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

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Date:	Tue, 14 Apr 2009 14:16:32 -0400
From:	Theodore Tso <tytso@....edu>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	Jens Axboe <jens.axboe@...cle.com>, 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 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?

					- Ted
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