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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 29 Nov 2012 14:36:38 -0800
Message-ID: <CA+55aFwoM3wje+XS60=kE3eVndWQD6szfgYa9MyfSp=itdr9Vg@mail.gmail.com>
Subject: Re: [PATCH v2] Do a proper locking for mmap and block size change
To: Chris Mason <chris.mason@fusionio.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Chris Mason <clmason@fusionio.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Jens Axboe <axboe@kernel.dk>,
        Jeff Chua <jeff.chua.linux@gmail.com>,
        Lai Jiangshan <laijs@cn.fujitsu.com>, Jan Kara <jack@suse.cz>,
        lkml <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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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
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