Re: [PATCH] mm: vmscan: never isolate more pages than necessary

[Michal Hocko]

On Mon 03-08-15 18:02:27, Vladimir Davydov wrote:
> If transparent huge pages are enabled, we can isolate many more pages
> than we actually need to scan, because we count both single and huge
> pages equally in isolate_lru_pages().
> 
> Since commit 5bc7b8aca942d ("mm: thp: add split tail pages to shrink
> page list in page reclaim"), we scan all the tail pages immediately
> after a huge page split (see shrink_page_list()). As a result, we can
> reclaim up to SWAP_CLUSTER_MAX * HPAGE_PMD_NR (512 MB) in one run!

512MB is really unexpected. But yeah, you are right. Mel has increased
SWAP_CLUSTER_MAX to 256 recently (mm: increase SWAP_CLUSTER_MAX to
batch TLB flushes) which I have missed. That has made the situation
potentially much worse. I guess this is worth mentioning in the
changelog because the original SWAP_CLUSTER_MAX (32) hasn't looked that
scary.
 
> This is easy to catch on memcg reclaim with zswap enabled. The latter
> makes swapout instant so that if we happen to scan an unreferenced huge
> page we will evict both its head and tail pages immediately, which is
> likely to result in excessive reclaim.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/vmscan.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5221e19e98f4..94092fd3b96b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1387,7 +1387,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	unsigned long nr_taken = 0;
>  	unsigned long scan;
>  
> -	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
> +	for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
> +					!list_empty(src); scan++) {
>  		struct page *page;
>  		int nr_pages;
>  
> -- 
> 2.1.4
> 

-- 
Michal Hocko
SUSE Labs
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