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

Chris Mason
2012-11-30T14:31:15Z

On Thu, Nov 29, 2012 at 07:49:10PM -0700, Dave Chinner wrote:
> On Thu, Nov 29, 2012 at 02:16:50PM -0800, Linus Torvalds wrote:
> > On Thu, Nov 29, 2012 at 1:29 PM, Chris Mason <chris.mason@fusionio.com> wrote:
> > >
> > > 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.
> >
> > Ugh. That's a big violation of how buffer-heads are supposed to work:
> > the block number is very much defined to be in multiples of b_size
> > (see for example "submit_bh()" that turns it into a sector number).
> >
> > But you're right. The direct-IO code really *is* violating that, and
> > knows that get_block() ends up being defined in i_blkbits regardless
> > of b_size.
>
> Same with mpage_readpages(), so it's not just direct IO that has
> this problem....

I guess the good news is that block devices don't have readpages. The
bad news would be that we can't put readpages in without much bigger
changes.

-chris