Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in page_cache_read

From: Michal Hocko <mhocko@suse.cz>
To: Dave Chinner <david@fromorbit.com>
Cc: Rik van Riel <riel@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@suse.de>, Neil Brown <neilb@suse.de>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Sage Weil <sage@inktank.com>, Mark Fasheh <mfasheh@suse.com>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in page_cache_read
Date: Mon, 23 Mar 2015 14:02:01 +0100	[thread overview]
Message-ID: <20150323130201.GB29084@dhcp22.suse.cz> (raw)
In-Reply-To: <20150320225139.GL28621@dastard>

On Sat 21-03-15 09:51:39, Dave Chinner wrote:
> On Fri, Mar 20, 2015 at 02:14:53PM +0100, Michal Hocko wrote:
> > On Fri 20-03-15 14:48:20, Dave Chinner wrote:
> > > On Thu, Mar 19, 2015 at 01:44:41PM +0100, Michal Hocko wrote:
> > > > > allocations where the caller cannot pass a gfp_mask need to obey
> > > > > the mapping_gfp_mask that is set by the mapping owner....
> > > > 
> > > > Hmm, I thought this is true only when the function might be called from
> > > > the fs path.
> > > 
> > > How do you know in, say, mpage_readpages, you aren't being called
> > > from a fs path that holds locks? e.g. we can get there from ext4
> > > doing readdir, so it is holding an i_mutex lock at that point.
> > > 
> > > Many other paths into mpages_readpages don't hold locks, but there
> > > are some that do, and those that do need functionals like this to
> > > obey the mapping_gfp_mask because it is set appropriately for the
> > > allocation context of the inode that owns the mapping....
> > 
> > What about the following?
> > ---
> > From 5d905cb291138d61bbab056845d6e53bc4451ec8 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@suse.cz>
> > Date: Thu, 19 Mar 2015 14:56:56 +0100
> > Subject: [PATCH 1/2] mm: do not ignore mapping_gfp_mask in page cache
> >  allocation paths
> 
> Looks reasonable, though I though there were more places that that
> in the mapping paths that need to be careful...

I have focused on those which involve page cache allocation because
those are obvious. We might need others but I do not see them right now.

I will include this patch for the next submit after I manage to wrap my
head around up-coming xfs changes and come up with something for
page_cache_read vs OOM killer issue.
-- 
Michal Hocko
SUSE Labs