linux-kernel - Re: [PATCH] Revert "writeback: limit write_cache

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Date:	Tue, 29 Jun 2010 12:27:57 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	ocfs2-devel@....oracle.com, Tao Ma <tao.ma@...cle.com>,
	Dave Chinner <dchinner@...hat.com>,
	Christoph Hellwig <hch@....de>, Mark Fasheh <mfasheh@...e.com>
Subject: Re: [PATCH] Revert "writeback: limit write_cache_pages integrity
 scanning to current EOF"

On Mon, Jun 28, 2010 at 07:04:20PM -0700, Joel Becker wrote:
> On Tue, Jun 29, 2010 at 11:56:15AM +1000, Dave Chinner wrote:
> > > 	Regarding XFS, how do you handle catching the tail of an
> > > allocation with an lseek(2)'d write?  That is, your current allocation
> > > has a few blocks outside of i_size, then I lseek(2) a gigabyte past EOF
> > > and write there.  The code has to recognize to zero around old_i_size
> > > before moving out to new_i_size, right?  I think that's where our old
> > > approaches had problems.
> > 
> > xfs_file_aio_write() handles both those cases for us via
> > xfs_zero_eof().  What it does is map the region from the old EOF to
> > the start of the new write and zeroes any allocated blocks that are
> > not marked unwritten that lie within the range. It does this via the
> > internal mapping interface because we hide allocated blocks past EOF
> > from the page cache and higher layers.
> 
> 	Makes sense as an approach.  We deliberately do this through the
> page cache to take advantage of its I/O patterns and tie in with JBD2.
> Also, we don't feel like maintaining an entire shadow page cache ;-)

Just to clarify any possible misunderstanding here, xfs_zero_eof()
also does it's IO through the page cache for similar reasons.  It's
just the mappings are found via the internal interfaces before the
zeroing is done via the anonymous pagecache_write_begin()/
pagecache_write_end() functions (in xfs_iozero()) rather than using
the generic block functions.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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