Re: [PATCH 1/4] vmscan: all_unreclaimable() usezone->all_unreclaimable as a name

From: Andrew Morton
Date: Mon Apr 11 2011 - 17:54:16 EST

On Mon, 11 Apr 2011 14:30:31 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> all_unreclaimable check in direct reclaim has been introduced at 2.6.19
> by following commit.
> 2006 Sep 25; commit 408d8544; oom: use unreclaimable info
> And it went through strange history. firstly, following commit broke
> the logic unintentionally.
> 2008 Apr 29; commit a41f24ea; page allocator: smarter retry of
> costly-order allocations
> Two years later, I've found obvious meaningless code fragment and
> restored original intention by following commit.
> 2010 Jun 04; commit bb21c7ce; vmscan: fix do_try_to_free_pages()
> return value when priority==0
> But, the logic didn't works when 32bit highmem system goes hibernation
> and Minchan slightly changed the algorithm and fixed it .
> 2010 Sep 22: commit d1908362: vmscan: check all_unreclaimable
> in direct reclaim path
> But, recently, Andrey Vagin found the new corner case. Look,
> struct zone {
> ..
> int all_unreclaimable;
> ..
> unsigned long pages_scanned;
> ..
> }
> zone->all_unreclaimable and zone->pages_scanned are neigher atomic
> variables nor protected by lock. Therefore zones can become a state
> of zone->page_scanned=0 and zone->all_unreclaimable=1. In this case,
> current all_unreclaimable() return false even though
> zone->all_unreclaimabe=1.
> Is this ignorable minor issue? No. Unfortunatelly, x86 has very
> small dma zone and it become zone->all_unreclamble=1 easily. and
> if it become all_unreclaimable=1, it never restore all_unreclaimable=0.
> Why? if all_unreclaimable=1, vmscan only try DEF_PRIORITY reclaim and
> a-few-lru-pages>>DEF_PRIORITY always makes 0. that mean no page scan
> at all!
> Eventually, oom-killer never works on such systems. That said, we
> can't use zone->pages_scanned for this purpose. This patch restore
> all_unreclaimable() use zone->all_unreclaimable as old. and in addition,
> to add oom_killer_disabled check to avoid reintroduce the issue of
> commit d1908362.

The above is a nice analysis of the bug and how it came to be
introduced. But we don't actually have a bug description! What was
the observeable problem which got fixed?

Such a description will help people understand the importance of the
patch and will help people (eg, distros) who are looking at a user's
bug report and wondering whether your patch will fix it.

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