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From: Chris Mason <chris.mason@fusionio.com>
Subject: Re: [PATCH v2] Do a proper locking for mmap and block size change
Date: Fri, 30 Nov 2012 09:31:10 -0500
Message-ID: <20121130143110.GD11004@shiny.int.fusionio.com>
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Cc: 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>
To: Dave Chinner <david@fromorbit.com>
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On Thu, Nov 29, 2012 at 07:49:10PM -0700, Dave Chinner wrote:
> On Thu, Nov 29, 2012 at 02:16:50PM -0800, Linus Torvalds wrote:
> > On Thu, Nov 29, 2012 at 1:29 PM, Chris Mason <chris.mason@fusionio.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.
> 
> Same with mpage_readpages(), so it's not just direct IO that has
> this problem....

I guess the good news is that block devices don't have readpages.  The
bad news would be that we can't put readpages in without much bigger
changes.

-chris