Date: Thu, 28 Jun 2007 04:44:43 +0200
From: Nick Piggin <npiggin@suse.de>
To: David Chinner <dgc@sgi.com>
Cc: Chris Mason <chris.mason@oracle.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linux Memory Management List <linux-mm@kvack.org>,
       linux-fsdevel@vger.kernel.org
Subject: Re: [RFC] fsblock
Message-ID: <20070628024443.GB6038@wotan.suse.de>
References: <20070624014528.GA17609@wotan.suse.de> <20070626030640.GM989688@sgi.com> <46808E1F.1000509@yahoo.com.au> <20070626092309.GF31489@sgi.com> <20070626123449.GM14224@think.oraclecorp.com> <20070627053245.GA6033@wotan.suse.de> <20070627115056.GW14224@think.oraclecorp.com> <20070627223548.GS989688@sgi.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20070627223548.GS989688@sgi.com>
User-Agent: Mutt/1.5.9i
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2474
Lines: 58

On Thu, Jun 28, 2007 at 08:35:48AM +1000, David Chinner wrote:
> On Wed, Jun 27, 2007 at 07:50:56AM -0400, Chris Mason wrote:
> > Lets look at a typical example of how IO actually gets done today,
> > starting with sys_write():
> > 
> > sys_write(file, buffer, 1MB)
> > for each page:
> >     prepare_write()
> > 	allocate contiguous chunks of disk
> >         attach buffers
> >     copy_from_user()
> >     commit_write()
> >         dirty buffers
> > 
> > pdflush:
> >     writepages()
> >         find pages with contiguous chunks of disk
> > 	build and submit large bios
> > 
> > So, we replace prepare_write and commit_write with an extent based api,
> > but we keep the dirty each buffer part.  writepages has to turn that
> > back into extents (bio sized), and the result is completely full of dark
> > dark corner cases.

That's true but I don't think an extent data structure means we can
become too far divorced from the pagecache or the native block size
-- what will end up happening is that often we'll need "stuff" to map
between all those as well, even if it is only at IO-time.

But the point is taken, and I do believe that at least for APIs, extent
based seems like the best way to go. And that should allow fsblock to
be replaced or augmented in future without _too_ much pain.

 
> Yup - I've been on the painful end of those dark corner cases several
> times in the last few months.
> 
> It's also worth pointing out that mpage_readpages() already works on
> an extent basis - it overloads bufferheads to provide a "map_bh" that
> can point to a range of blocks in the same state. The code then iterates
> the map_bh range a page at a time building bios (i.e. not even using
> buffer heads) from that map......

One issue I have with the current nobh and mpage stuff is that it
requires multiple calls into get_block (first to prepare write, then
to writepage), it doesn't allow filesystems to attach resources
required for writeout at prepare_write time, and it doesn't play nicely
with buffers in general. (not to mention that nobh error handling is
buggy).

I haven't done any mpage-like code for fsblocks yet, but I think they
wouldn't be too much trouble, and wouldn't have any of the above
problems...
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/