linux-ext4 - Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

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Date:	Tue, 2 Mar 2010 10:34:31 +0100
From:	Christoph Hellwig <hch@....de>
To:	Jan Kara <jack@...e.cz>
Cc:	Dmitry Monakhov <dmonakhov@...nvz.org>,
	Camille Moncelier <pix@...life.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>, hch@....de,
	viro@...iv.linux.org.uk
Subject: Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

On Wed, Feb 24, 2010 at 05:56:46PM +0100, Jan Kara wrote:
>   OK, I see that in theory a process can open file for writing after
> fs_may_remount_ro() before MS_RDONLY flag gets set. That could be really
> nasty.

Not just in theory, but also in practice.  We can easily hit this under
load with XFS.

> But by no means we should solve this VFS problem by spilling error
> messages from the filesystem.

Exactly.

>   Al, Christoph, do I miss something or there is really nothing which
> prevents a process from opening a file after the fs_may_remount_ro() check
> in do_remount_sb()?

No, there is nothing.  We really do need a multi-stage remount read-only
process:

 1) stop any writes from userland, that is opening new files writeable
 2) stop any periodic writeback from the VM or filesystem-internal
 3) write out all filesystem data and metadata
 4) mark the filesystem fully read-only

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