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

[Jaewon Kim]

On 2020ë 04ì 20ì 15:19, Jaewon Kim wrote:
>
> 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()
Hello Minchan

Are you preparing a complete patch for this issue?
Sorry if I am bugging you.
>
> Thank you
> Jaewon Kim
>
>