linux-kernel - Re: [PATCHSET] block: fix merge of requests with different failfast settings

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Message-ID: <20090703105431.GN23611@kernel.dk>
Date:	Fri, 3 Jul 2009 12:54:31 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Niel Lambrechts <niel.lambrechts@...il.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: [PATCHSET] block: fix merge of requests with different
	failfast settings

On Fri, Jul 03 2009, Tejun Heo wrote:
> Hello,
> 
> Block layer didn't consider failfast status while merging requests and
> it led to premature failure of normal (non-failfast) IOs.  Niel
> Lambrechts could trigger the problem semi-reliably on ext4 when
> resuming from STR.  ext4 uses readahead when reading inodes and
> combined with the deterministic extra SATA PHY exception cycle during
> resume on the specific configuration, non-readahead inode read would
> fail causing ext4 errors.  Please read the following thread for
> details.
> 
>   http://lkml.org/lkml/2009/5/23/21
> 
> This patchset contains the following four patches to fix the problem.
> 
>  0001-block-don-t-merge-requests-of-different-failfast-se.patch
>  0002-block-use-the-same-failfast-bits-for-bio-and-reques.patch
>  0003-block-implement-mixed-merge-of-different-failfast-r.patch
>  0004-scsi-block-update-SCSI-to-handle-mixed-merge-failur.patch
> 
> 0001 disallows merge between requests with different failfast
> settings.  This one is the quick fix and should go into 2.6.31 and
> later to -stable as the bug is pretty serious and may lead to data
> loss.
> 
> 0002 preps for later changes.
> 
> 0003-0004 implements and applies mixed merge.  Requests of different
> failfast settings are merged as before but failure handling is updated
> such that parts which shouldn't fail without retrial are properly
> retried.
> 
> I spent quite some time thinking about and testing it but I'd really
> like more pairs of eyes on this patchset as dangerous bugs can go
> unnoticed for quite a while in this area (anyone knows when the
> failfast bug was introduced?).

It must have been several releases ago. So while the bug is indeed very
nasty, I don't think there's been much fallout from it.

> Jens, I think the best way to merge this is to first push 0001 to
> Linus's tree and then pull it into for-next and then apply the rest on
> top of them.

Yeah I'll do that, #1 for 2.6.31 and the rest for .32. Thanks for
finding and fixing this bug!

-- 
Jens Axboe

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