linux-kernel - Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP

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Date:	Tue, 4 Aug 2015 11:51:58 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Nikolay Borisov <kernel@...p.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Marian Marinov <mm@...com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations

On Mon 03-08-15 23:32:00, Hugh Dickins wrote:
[...]
> But I have modified it a little, I don't think you'll mind.  As you
> suggested yourself, I actually prefer to test may_enter_fs there, rather
> than __GFP_FS: not a big deal, I certainly wouldn't want to delay the
> fix if someone thinks differently; but I tend to feel that may_enter_fs
> is what we already use for such decisions there, so better to use it.
> (And the SwapCache case immune to ext4 or xfs IO submission pattern.)

I am not opposed. This is closer to what we had before.

[...]
> (I was tempted to add in
> my unlock_page there, that we discussed once before: but again thought
> it better to minimize the fix - it is "selfish" not to unlock_page,
> but I think that anything heading for deadlock on the locked page would
> in other circumstances be heading for deadlock on the writeback page -
> I've never found that change critical.)

I agree. It would deserve a separate patch.

> And I've done quite a bit of testing.  The loads that hung at the
> weekend have been running nicely for 24 hours now, no problem with the
> writeback hang and no problem with the dcache ENOTDIR issue.  Though
> I've no idea of what recent VM change turned this into a hot issue.
> 
> And more testing on the history of it, considering your stable 3.6+
> designation that I wasn't satisfied with.  Getting out that USB stick
> again, I find that 3.6, 3.7 and 3.8 all OOM if their __GFP_IO test
> is updated to a may_enter_fs test; but something happened in 3.9
> to make it and subsequent releases safe with the may_enter_fs test.

Interesting. I would have guessed that 3.12 would make a difference (as
mentioned in the changelog). Why would 3.9 make a difference is not
entirely clear to me.

> You can certainly argue that the remote chance of a deadlock is
> worse than the fair chance of a spurious OOM; but if you insist
> on 3.6+, then I think it would have to go back even further,
> because we marked that commit for stable itself.  I suggest 3.9+.

Agreed and thanks!
-- 
Michal Hocko
SUSE Labs
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