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

[Rik van Riel <riel@redhat.com>]

On 01/20/2013 08:52 PM, Minchan Kim wrote:

>  From 94086dc7152359d052802c55c82ef19509fe8cce Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Mon, 21 Jan 2013 10:43:43 +0900
> Subject: [PATCH] mm: Use up free swap space before reaching OOM kill
>
> 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.
> He said there was no problem when he disabled laptop_mode.
> The problem when I investigate problem is following as.
>
> Assumption for easy explanation: There are no page cache page in system
> because they all are already reclaimed.
>
> 1. try_to_free_pages disable may_writepage when laptop_mode is enabled.
> 2. shrink_inactive_list isolates victim pages from inactive anon lru list.
> 3. shrink_page_list adds them to swapcache via add_to_swap but it doesn't
>     pageout because sc->may_writepage is 0 so the page is rotated back into
>     inactive anon lru list. The add_to_swap made the page Dirty by SetPageDirty.
> 4. 3 couldn't reclaim any pages so do_try_to_free_pages increase priority and
>     retry reclaim with higher priority.
> 5. shrink_inactlive_list try to isolate victim pages from inactive anon lru list
>     but got failed because it try to isolate pages with ISOLATE_CLEAN mode but
>     inactive anon lru list is full of dirty pages by 3 so it just returns
>     without  any reclaim progress.
> 6. do_try_to_free_pages doesn't set may_writepage due to zero total_scanned.
>     Because sc->nr_scanned is increased by shrink_page_list but we don't call
>     shrink_page_list in 5 due to short of isolated pages.
>
> Above loop is continued until OOM happens.
> The problem didn't happen before [1] was merged because old logic's
> isolatation in shrink_inactive_list was successful and tried to call
> shrink_page_list to pageout them but it still ends up failed to page out
> by may_writepage. But important point is that sc->nr_scanned was increased
> although we couldn't swap out them so do_try_to_free_pages could set
> may_writepages.
>
> Since [1] was introduced, it's not a good idea any more to depends on
> only the number of scanned pages for setting may_writepage. So this patch
> adds new trigger point of setting may_writepage as below DEF_PRIOIRTY - 2
> which is used to show the significant memory pressure in VM so it's good
> fit for our purpose which would be better to lose power saving or clickety
> rather than OOM killing.
>
> [1] f80c067[mm: zone_reclaim: make isolate_lru_page() filter-aware]
>
> Reported-by: Luigi Semenzato <semenzato@google.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

Your patch is a nice simplification.  I am ok with the
change, provided it works for Luigi :)

Acked-by: Rik van Riel <riel@redhat.com>


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