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

[Joel Becker <Joel.Becker@oracle.com>]

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 ;-)

Joel

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Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127