Re: [PATCH RFC] O_DIRECT reads and writes without i_sem

[Chris Mason <mason@suse.com>]

On Mon, 2004-11-01 at 16:08 +0000, Christoph Hellwig wrote:
> On Mon, Nov 01, 2004 at 10:32:07AM -0500, Chris Mason wrote:

> > Right now, O_DIRECT reads and writes on regular files have to take i_sem
> > while reading file metadata in order to make sure we don't race with
> > hole filling.
[ ... ]
> This gets too complicated for it's own sake.  What about going down the
> XFS route and making i_sem a r/w semaphore that's taken only shared
> during read and write I/O, but exclusive while setting up write I/O
> outside of i_size?  Alternatively just move the I/O locking into the
> filesytem.
> 

Nod, it is too complex, this is why I posted early ;)  The only place in
the FS specific code for the locking I added to fs/direct-io.c would be
in each filesystem get_blocks call.  I went for direct-io.c because
that's where all the other locking already was.  shrug.

If we do down_read(i_rw_sem) for all cases except growing the file, then
we still have no locking for O_DIRECT while we are filling holes in the
file.  The filesystem write function could do this:

	down_read(i_rw_sem)
	read fs metadata
	if (filling hole) {
		up_read(i_rw_sem)
		down_write(i_rw_sem)
		goto retry
	}

But it also gets nasty in a hurry.  The second alternative is just to
make O_DIRECT lock pages (or stub pages if they aren't in cache) in the
filesystem address space, but this might add significant overhead in
radix tree operations during O_DIRECT.

-chris