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

[Vlastimil Babka]

On 08/05/2015 02:52 AM, Jaewon Kim wrote:
> On 2015ë 08ì 05ì 08:31, Minchan Kim wrote:
> > 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@xxxxxxxxxxx> 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?

The only user that will see the change wrt migration is 
__alloc_contig_migrate_range() which is explicit about isolating mlocked 
page for migration (isolate_migratepages_range() calls 
isolate_migratepages_block() with ISOLATE_UNEVICTABLE). So this will 
make the migration work for clean page cache too.

> > 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.

This should be more explicit in the changelog. The first issue is not 
mentioned at all. The second is not clear from the description.

> > So, I think it's stable material.
> >
> > Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> >
> Hello
>
> Traditional shrink_inactive_list will put back the unevictable pages as it does through putback_inactive_pages.
> However as Minchan Kim said, cma_alloc will be more successful by migrating unevictable pages.
> In current kernel, I think, cma_alloc is already trying to migrate unevictable pages except clean page cache.
> This patch will allow clean page cache also to be migrated in cma_alloc.
>
> Thank you
>
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