linux-ext4 - Re: [Cluster-devel] [PATCH] fs: push sync_filesystem() down to the file system's remount

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Message-ID: <20140313231506.GB16785@thunk.org>
Date:	Thu, 13 Mar 2014 19:15:06 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, jfs-discussion@...ts.sourceforge.net,
	Anders Larsen <al@...rsen.net>, cluster-devel@...hat.com,
	linux-mtd@...ts.infradead.org,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Petr Vandrovec <petr@...drovec.name>,
	codalist@...EMANN.coda.cs.cmu.edu, linux-cifs@...r.kernel.org,
	linux-fsdevel@...nk.org,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Evgeniy Dushistov <dushistov@...l.ru>,
	Kees Cook <keescook@...omium.org>,
	fuse-devel@...ts.sourceforge.net, reiserfs-devel@...r.kernel.org,
	xfs@....sgi.com, linux-nilfs@...r.kernel.org,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	linux-nfs@...r.kernel.org, Artem Bityutskiy <dedekind1@...il.com>,
	linux-ntfs-dev@...ts.sourceforge.net,
	samba-technical@...ts.samba.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-f2fs-devel@...ts.sourceforge.net, ocfs2-devel@....oracle.com,
	linux-fsdevel@...r.kernel.org,
	Phillip Lougher <phillip@...ashfs.org.uk>,
	linux-btrfs@...r.kernel.org
Subject: Re: [Cluster-devel] [PATCH] fs: push sync_filesystem() down to the
 file system's remount_fs()

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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