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

[Minchan Kim]

Hi Hannes,

On Wed, Jan 09, 2013 at 01:56:12AM -0500, Johannes Weiner wrote:
> On Wed, Jan 09, 2013 at 03:21:13PM +0900, Minchan Kim 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.
> > 
> > Reported-by: Luigi Semenzato <semenzato@xxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> We used to ignore the page's writeback state on isolation in the past,
> could you include a reference to since when this problem has been in

Good idea.
It has existed since f80c067[mm: zone_reclaim: make isolate_lru_page() filter-aware]
I will write down it in changelog.

> the tree?  Also, would it make sense to tag it for one of the stable
> trees?

If Luigi confirmed it, I will Cc stable@xxxxxxxxxxxxxxx in next spin.
Thanks!

> 
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-- 
Kind regards,
Minchan Kim
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