Re: [PATCH v2 1/2] SUNRPC: Fix memory reclaim deadlocks in rpciod

From: Michal Hocko <mhocko@suse.cz>
To: NeilBrown <neilb@suse.de>
Cc: Mel Gorman <mgorman@suse.com>,
	Trond Myklebust <trond.myklebust@primarydata.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Junxiao Bi <junxiao.bi@oracle.com>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
	Devel FS Linux <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] SUNRPC: Fix memory reclaim deadlocks in rpciod
Date: Wed, 10 Sep 2014 15:48:43 +0200	[thread overview]
Message-ID: <20140910134842.GG25219@dhcp22.suse.cz> (raw)
In-Reply-To: <20140909123346.434f0443@notabene.brown>

On Tue 09-09-14 12:33:46, Neil Brown wrote:
> On Thu, 4 Sep 2014 15:54:27 +0200 Michal Hocko <mhocko@suse.cz> wrote:
> 
> > [Sorry for jumping in so late - I've been busy last days]
> > 
> > On Wed 27-08-14 16:36:44, Mel Gorman wrote:
> > > On Tue, Aug 26, 2014 at 08:00:20PM -0400, Trond Myklebust wrote:
> > > > On Tue, Aug 26, 2014 at 7:51 PM, Trond Myklebust
> > > > <trond.myklebust@primarydata.com> wrote:
> > > > > On Tue, Aug 26, 2014 at 7:19 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > [...]
> > > > >> wait_on_page_writeback() is a hammer, and we need to be better about
> > > > >> this once we have per-memcg dirty writeback and throttling, but I
> > > > >> think that really misses the point.  Even if memcg writeback waiting
> > > > >> were smarter, any length of time spent waiting for yourself to make
> > > > >> progress is absurd.  We just shouldn't be solving deadlock scenarios
> > > > >> through arbitrary timeouts on one side.  If you can't wait for IO to
> > > > >> finish, you shouldn't be passing __GFP_IO.
> > 
> > Exactly!
> 
> This is overly simplistic.
> The code that cannot wait may be further up the call chain and not in a
> position to avoid passing __GFP_IO.
> In many case it isn't that "you can't wait for IO" in general, but that you
> cannot wait for one specific IO request.

Could you be more specific, please? Why would a particular IO make any
difference to general IO from the same path? My understanding was that
once the page is marked PG_writeback then it is about to be written to
its destination and if there is any need for memory allocation it should
better not allow IO from reclaim.

> wait_on_page_writeback() waits for a specific IO and so is dangerous.
> congestion_wait() or similar waits for IO in general and so is much safer.

congestion_wait was actually not sufficient to prevent from OOM with
heavy writer in a small memcg. We simply do not know how long will the
IO last so any "wait for a random timeout" will end up causing some
troubles.
-- 
Michal Hocko
SUSE Labs