From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [Cluster-devel] [PATCH] fs: push
 sync_filesystem() down to the file system's remount_fs()
Date: Thu, 13 Mar 2014 19:15:06 -0400
Message-ID: <20140313231506.GB16785@thunk.org>
References: <20140313073936.GA14663@infradead.org>
	<1394720456-16629-1-git-send-email-tytso@mit.edu>
	<20140313162319.GA504@quack.suse.cz>
	<1394728103.2767.32.camel@menhir>
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Cc: jfs-discussion@lists.sourceforge.net, Jan Kara <jack@suse.cz>,
	Anders Larsen <al@alarsen.net>, linux-mtd@lists.infradead.org,
	Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>,
	Petr Vandrovec <petr@vandrovec.name>, linux-cifs@vger.kernel.org,
	linux-fsdevel@thunk.org, cluster-devel@redhat.com,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	Evgeniy Dushistov <dushistov@mail.ru>, Kees Cook <keescook@chromium.org>,
	fuse-devel@lists.sourceforge.net, reiserfs-devel@vger.kernel.org,
	xfs@oss.sgi.com, linux-nilfs@vger.kernel.org,
	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
	codalist@TELEMANN.coda.cs.cmu.edu, linux-nfs@vger.kernel.org,
	Artem Bityutskiy <dedekind1@gmail.com>,
	linux-ntfs-dev@lists.sourceforge.net, samba-technical@lists.samba.org,
	Adrian Hunter <adrian.hunter@intel.com>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-btrfs@vger.kernel.org, linux-fsdevel@vger.k
To: Steven Whitehouse <swhiteho@redhat.com>
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In-Reply-To: <1394728103.2767.32.camel@menhir>
Errors-To: jfs-discussion-bounces@lists.sourceforge.net

On Thu, Mar 13, 2014 at 04:28:23PM +0000, Steven Whitehouse wrote:
> 
> I guess the same is true for other file systems which are mounted ro
> too. So maybe a check for MS_RDONLY before doing the sync in those
> cases?

My original patch moved the sync_filesystem into the check for
MS_RDONLY in the core VFS code.  The objection was raised that there
might be some file system out there that might depend on this
behaviour.  I can't imagine why, but I suppose it's at least
theoretically possible.

So the idea is that this particular patch is *guaranteed* not to make
any difference.  That way there can be no question about the patch'es
correctness.

I'm going to follow up with a patch for ext4 that does exactly that,
but the idea is to allow each file system maintainer to do that for
their own file system.

I could do that as well for file systems that are "obviously"
read-only, but then I'll find out that there's some wierd case where
the file system can be used in a read-write fashion.  (Example: UDF is
normally used for DVD's, but at least in theory it can be used
read/write --- I'm told that Windows supports read-write UDF file
systems on USB sticks, and at least in theory it could be used as a
inter-OS exchange format in situations where VFAT and exFAT might not
be appropriate for various reasons.)

Cheers,

						- Ted

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