim wrote:
>>>> This patch fix nr_isolate_* mismatch problem between cma and dirty
>>>> lazyfree page.
>>>>
>>>> If try_to_unmap_one is used for reclaim and it detects a dirty lazyfree
>>>> page, then the lazyfree page is changed to a normal anon page having
>>>> SwapBacked by commit 18863d3a3f59 ("mm: remove SWAP_DIRTY in ttu"). Even
>>>> with the change, reclaim context correctly counts isolated files because
>>>> it uses is_file_lru to distinguish file. And the change to anon is not
>>>> happened if try_to_unmap_one is used for migration. So migration context
>>>> like compaction also correctly counts isolated files even though it uses
>>>> page_is_file_lru insted of is_file_lru. Recently page_is_file_cache was
>>>> renamed to page_is_file_lru by commit 9de4f22a60f7 ("mm: code cleanup for
>>>> MADV_FREE").
>>>>
>>>> But the nr_isolate_* mismatch problem happens on cma alloc. There is
>>>> reclaim_clean_pages_from_list which is being used only by cma. It was
>>>> introduced by commit 02c6de8d757c ("mm: cma: discard clean pages during
>>>> contiguous allocation instead of migration") to reclaim clean file pages
>>>> without migration. The cma alloc uses both reclaim_clean_pages_from_list
>>>> and migrate_pages, and it uses page_is_file_lru to count isolated
>>>> files. If there are dirty lazyfree pages allocated from cma memory
>>>> region, the pages are counted as isolated file at the beginging but are
>>>> counted as isolated anon after finished.
>>>>
>>>> Mem-Info:
>>>> Node 0 active_anon:3045904kB inactive_anon:611448kB active_file:14892kB inactive_file:205636kB unevictable:10416kB isolated(anon):0kB isolated(file):37664kB mapped:630216kB dirty:384kB writeback:0kB shmem:42576kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
>>>>
>>>> Like log above, there was too much isolated file, 37664kB, which
>>>> triggers too_many_isolated in reclaim when there is no isolated file in
>>>> system wide. It could be reproducible by running two programs, doing
>>>> MADV_FREE, writing and doing cma alloc, respectively. Although isolated
>>>> anon is 0, I found that the internal value of isolated anon was the
>>>> negative value of isolated file.
>>>>
>>>> Fix this by skipping anon pages on reclaim_clean_pages_from_list. The
>>>> lazyfree page can be checked by both PageAnon(page) and
>>>> page_is_file_lru. But in this case, PageAnon is enough to skip all
>>>> anon pages.
>>>>
>>>> Reported-by: Yong-Taek Lee <ytk.lee@xxxxxxxxxxx>
>>>> Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
>>> Thanks for the investigation!
>>> The thing is MADV_FREEed page since supporting swapless could change
>>> his LRU status between reclaim.
>>>
>>> I am worry about voiding the optimization we have kept in CMA but
>>> also don't have good idea, either so I tend to agree with this.
>>>
>>> Let me Cc Johannes who might have better idea.
>>>
>>>> ---
>>>>  mm/vmscan.c | 3 +++
>>>>  1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index b06868fc4926..9380a18eef5e 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -1497,6 +1497,9 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>>>>  	LIST_HEAD(clean_pages);
>>>>  
>>>>  	list_for_each_entry_safe(page, next, page_list, lru) {
>>>> +		/* to avoid race with MADV_FREE anon page */
>>>> +		if (PageAnon(page))
>>>> +			continue;
>>>>  		if (page_is_file_lru(page) && !PageDirty(page) &&
>>>>  		    !__PageMovable(page) && !PageUnevictable(page)) {
>>>>  			ClearPageActive(page);
>>>> -- 
>>>> 2.13.7
>>>>
>> Hi Jaewon,
>>
>> How about this idea? I think it could solve the issue with keeping
>> CMA alloc latency optimization.
> Hello Minchan
>
> It looks good to me except compilation error.
>
> And to apply this patch on other stable branches, we may need some other
> dependent patches though.
>
> Thank you
>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
>> index 292485f3d24d..10cc932e209a 100644
>> --- a/include/linux/vmstat.h
>> +++ b/include/linux/vmstat.h
>> @@ -29,6 +29,7 @@ struct reclaim_stat {
>>  	unsigned nr_activate[2];
>>  	unsigned nr_ref_keep;
>>  	unsigned nr_unmap_fail;
>> +	unsigned nr_lazyfree_fail;
>>  };
>>  
>>  enum writeback_stat_item {
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 4c8a1cdccbba..b390f6094f2f 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1296,11 +1296,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>  		 */
>>  		if (page_mapped(page)) {
>>  			enum ttu_flags flags = ttu_flags | TTU_BATCH_FLUSH;
>> +			bool lazyfree = PageAnon(page) && !PageSwapBacked(page);
>>  
>>  			if (unlikely(PageTransHuge(page)))
>>  				flags |= TTU_SPLIT_HUGE_PMD;
>> +
>>  			if (!try_to_unmap(page, flags)) {
>>  				stat->nr_unmap_fail += nr_pages;
>> +				if (lazyfree && PageSwapBacked(page))
>> +					stat->nr_lazyfree_fail += nr_pages;
>>  				goto activate_locked;
>>  			}
>>  		}
>> @@ -1492,8 +1496,8 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>>  		.priority = DEF_PRIORITY,
>>  		.may_unmap = 1,
>>  	};
>> -	struct reclaim_stat dummy_stat;
>> -	unsigned long ret;
>> +	struct reclaim_stat stat;
>> +	unsigned long reclaimed;
>>  	struct page *page, *next;
>>  	LIST_HEAD(clean_pages);
>>  
>> @@ -1505,11 +1509,21 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>>  		}
>>  	}
>>  
>> -	ret = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
>> -			TTU_IGNORE_ACCESS, &dummy_stat, true);
>> +	reclaimed = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
>> +			TTU_IGNORE_ACCESS, &stat, true);
>>  	list_splice(&clean_pages, page_list);
>> -	mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -ret);
>> -	return ret;
>> +	mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -reclaimed);
>> +	/*
>> +	 * Since lazyfree pages are isolated from file LRU from the beginning,
>> +	 * they will rotate back to anonymous LRU in the end if it failed to
>> +	 * discard so isolated count will be mismatched.
>> +	 * Compensate the isolated count for both LRU lists.
>> +	 */
>> +	mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_ANON,
>> +					stat->nr_lazyfree_fail);
>> +	mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE,
>> +					-stat->nr_lazyfree_fail);
> should be stat.nr_lazyfree_fail and -stat.nr_lazyfree_fail instead of ->
>> +	return reclaimed;
>>  }
>>  
>>  /*

Let me just CC Shaohua Li for commit 802a3a92ad7a ("mm: reclaim MADV_FREE pages")
, because I missed him/her on other mail thread 
 : Sorry, I think I pointed a wrong commit, the SwapBacked was recovered
 : by commit 802a3a92ad7a ("mm: reclaim MADV_FREE pages").


Hello Minchan

I tested on my v4.19 based source tree and it seems to work.

Prior to your patch I applied commit 060f005f0747 (
"mm/vmscan.c: do not allocate duplicate stack variables in shrink_page_list()"
for struct reclaim_stat.

I considered other dependent changes below to follow code changes, not really needed for this issue though.
v5.3 98879b3b9edc mm: vmscan: correct some vmscan counters for THP swapout
v5.2 886cf1901db9 mm: move recent_rotated pages calculation to shrink_inactive_list()

Thank you
Jaewon Kim