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

Chris Mason
2012-11-29T21:29:36Z

On Thu, Nov 29, 2012 at 01:52:22PM -0700, Linus Torvalds wrote:
> On Thu, Nov 29, 2012 at 11:48 AM, Chris Mason <chris.mason@fusionio.com> wrote:
> >
> > It was all a trick to get you to say the AIO code was sane.
>
> It's only sane compared to the DIO code.
>
> That said, I hate AIO much less these days that we've largely merged
> the code with the regular IO. It's still a horrible interface, but at
> least it is no longer a really disgusting separate implementation in
> the kernel of that horrible interface.
>
> So yeah, I guess AIO really is pretty sane these days.
>
> > It looks like we could use the private copy of i_blkbits that DIO is
> > already recording.
>
> Yes. But that didn't fix the blkdev_get_blocks() mess you pointed out.
>
> I've pushed out two more commits to the 'block-dev' branch at
>
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux block-dev
>
> in case anybody wants to take a look.
>
> It is - as usual - entirely untested. It compiles, and I *think* that
> blkdev_get_blocks() makes a whole lot more sense this way - as you
> said, it should be byte-based (although it actually does the block
> number conversion because I worried about overflow - probably
> unnecessarily).
>
> Comments?

Your blkdev_get_blocks emails were great reading while at the dentist,
thanks for helping me pass the time.

Just reading the new blkdev_get_blocks, it looks like we're mixing
shifts. In direct-io.c map_bh->b_size is how much we'd like to map, and
it has no relation at all to the actual block size of the device. The
interface is abusing b_size to ask for as large a mapping as possible.

Most importantly, it has no relation to the fs_startblk that we pass in,
which is based on inode->i_blkbits.

So your new check in blkdev_get_blocks:

if (iblock >= end_block) {

Is wrong because iblock and end_block are based on different sizes. I
think we have to do the eof checks inside fs/direct-io.c or change the
get_blocks interface completely.

I really thought fs/direct-io.c was already doing eof checks, but I'm
reading harder.

-chris