Re: [PATCH 1/5] mm, vmscan: Do not account skipped pages as scanned

[Minchan Kim <minchan@kernel.org>]

On Wed, Jul 20, 2016 at 04:21:47PM +0100, Mel Gorman wrote:
> Page reclaim determines whether a pgdat is unreclaimable by examining how
> many pages have been scanned since a page was freed and comparing that
> to the LRU sizes. Skipped pages are not considered reclaim candidates but
> contribute to scanned. This can prematurely mark a pgdat as unreclaimable
> and trigger an OOM kill.
> 
> While this does not fix an OOM kill message reported by Joonsoo Kim,
> it did stop pgdat being marked unreclaimable.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
>  mm/vmscan.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 22aec2bcfeec..b16d578ce556 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1415,7 +1415,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	LIST_HEAD(pages_skipped);
>  
>  	for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
> -					!list_empty(src); scan++) {
> +					!list_empty(src);) {
>  		struct page *page;
>  
>  		page = lru_to_page(src);
> @@ -1429,6 +1429,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  			continue;
>  		}
>  
> +		/* Pages skipped do not contribute to scan */

The comment should explain why.

/* Pages skipped do not contribute to scan to prevent premature OOM */


> +		scan++;
> +


The one of my concern about node-lru is to add more lru lock contetion
in multiple zone system so such unbounded skip scanning under the lock
should have a limit to prevent latency spike and serialization of
current reclaim work.

Another concern is big mismatch between the number of pages from list and
LRU stat count because lruvec_lru_size call sites don't take the stat
under the lock while isolate_lru_pages moves many pages from lru list
to temporal skipped list.


>  		switch (__isolate_lru_page(page, mode)) {
>  		case 0:
>  			nr_pages = hpage_nr_pages(page);
> -- 
> 2.6.4
> 
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