linux-kernel - Re: [dm-devel] [PATCH] dm: dm-cache fails to write the cache device in writethrough mode

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Date:	Sat, 23 Mar 2013 03:00:44 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	Heinz Mauelshagen <heinzm@...hat.com>,
	Joe Thornber <ejt@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [dm-devel] [PATCH] dm: dm-cache fails to write the cache
	device in writethrough mode

On Fri, Mar 22, 2013 at 01:11:51PM -0700, Darrick J. Wong wrote:
> The new writethrough strategy for dm-cache issues a bio to the origin device,
> remaps the bio to the cache device, and issues the bio to the cache device.
> However, the block layer modifies bi_sector and bi_size, so we need to preserve
> these or else nothing gets written to the cache (bi_size == 0).  This fixes the
> problem where someone writes a block through the cache, but a subsequent reread
> (from the cache) returns old contents.

If this needs doing here, please use dm_bio_record() and dm_bio_restore().
E.g. Look at how dm-raid1.c does something similar.
 
Alasdair

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