Re: [rfc][patch] mm: direct io less aggressive syncs and invalidates

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 29, 2008 at 09:30:12AM -0400, Jeff Moyer wrote:
> Jamie Lokier <jamie@shareable.org> writes:
> 
> > Jeff Moyer wrote:
> >> > Index: linux-2.6/mm/filemap.c
> >> > ===================================================================
> >> > --- linux-2.6.orig/mm/filemap.c	2008-10-03 11:21:31.000000000 +1000
> >> > +++ linux-2.6/mm/filemap.c	2008-10-03 12:00:17.000000000 +1000
> >> > @@ -1304,11 +1304,8 @@ generic_file_aio_read(struct kiocb *iocb
> >> >  			goto out; /* skip atime */
> >> >  		size = i_size_read(inode);
> >> >  		if (pos < size) {
> >> > -			retval = filemap_write_and_wait(mapping);
> >> > -			if (!retval) {
> >> > -				retval = mapping->a_ops->direct_IO(READ, iocb,
> >> > +			retval = mapping->a_ops->direct_IO(READ, iocb,
> >> >  							iov, pos, nr_segs);
> >> > -			}
> >> 
> >> So why is it safe to get rid of this?  Can't this result in reading
> >> stale data from disk?
> >
> > It seems that could be easily tested in one of the test suites, by
> > writing a page without O_DIRECT to make a dirty page, then reading the
> > same page with O_DIRECT.  Do it a few times to be sure.
> 
> Sure, are you volunteering to write this? =)  For completeness, it
> should probably do this via mmap, too.

Making fsx mix buffered and direct I/O is the anwer to this
problem.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com