Date: Thu, 29 Nov 2012 13:23:01 -0500 (EST)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
cc: 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>
Subject: Re: [PATCH v2] Do a proper locking for mmap and block size change
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On Thu, 29 Nov 2012, Linus Torvalds wrote:

> On Wed, Nov 28, 2012 at 2:01 PM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> >
> > This sounds sensible. I'm sending this patch.
> 
> This looks much better.
> 
> I think I'll apply this for 3.7 (since it's too late to do anything
> fancier), and then for 3.8 I will rip out all the locking entirely,
> because looking at the fs/buffer.c patch I wrote up, it's all totally
> unnecessary.
> 
> Adding a ACCESS_ONCE() to the read of the i_blkbits value (when
> creating new buffers) simply makes the whole locking thing pointless.
> Just make the page lock protect the block size, and make it per-page,
> and we're done.
> 
> No RCU grace period crap, no expedited mess, no nothing.
> 
>                       Linus

Yes.

If you remove that percpu rw lock, you also need to rewrite direct i/o 
code.

In theory, block device direct i/o doesn't need buffer block size at all. 
But in practice, it shares a lot of code with filesystem direct i/o, it 
reads the block size multiple times and it crashes if it changes.

Mikulas
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