Date:   Wed, 22 Apr 2020 17:39:05 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>, mgorman@...e.de,
        m.szyprowski@...sung.com, mina86@...a86.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jaewon31.kim@...il.com,
        ytk.lee@...sung.com, shli@...com
Subject: Re: [PATCH] mm/vmscan: skip layzfree page on
 reclaim_clean_pages_from_list



On 2020년 04월 22일 14:40, Minchan Kim wrote:
> On Tue, Apr 21, 2020 at 09:06:37PM +0900, Jaewon Kim wrote:
>>
>> On 2020년 04월 20일 15:19, Jaewon Kim wrote:
>>> On 2020년 04월 18일 08:45, Jaewon Kim wrote:
>>>> On 2020년 04월 18일 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@...sung.com>
>>>>>>> Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
>>>>>> 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.
> Hi Jaewon
>
> Sorry for the late. You catched the bug and gave good description with
> the solution. What I did was just suggestion for alternative so feel
> free to send the patch with your SoB. You could use my Suggested-by
> and Acked-by. It's totally your credit!
Thank you for your support and generosity.
>
> Thanks!
>
> Thanks!
>
>

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