From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <3D22D1CE.1C0A4906@zip.com.au> From: Andrew Morton MIME-Version: 1.0 Subject: Re: [linux-lvm] LVM2 modifies the buffer_head struct? References: <20020702141702.GA9769@fib011235813.fsnet.co.uk> <20020703100838.GH14097@suse.de> Content-Transfer-Encoding: 7bit Sender: linux-lvm-admin@sistina.com Errors-To: linux-lvm-admin@sistina.com Reply-To: linux-lvm@sistina.com List-Help: List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: Date: Wed Jul 3 05:23:01 2002 List-Id: Content-Type: text/plain; charset="us-ascii" To: Jens Axboe Cc: Joe Thornber , linux-lvm@sistina.com, linux-kernel@vger.kernel.org Jens Axboe wrote: > > On Tue, Jul 02 2002, Joe Thornber wrote: > > Tom, > > > > On Tue, Jul 02, 2002 at 09:40:56AM -0400, Tom Walcott wrote: > > > Hello, > > > > > > Browsing the patch submitted for 2.4 inclusion, I noticed that LVM2 > > > modifies the buffer_head struct. Why does LVM2 require the addition of it's > > > own private field in the buffer_head? It seems that it should be able to > > > use the existing b_private field. > > > > This is a horrible hack to get around the fact that ext3 uses the > > b_private field for its own purposes after the buffer_head has been > > handed to the block layer (it doesn't just use b_private when in the > > b_end_io function). Is this acceptable behaviour ? Other filesystems > > do not have similar problems as far as I know. > > > > device-mapper uses the b_private field to 'hook' the buffer_heads so > > it can keep track of in flight ios (essential for implementing > > suspend/resume correctly). See dm.c:dec_pending() > > Your driver is required to properly stack b_private uses, however if > ext3 (well jbd really) over writes b_private after bh i/o submission I > would say that it is broken. That breaks more than just device mapper, > that will break any stacked driver (such as loop, for instance). It requires that b_private be stable across the lifetime of the buffer. hmm. -