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

[Jamie Lokier <jamie@shareable.org>]

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.

Do the test suites verify O_DIRECT / page-cache coherency?

Cheers,
-- Jamie