Re: [PATCH 5/5] mm, vmscan: Account for skipped pages as a partial scan

[Joonsoo Kim]

On Thu, Jul 21, 2016 at 03:11:01PM +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 reclaim candidates but contribute to
> scanned. This can prematurely mark a pgdat as unreclaimable and trigger
> an OOM kill.
> 
> This patch accounts for skipped pages as a partial scan so that an
> unreclaimable pgdat will still be marked as such but by scaling the cost
> of a skip, it'll avoid the pgdat being marked prematurely.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6810d81f60c7..e5af357dd4ac 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1424,7 +1424,7 @@ 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);
> @@ -1438,6 +1438,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  			continue;
>  		}
>  
> +		/*
> +		 * Account for scanned and skipped separetly to avoid the pgdat
> +		 * being prematurely marked unreclaimable by pgdat_reclaimable.
> +		 */
> +		scan++;
> +

This logic has potential unbounded retry problem. src would not become
empty if __isolate_lru_page() return -EBUSY since we move failed page
to src list again in this case.

Thanks.

>  		switch (__isolate_lru_page(page, mode)) {
>  		case 0:
>  			nr_pages = hpage_nr_pages(page);
> @@ -1465,14 +1471,24 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	 */
>  	if (!list_empty(&pages_skipped)) {
>  		int zid;
> +		unsigned long total_skipped = 0;
>  
> -		list_splice(&pages_skipped, src);
>  		for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>  			if (!nr_skipped[zid])
>  				continue;
>  
>  			__count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
> +			total_skipped += nr_skipped[zid];
>  		}
> +
> +		/*
> +		 * Account skipped pages as a partial scan as the pgdat may be
> +		 * close to unreclaimable. If the LRU list is empty, account
> +		 * skipped pages as a full scan.
> +		 */
> +		scan += list_empty(src) ? total_skipped : total_skipped >> 2;
> +
> +		list_splice(&pages_skipped, src);
>  	}
>  	*nr_scanned = scan;
>  	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,
> -- 
> 2.6.4
> 
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