Re: O_DIRECT and barriers

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 21, 2009 at 03:45:18PM -0700, Joel Becker wrote:
> 	In the section on perhaps not waiting for buffered fallback, we
> need to clarify that O_DIRECT reads need to know to look in the
> pagecache.  That is, if we decide that extending O_DIRECT writes without
> fsync can return before the data hits the storage, the caller shouldn't
> also have to call fsync() just to call read() of data they just wrote!

The way the O_DIRECT fallback is implemented currenly is that data does
hit the disk before return, thanks to a:

	err = do_sync_mapping_range(file->f_mapping, pos, endbyte,
					SYNC_FILE_RANGE_WAIT_BEFORE|
					SYNC_FILE_RANGE_WRITE|
					SYNC_FILE_RANGE_WAIT_AFTER);

which I expected to also sync the required metdata to disk, which
it doesn't.    Which btw are really horrible semantics given that
we export that beast to userspace as a separate system call.