Re: [PATCH v2] xfs: remove kmem_zalloc_greedy

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Brian Foster <bfoster@redhat.com>,
	Michal Hocko <mhocko@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Xiong Zhou <xzhou@redhat.com>,
	linux-xfs@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
	Dave Chinner <david@fromorbit.com>,
	sebastian.parschauer@suse.com, AlNovak@suse.com, jack@suse.cz
Subject: Re: [PATCH v2] xfs: remove kmem_zalloc_greedy
Date: Tue, 14 Mar 2017 11:07:38 -0700	[thread overview]
Message-ID: <20170314180738.GV5280@birch.djwong.org> (raw)
In-Reply-To: <20170314165745.GB28800@wotan.suse.de>

On Tue, Mar 14, 2017 at 05:57:45PM +0100, Luis R. Rodriguez wrote:
> On Tue, Mar 07, 2017 at 04:35:28PM -0800, Darrick J. Wong wrote:
> > The sole remaining caller of kmem_zalloc_greedy is bulkstat, which uses
> > it to grab 1-4 pages for staging of inobt records.  The infinite loop in
> > the greedy allocation function is causing hangs[1] in generic/269, so
> > just get rid of the greedy allocator in favor of kmem_zalloc_large.
> > This makes bulkstat somewhat more likely to ENOMEM if there's really no
> > pages to spare, but eliminates a source of hangs.
> > 
> > [1] http://lkml.kernel.org/r/20170301044634.rgidgdqqiiwsmfpj%40XZHOUW.usersys.redhat.com
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> > v2: remove single-page fallback
> > ---
> 
> Since this fixes a hang how about *at the very least* a respective Fixes tag ?
> This fixes an existing hang so what are the stable considerations here ? I
> realize the answer is not easy but figured its worth asking.

I didn't think it was appropriate to "Fixes: 77e4635ae1917" since we're
not fixing _greedy so much as we are killing it.  The patch fixes an
infinite retry hang when bulkstat tries a memory allocation that cannot
be satisfied; and having done that, realizes there are no remaining
callers of _greedy and garbage collects it.  The code that was there
before also seems capable of sleeping forever, I think.

So the minimally invasive fix is to apply the allocation conversion in
bulkstat, and if there aren't any other callers of _greedy then you can
get rid of it too.

> FWIW I trace kmem_zalloc_greedy()'s introduction back to 2006 77e4635ae1917
> ("[XFS] Add a greedy allocation interface, allocating within a min/max size
> range.") through v2.6.19 days...

--D

> 
>   Luis
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