Re: [PATCH] fs: push sync_filesystem() down to the file system's remount_fs()

From: Theodore Ts'o <tytso@mit.edu>
To: Steve French <smfrench@gmail.com>
Cc: jfs-discussion@lists.sourceforge.net, Jan Kara <jack@suse.cz>,
	Anders Larsen <al@alarsen.net>,
	cluster-devel@redhat.com, linux-mtd@lists.infradead.org,
	Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>,
	Petr Vandrovec <petr@vandrovec.name>,
	codalist@TELEMANN.coda.cs.cmu.edu,
	"linux-cifs@vger.kernel.org" <linux-cifs@vger.kernel.org>,
	linux-fsdevel@thunk.org, Christoph Hellwig <hch@infradead.org>,
	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>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
	Artem Bityutskiy <dedekind1@gmail.com>,
	linux-ntfs-dev@lists.sourceforge.net,
	samba-technical <samba-technical@lists.samba.org>,
	Adrian
Subject: Re: [PATCH] fs: push sync_filesystem() down to the file system's remount_fs()
Date: Thu, 13 Mar 2014 21:23:00 -0400	[thread overview]
Message-ID: <20140314012300.GA8282@thunk.org> (raw)
In-Reply-To: <CAH2r5mvtLZYt=L9HHYOYGBB-kZNXrPiMQL-agzD_VRGCW4vMcw@mail.gmail.com>

On Thu, Mar 13, 2014 at 07:33:02PM -0500, Steve French wrote:
> On Thu, Mar 13, 2014 at 9:20 AM, Theodore Ts'o <tytso@mit.edu> wrote:
> > Previously, the no-op "mount -o mount /dev/xxx" operation when the
> > file system is already mounted read-write causes an implied,
> > unconditional syncfs().  This seems pretty stupid, and it's certainly
> > documented or guaraunteed to do this, nor is it particularly useful,
> > except in the case where the file system was mounted rw and is getting
> > remounted read-only.
> 
> Is there a case where a file system, not mounted read-only,
> would want to skip the syncfs on remount?  I don't know
> of any particular reason to do a syncfs on remount unless
> caching behavior is changing (or moving to read-only mount),
> but if as you say it is documented and guaranteed...

If the file system is mounted read-write, and it is transitioning to
read-only, i.e. "mount -o remount,ro /" then you do want to write out
all dirty data before you transition it to be read-only (otherwise you
would lose data).

It is my belief that this is the _only_ data integrity issue which is
implied by remount (and this is more about not losing work done by
previous system calls).

The background reason for this commit is that a user complained on the
ext4 list that he is using containers in a virtualization environment,
and due to the init scripts which the user doesn't want to change, it
is causing gazillions of no-op remounts, and this is causing ext4 (and
xfs) to do send CACHE FLUSH commands because it is required by the
syncfs(2) system call, which also calls sync_filesystem.  These CACHE
FLUSH commands are unneeded for anything, especially for these no-op
remounts, and so I want them to go away for remounts, but they should
still be there in response to syncfs(2) requests.

Cheers,

					- Ted

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