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

Linus Torvalds
2012-11-29T22:37:01Z

On Thu, Nov 29, 2012 at 2:16 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> 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.

It turns out fs/ioctl.c does the same - it fills in the buffer head
with some random bh->b_size too. I think it's not even a power of two
in that case.

And I guess it's understandable - they don't actually *use* the
buffer, they just want the offset. So the b_size field really is just
random crap to the users of the get_block interfaces, since they've
never cared before.

Ugh, this was definitely a dark and disgusting underbelly of the VFS
layer. We've not had to really touch it for a *looong* time..

Linus