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 03:36:38PM -0700, Linus Torvalds wrote:
> 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..

I searched through filemap.c for the magic i_size check that would let
us get away with ignoring i_blkbits in get_blocks, but its just not
there.  The whole fallback-to-buffered scheme seems to rely on
get_blocks checking for i_size.  I really hope I'm just missing
something.

If we're going to change this, I'd vote for something non-bh based.  I
didn't check every single FS, but I don't think direct-IO really wants
or needs buffer heads at all.

One less wart in direct-io.c would really be nice, but I'm assuming
it'll take us at least one full release to hammer out a shiny new
get_blocks.  Passing i_blkbits would be more mechanical, since all the
filesystems would just ignore it.

-chris