Re: [PATCH 1/2] mm: prevent to add a page to swap if may_writepage is unset

[Minchan Kim <minchan@kernel.org>]

Hi Andrew,

On Wed, Jan 09, 2013 at 04:18:54PM -0800, Andrew Morton wrote:
> On Wed,  9 Jan 2013 15:21:13 +0900
> Minchan Kim <minchan@kernel.org> wrote:
> 
> > Recently, Luigi reported there are lots of free swap space when
> > OOM happens. It's easily reproduced on zram-over-swap, where
> > many instance of memory hogs are running and laptop_mode is enabled.
> > 
> > Luigi reported there was no problem when he disabled laptop_mode.
> > The problem when I investigate problem is following as.
> > 
> > try_to_free_pages disable may_writepage if laptop_mode is enabled.
> > shrink_page_list adds lots of anon pages in swap cache by
> > add_to_swap, which makes pages Dirty and rotate them to head of
> > inactive LRU without pageout. If it is repeated, inactive anon LRU
> > is full of Dirty and SwapCache pages.
> > 
> > In case of that, isolate_lru_pages fails because it try to isolate
> > clean page due to may_writepage == 0.
> > 
> > The may_writepage could be 1 only if total_scanned is higher than
> > writeback_threshold in do_try_to_free_pages but unfortunately,
> > VM can't isolate anon pages from inactive anon lru list by
> > above reason and we already reclaimed all file-backed pages.
> > So it ends up OOM killing.
> > 
> > This patch prevents to add a page to swap cache unnecessary when
> > may_writepage is unset so anoymous lru list isn't full of
> > Dirty/Swapcache page. So VM can isolate pages from anon lru list,
> > which ends up setting may_writepage to 1 and could swap out
> > anon lru pages. When OOM triggers, I confirmed swap space was full.
> > 
> > ...
> >
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -780,6 +780,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		if (PageAnon(page) && !PageSwapCache(page)) {
> >  			if (!(sc->gfp_mask & __GFP_IO))
> >  				goto keep_locked;
> > +			if (!sc->may_writepage)
> > +				goto keep_locked;
> >  			if (!add_to_swap(page))
> >  				goto activate_locked;
> >  			may_enter_fs = 1;
> 
> I'm not really getting it, and the description is rather hard to follow :(

It seems I don't have a talent about description. :(
I hope it would be better this year. :)

> 
> We should be adding anon pages to swapcache even when laptop_mode is
> set.  And we should be writing them to swap as well, then reclaiming
> them.  The only thing laptop_mode shouild do is make the disk spin up
> less frequently - that doesn't mean "not at all"!

So it seems your rationale is that let's save power in only system has
enough memory so let's remove may_writepage in reclaim path?

If it is, I love it because I didn't see any number about power saving
through reclaiming throttling(But surely there was reason to add it)
and not sure it works well during long time because we have tweaked
reclaim part too many.

> 
> So something seems screwed up here and the patch looks like a
> heavy-handed workaround.  Why aren't these anon pages getting written
> out in laptop_mode?

Don't know. It was there long time and I don't want to screw it up.
If we decide paging out in reclaim path regardless of laptop_mode,
it makes the problem easy without ugly workaround.

Remove may_writepage? If it's too agressive, we can remove it in only
direct reclaim path.

> 
> 
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-- 
Kind regards,
Minchan Kim

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