linux-kernel - Re: [PATCH 0/6] Handle bio

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Message-ID: <20090414182049.GI5178@kernel.dk>
Date:	Tue, 14 Apr 2009 20:20:49 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Nikanth Karthikesan <knikanth@...e.de>, Neil Brown <neilb@...e.de>,
	linux-kernel@...r.kernel.org, Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <shaggy@...tin.ibm.com>, xfs-masters@....sgi.com
Subject: Re: [PATCH 0/6] Handle bio_alloc failure

On Tue, Apr 14 2009, Theodore Tso wrote:
> On Tue, Apr 14, 2009 at 05:11:19PM +0530, Nikanth Karthikesan wrote:
> > On Tuesday 14 April 2009 16:48:38 Jens Axboe wrote:
> > >
> > > It will not fail as long as __GFP_WAIT is set, which it is for all 6 of
> > > your patches.
> 
> Um, before we take out the checks, can we please make sure this is a
> guaranteed, documented behaviour?  In include/linux/page_alloc.h,
> __GFP_NOFAIL is documented as "will never fail", but it says
> absolutely nothing about __GFP_WAIT.
> 
> Some day, someone will create a static checker that will flag warnings
> when people fail to check for allocation failures, and it would be
> good if the formal semantics for __GFP_WAIT, and hence for GFP_NOFS,
> GFP_KERNEL, and GFP_USER, et. al. are defined.
> 
> We have code in fs/jbd2/transaction.c that calls kzalloc with
> GFP_NOFS|__GFP_NOFAIL, since I and many other people had the
> assumption that without __GFP_NOFAIL, an GFP_NOFS allocation could
> very well fail.
> 
> Or is this special-case behaviour which bio_alloc() guarantees, but
> not necessarily any other allocation function?

It's a bio_alloc() guarantee, it uses a mempool backing. And if you use
a mempool backing, any allocation that can wait will always be
satisfied.

-- 
Jens Axboe

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