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

[Andrew Morton <akpm@linux-foundation.org>]

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 :(

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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