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Date: Fri, 8 Oct 2010 11:24:27 +0200
From: Christoph Hellwig <hch@lst.de>
To: McPacino <mcpacino@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>, Daniel Phillips <phillips@phunq.net>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        dm-devel@redhat.com, Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Nick Piggin <npiggin@kernel.dk>
Subject: Re: introduce dm-snap-mv
Message-ID: <20101008092427.GA5426@lst.de>
References: <20101006083150.GA15758@zhongling> <201010071431.14937.phillips@phunq.net> <20101008082231.GA2054@lst.de> <AANLkTim9KFuP1_mrUUsMOEspxkfA=SZW2dmTX4WJbfep@mail.gmail.com>
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On Fri, Oct 08, 2010 at 05:14:27PM +0800, McPacino wrote:
> Hi Christoph,
> 
> I have to take care the cache problem If using the bio directly.
> BHs can be released by kernel when necessary.
> 
> Is there any existing code using bio to read/write metadata
> blocks? How do they handle the timing freeing bios? I really
> wish to learn something form it.

If you actually need caching just use the pagecache, e.g.
read_mapping_page to read in your data.  That completely abstracts
away the underlying block size.