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Date: Fri, 27 Mar 2015 08:43:54 +1100
From: Dave Chinner <david@fromorbit.com>
To: Michal Hocko <mhocko@suse.cz>
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
Message-ID: <20150326214354.GG28129@dastard>
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On Thu, Mar 26, 2015 at 10:53:02AM +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:
> [...]
> > > Or did I miss your point? Are you concerned about some fs overloading
> > > filemap_fault and do some locking before delegating to filemap_fault?
> > 
> > The latter:
> > 
> > https://git.kernel.org/cgit/linux/kernel/git/dgc/linux-xfs.git/commit/?h=xfs-mmap-lock&id=de0e8c20ba3a65b0f15040aabbefdc1999876e6b
> 
> Hmm. I am completely unfamiliar with the xfs code but my reading of
> 964aa8d9e4d3..723cac484733 is that the newly introduced lock should be
> OK from the reclaim recursion POV. It protects against truncate and
> punch hole, right? Or are there any internal paths which I am missing
> and would cause problems if we do GFP_FS with XFS_MMAPLOCK_SHARED held?

It might be OK, but you're only looking at the example I gave you,
not the fundamental issue it demonstrates. That is: filesystems may
have *internal dependencies that are unknown to the page cache or mm
subsystem*. Hence the page cache or mm allocations cannot
arbitrarily ignore allocation constraints the filesystem assigns to
mapping operations....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com