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

[Johannes Weiner <hannes@cmpxchg.org>]

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@google.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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