Re: [PATCH v2] vmscan: fix increasing nr_isolated incurred by putback unevictable pages

Minchan Kim
2015-08-04T23:30:55Z

Hello,

On Tue, Aug 04, 2015 at 03:09:37PM -0700, Andrew Morton wrote:
> On Tue, 04 Aug 2015 19:40:08 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote:
>
> > reclaim_clean_pages_from_list() assumes that shrink_page_list() returns
> > number of pages removed from the candidate list. But shrink_page_list()
> > puts back mlocked pages without passing it to caller and without
> > counting as nr_reclaimed. This incurrs increasing nr_isolated.
> > To fix this, this patch changes shrink_page_list() to pass unevictable
> > pages back to caller. Caller will take care those pages.
> >
> > ..
> >
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1157,7 +1157,7 @@ cull_mlocked:
> > if (PageSwapCache(page))
> > try_to_free_swap(page);
> > unlock_page(page);
> > - putback_lru_page(page);
> > + list_add(&page->lru, &ret_pages);
> > continue;
> >
> > activate_locked:
>
> Is this going to cause a whole bunch of mlocked pages to be migrated
> whereas in current kernels they stay where they are?
>

It fixes two issues.

1. With unevictable page, cma_alloc will be successful.

Exactly speaking, cma_alloc of current kernel will fail due to unevictable pages.

2. fix leaking of NR_ISOLATED counter of vmstat

With it, too_many_isolated works. Otherwise, it could make hang until
the process get SIGKILL.

So, I think it's stable material.

Acked-by: Minchan Kim <minchan@kernel.org>