Re: [PATCH v2] Do a proper locking for mmap and block size change

[Chris Mason <chris.mason@fusionio.com>]

On Thu, Nov 29, 2012 at 12:02:17PM -0700, Linus Torvalds wrote:
> On Thu, Nov 29, 2012 at 9:19 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > I think I'll apply this for 3.7 (since it's too late to do anything
> > fancier), and then for 3.8 I will rip out all the locking entirely,
> > because looking at the fs/buffer.c patch I wrote up, it's all totally
> > unnecessary.
> >
> > Adding a ACCESS_ONCE() to the read of the i_blkbits value (when
> > creating new buffers) simply makes the whole locking thing pointless.
> > Just make the page lock protect the block size, and make it per-page,
> > and we're done.
> 
> There's a 'block-dev' branch in my git tree, if you guys want to play
> around with it.
> 
> It actually reverts fs/block-dev.c back to the 3.6 state (except for
> some whitespace damage that I refused to re-introduce), so that part
> of the changes should be pretty safe and well tested.
> 
> The fs/buffer.c changes, of course, are new. It's largely the same
> patch I already sent out, with a small helper function to simplify it,
> and to keep the whole ACCESS_ONCE() thing in just a single place.

The fs/buffer.c part makes sense during a quick read.  But
fs/direct-io.c plays with i_blkbits too.  The semaphore was fixing real
bugs there.

-chris