linux-ext4 - Re: [PATCH] Improve buffered streaming write ordering

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Message-ID: <20081007132911.GA6905@mit.edu>
Date:	Tue, 7 Oct 2008 09:29:11 -0400
From:	Theodore Tso <tytso@....edu>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] Improve buffered streaming write ordering

On Tue, Oct 07, 2008 at 03:32:57PM +0530, Aneesh Kumar K.V wrote:
> On Tue, Oct 07, 2008 at 05:05:54AM -0400, Christoph Hellwig wrote:
> > On Tue, Oct 07, 2008 at 02:15:31PM +0530, Aneesh Kumar K.V wrote:
> > > +static int ext4_write_cache_pages(struct address_space *mapping,
> > > +		      struct writeback_control *wbc, writepage_t writepage,
> > > +		      void *data)
> > > +{
> > 
> > Looking at this functions the only difference is killing the
> > writeback_index and range_start updates.  If they are bad why would we
> > only remove them from ext4?
> 
> I am also not updating wbc->nr_to_write.
    ...
> I don't think other filesystem have this requirement.

That's true, but there is a lot of code duplication, which means that
bugs or changes in write_cache_pages() would need to be fixed in
ext4_write_cache_pages().  So another approach that might be better
from a long-term code maintenance point of view is to add a flag in
struct writeback_control that tells write_cache_pages() not to update
those fields, and avoid duplicating approximately 95 lines of code.
It means a change in a core mm function, though, so if folks thinks
its too ugly, we can make our own copy in fs/ext4.

Opinions?  Andrew, as someone who often weighs in on fs and mm issues,
what do you think?  My preference would be to make the change to
mm/page-writeback.c, controlled by a flag which ext4 would set be set
by fs/ext4 before it calls write_cache_pages().

						- Ted
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