linux-kernel - Re: [PATCH RFC] ext4: use __GFP_NOFAIL in ext4_free

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Message-ID: <20160225090839.GC17573@dhcp22.suse.cz>
Date:	Thu, 25 Feb 2016 10:08:39 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Nikolay Borisov <kernel@...p.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Dmitry Monakhov <dmonakhov@...tuozzo.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH RFC] ext4: use __GFP_NOFAIL in ext4_free_blocks()

On Thu 25-02-16 11:01:32, Nikolay Borisov wrote:
> 
> 
> On 02/24/2016 07:09 PM, Konstantin Khlebnikov wrote:
> > This might be unexpected but pages allocated for sbi->s_buddy_cache are
> > charged to current memory cgroup. So, GFP_NOFS allocation could fail if
> > current task has been killed by OOM or if current memory cgroup has no
> > free memory left. Block allocator cannot handle such failures here yet.
> > 
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> 
> Adding new users of GFP_NOFAIL is deprecated.

This is not true. GFP_NOFAIL should be used where the allocation failure
is no tolleratable and it is much more preferrable to doing an opencoded
endless loop over page allocator.

> Where exactly does the
> block allocator fail, I skimmed the code and failing ext4_mb_load_buddy
> seems to be handled at all call sites. There are some BUG_ONs but from
> the comments there I guess they should occur when we try to find a page
> and not allocate a new one?

I have posted a similar patch last year:
http://lkml.kernel.org/r/1438768284-30927-6-git-send-email-mhocko@kernel.org
because I could see emergency reboots when GFP_NOFS allocations were
allowed to fail.

-- 
Michal Hocko
SUSE Labs