linux-kernel - Re: [RFC 0/2] mapping_gfp_mask from the page fault path

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Message-Id: <20150602132241.26fbbc98be71920da8485b73@linux-foundation.org>
Date:	Tue, 2 Jun 2015 13:22:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	<linux-mm@...ck.org>, Dave Chinner <david@...morbit.com>,
	Neil Brown <neilb@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 0/2] mapping_gfp_mask from the page fault path

On Mon,  1 Jun 2015 15:00:01 +0200 Michal Hocko <mhocko@...e.cz> wrote:

> I somehow forgot about these patches. The previous version was
> posted here: http://marc.info/?l=linux-mm&m=142668784122763&w=2. The
> first attempt was broken but even when fixed it seems like ignoring
> mapping_gfp_mask in page_cache_read is too fragile because
> filesystems might use locks in their filemap_fault handlers
> which could trigger recursion problems as pointed out by Dave
> http://marc.info/?l=linux-mm&m=142682332032293&w=2.
> 
> The first patch should be straightforward fix to obey mapping_gfp_mask
> when allocating for mapping. It can be applied even without the second
> one.

I'm not so sure about that.  If only [1/2] is applied then those
filesystems which are setting mapping_gfp_mask to GFP_NOFS will now
actually start using GFP_NOFS from within page_cache_read() etc.  The
weaker allocation mode might cause problems.

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