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From: Michal Hocko <mhocko@kernel.org>
To: Jan Kara <jack@suse.cz>
Cc: lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] proposals for topics
Date: Mon, 25 Jan 2016 15:40:23 +0100	[thread overview]
Message-ID: <20160125144023.GF23934@dhcp22.suse.cz> (raw)
In-Reply-To: <20160125142139.GF24938@quack.suse.cz>

On Mon 25-01-16 15:21:39, Jan Kara wrote:
> Hi!
> 
> On Mon 25-01-16 14:33:57, Michal Hocko wrote:
[...
> >   Another issue is that GFP_NOFS is quite often used without any obvious
> >   reason. It is not clear which lock is held and could be taken from
> >   the reclaim path. Wouldn't it be much better if the no-recursion
> >   behavior was bound to the lock scope rather than particular allocation
> >   request? We already have something like this for PM
> >   pm_res{trict,tore}_gfp_mask resp. memalloc_noio_{save,restore}. It
> >   would be great if we could unify this and use the context based NOFS
> >   in the FS.
> 
> I like the idea that we'd protect lock scopes from reclaim recursion but the
> effort to do so would be IMHO rather big. E.g. there are ~75 instances of
> GFP_NOFS allocation in ext4/jbd2 codebase and making sure all are properly
> covered will take quite some auditing... I'm not saying we shouldn't do
> something like this, just you will have to be good in selling the benefits
> :).

My idea was that the first step would be using the helpers to mark
scopes and other usage of the ~__GFP_FS inside such a scope could be
identified much easier (e.g. a debugging WARN_ON or something like
that). That can be done in a longer term. Then I would hope for reducing
GFP_NOFS usage from mapping_gfp_mask.

I realize this is a lot of work but I believe this will pay of long
term. And especially the first step shouldn't be that hard because locks
used from the reclaim path shouldn't be that hard to identify.

GFP_NOFS is a mess these days and it is far from trivial to tell wether
it should be used or not from some paths.
-- 
Michal Hocko
SUSE Labs

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  reply	other threads:[~2016-01-25 14:40 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-25 13:33 [LSF/MM TOPIC] proposals for topics Michal Hocko
2016-01-25 14:21 ` [Lsf-pc] " Jan Kara
2016-01-25 14:40   ` Michal Hocko [this message]
2016-01-25 15:08 ` Tetsuo Handa
2016-01-26  9:43   ` Michal Hocko
2016-01-27 13:44     ` Tetsuo Handa
2016-01-27 14:33       ` [Lsf-pc] " Jan Kara
2016-01-25 18:45 ` Johannes Weiner
2016-01-26  9:50   ` Michal Hocko
2016-01-26 17:17     ` Vlastimil Babka
2016-01-26 17:20       ` [Lsf-pc] " Jan Kara
2016-01-27  9:08         ` Michal Hocko
2016-01-28 20:55     ` Dave Chinner
2016-01-28 22:04       ` Michal Hocko
2016-01-31 23:29         ` Dave Chinner
2016-02-01 12:24           ` Vlastimil Babka
2016-01-26 17:07   ` Vlastimil Babka
2016-01-26 18:09     ` Johannes Weiner
2016-01-30 18:18   ` Greg Thelen

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