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

[Minchan Kim <minchan@kernel.org>]

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@samsung.com>
> >>>>> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.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!

Thanks!

Thanks!