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

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Date:	Thu, 29 Nov 2012 14:16:50 -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 1:29 PM, Chris Mason <chris.mason@...ionio.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.

What a crock. That direct-IO code is hack-upon-hack. Whoever wrote it
should be shot.

I think the only sane way to fix is is to pass in the block size to
get_blocks(). Which we admittedly should have done long ago, so that's
not a bad fix, but without actually looking at what it involves, I
think it's going to be pretty big patch. All the filesystems that
support the interface need to update it, even if they can then ignore
it, because direct-IO does all these hacks only for the raw device.

And I think it will improve the interface, but damn, direct-IO is
still horrible for playing these kinds of games.

                    Linus
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