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

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 29, 2008 at 09:12:24AM -0400, Jeff Moyer wrote:
> Nick Piggin <npiggin@suse.de> writes:
> 
> > On Tue, Oct 28, 2008 at 05:11:02PM -0400, Jeff Moyer wrote:
> >> Nick Piggin <npiggin@suse.de> writes:
> 
> >> > 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?
> >
> > AFAIKS, __blockdev_direct_IO is doing the same thing for us, when it
> > encounters a READ. I should have documented this change. This is one
> > thing I'm not *quite* sure of there  might be a path do the block device
> > that I haven't considered, and which does not do the sync...
> 
> Well, that's if dio_lock_type != DIO_NO_LOCKING.  cscope shows the
> following callers of blockdev_direct_IO_no_locking:
> 
> gfs2_direct_IO
> ocfs2_direct_IO
> xfs_vm_direct_IO

XFS does it's own flush and invalidate before calling into the
generic direct I/O code, so the above patch is safe from an
XFS perspective.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com