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From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2] Do a proper locking for mmap and block size change
Date: Fri, 30 Nov 2012 11:36:01 -0500
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Chris Mason <chris.mason@fusionio.com>,
	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 Fri, Nov 30, 2012 at 01:49:10PM +1100, Dave Chinner wrote:
> > 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....

The mpage code may actually fall back to BHs.

I have a version of the direct I/O code that uses the iomap_ops from the
multi-page write code that you originally started.  It uses the new op
as primary interface for direct I/O and provides a helper for
filesystems that still use buffer heads internally.  I'll try to dust it
off and send out a version for the current kernel.