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

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Message-ID: <CA+55aFzpKH2FkU+9hdhZ5L9n8Mqr1vNO3Tdjmu1n+WmcHG1sNg@mail.gmail.com>
Date:	Thu, 29 Nov 2012 10:46:06 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	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 10:23 AM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>
> 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.

If it's a filesystem, then the size will never change while it is mounted.

So only the direct-block-device case needs to be worried about, no?

And that uses __generic_file_aio_write() and friends, which in turn
use the readpage/writepage functions.

So for block devices, it should be sufficient to make
readpage/writepage (with the writing obviously having all the
"write_begin/write_end/fullpage" variants) be safe as far as I can
see.

                 Linus
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