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

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Date:	Thu, 29 Nov 2012 14:36:38 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <chris.mason@...ionio.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clmason@...ionio.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2] Do a proper locking for mmap and block size change

On Thu, Nov 29, 2012 at 2:16 PM, Linus Torvalds
<torvalds@...ux-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
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