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

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Message-ID: <20121129194840.GC3490@shiny>
Date:	Thu, 29 Nov 2012 14:48:40 -0500
From:	Chris Mason <chris.mason@...ionio.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	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 12:26:06PM -0700, Linus Torvalds wrote:
> On Thu, Nov 29, 2012 at 11:15 AM, Chris Mason <chris.mason@...ionio.com> wrote:
> >
> > The fs/buffer.c part makes sense during a quick read.  But
> > fs/direct-io.c plays with i_blkbits too.  The semaphore was fixing real
> > bugs there.
> 
> Ugh. I _hate_ direct-IO. What a mess. And yeah, it seems to be
> incestuously playing games that should be in fs/buffer.c. I thought it
> was doing the sane thing with the page cache.
> 
> (I now realize that Mikulas was talking about this mess, while I
> thought he was talking about the AIO code which is largely sane).

It was all a trick to get you to say the AIO code was sane.

It looks like we could use the private copy of i_blkbits that DIO is
already recording.

blkdev_get_blocks (called during DIO) is also checking i_blkbits, but I
really don't get why that isn't byte based instead.  DIO is already
doing the shift & mask game.

I think only clean_blockdev_aliases is intentionally using the inode's
i_blkbits, but again that shouldn't be changing for filesystems so it
seems safe to use the DIO copy.

-chris

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