linux-kernel - Re: [RFC PATCH] block: warn if blk_stack

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Message-ID: <yq1635nop2j.fsf@sermon.lab.mkp.net>
Date:	Tue, 23 Feb 2010 12:10:44 -0500
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] block: warn if blk_stack_limits() undermines atomicity

>>>>> "Mike" == Mike Snitzer <snitzer@...hat.com> writes:

Mike> For instance, a 512 byte device and a 4K device may be combined
Mike> into a single logical DM device; the resulting DM device would
Mike> have a logical_block_size of 4K.  Filesystems layered on such a
Mike> hybrid device assume that 4K will be written atomically but in
Mike> reality that 4K will be split into 8 512 byte IOs when issued to
Mike> the 512 byte device.

Not really.  It'll be issued as one I/O with a smaller LBA count but an
identical data payload.

Mike> Using a 4K logical_block_size for the higher-level DM device
Mike> increases potential for a partial write to the 512b device if
Mike> there is a system crash.

That's a definite maybe :)

Mike> [NOTE: setting "misaligned" for this warning is somewhat awkward
Mike> but blk_stack_limits() return of -1 can be viewed as there was an
Mike> "alignment inconsistency".  Would it be better to return -1 but
Mike> avoid setting t->misaligned?]

I don't have a problem with printing a warning but I don't think this
qualifies as misalignment on the grounds that the error scenario is in
the hypothetical bucket and not a deterministic thing.

-- 
Martin K. Petersen	Oracle Linux Engineering
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