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

[Minchan Kim]

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
>