From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 28 Jun 2007 04:44:43 +0200 From: Nick Piggin 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> Sender: owner-linux-mm@kvack.org Return-Path: To: David Chinner Cc: Chris Mason , Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , linux-fsdevel@vger.kernel.org List-ID: 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org