Re: [PATCH] Revert "writeback: limit write_cache_pages integrityscanning to current EOF"

From: Dave Chinner
Date: Mon Jun 28 2010 - 22:28:50 EST

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.


Dave Chinner
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