linux-kernel - Re: [PATCH 3.5 29/64] fs: buffer: move allocation failure loop into the allocator

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Message-Id: <20131031090353.a67265bf.akpm@linux-foundation.org>
Date:	Thu, 31 Oct 2013 09:03:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Luis Henriques <luis.henriques@...onical.com>,
	linux-kernel@...r.kernel.org, kernel-team@...ts.ubuntu.com,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 3.5 29/64] fs: buffer: move allocation failure loop into
 the allocator

On Thu, 31 Oct 2013 15:48:48 +0100 Jan Kara <jack@...e.cz> wrote:

> On Thu 31-10-13 10:00:08, Johannes Weiner wrote:
> > On Mon, Oct 28, 2013 at 02:47:48PM +0000, Luis Henriques wrote:
> > > 3.5.7.24 -stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Johannes Weiner <hannes@...xchg.org>
> > > 
> > > commit 84235de394d9775bfaa7fa9762a59d91fef0c1fc upstream.
> > > 
> > > Buffer allocation has a very crude indefinite loop around waking the
> > > flusher threads and performing global NOFS direct reclaim because it can
> > > not handle allocation failures.
> > > 
> > > The most immediate problem with this is that the allocation may fail due
> > > to a memory cgroup limit, where flushers + direct reclaim might not make
> > > any progress towards resolving the situation at all.  Because unlike the
> > > global case, a memory cgroup may not have any cache at all, only
> > > anonymous pages but no swap.  This situation will lead to a reclaim
> > > livelock with insane IO from waking the flushers and thrashing unrelated
> > > filesystem cache in a tight loop.
> > > 
> > > Use __GFP_NOFAIL allocations for buffers for now.  This makes sure that
> > > any looping happens in the page allocator, which knows how to
> > > orchestrate kswapd, direct reclaim, and the flushers sensibly.  It also
> > > allows memory cgroups to detect allocations that can't handle failure
> > > and will allow them to ultimately bypass the limit if reclaim can not
> > > make progress.
>   So I was under the impression that __GFP_NOFAIL is going away, doesn't
> it? At least about an year ago there was some effort to remove its users so
> we ended up creating loops like the above one (and similar ones for
> jbd/jbd2) in cases where handling the failure wasn't easily possible. And now
> it seems we are going in the opposite direction... At least we have a
> steady flow of patches guaranteed :)

Argh.  The whole point behind __GFP_NOFAIL was to centralise the
open-coded infinite-retry loops into the MM core.  So they can be
easily located and fixed up.

Yes, __GFP_NOFAIL *should* go away, once all those infinite-retry loops
are fixed to handle allocation failures.  But it sounds like this
"effort" was just undoing

: commit f3615244f15c8bee5783fcf032717ffdfd56e219
: Author:     akpm <akpm>
: AuthorDate: Sun Apr 20 21:28:12 2003 +0000
: Commit:     akpm <akpm>
: CommitDate: Sun Apr 20 21:28:12 2003 +0000
: 
:     [PATCH] implement __GFP_REPEAT, __GFP_NOFAIL, __GFP_NORETRY

and thereby hiding the bad code from grep again :(
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