On Wed, 9 Jan 2013 15:21:13 +0900
Minchan Kim <minchan@xxxxxxxxxx> 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 :(
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 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?
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