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

[Jeff Moyer <jmoyer@redhat.com>]

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

and of course

blkdev_direct_IO

I can't say whether all of these callers are safe.  They certainly don't
appear to be safe to me.

Cheers,

Jeff