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Date: Fri, 27 Apr 2007 11:10:12 +0100
From: Christoph Hellwig <hch@infradead.org>
To: Theodore Tso <tytso@mit.edu>, Nigel Cunningham <nigel@nigel.suspend2.net>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Xavier Bestel <xavier.bestel@free.fr>,
       Pekka Enberg <penberg@cs.helsinki.fi>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: Back to the future.
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On Thu, Apr 26, 2007 at 05:38:07PM -0400, Theodore Tso wrote:
> On Fri, Apr 27, 2007 at 06:08:01AM +1000, Nigel Cunningham wrote:
> > We tried that. It would need some work. IIRC remounting filesystems
> > read-only makes files become marked read-only. Perfectly sensible,
> > except that if you then remount the filesystem rw at resume time, all
> > those files are still marked ro and userspace crashes and burns. Not
> > unfixable, I'll agree, but there is more work to do there.
> 
> There are other solutions, though.  One is that we could export a
> system call interface which freezes a filesystem and prevents any
> further I/O.  We mostly have something like that right now (via the
> the write_super_lockfs function in the superblock operations
> structure), but we haven't exported it to userspace.

It is exported on XFS ;-)

> We would also need a similar interface to freeze any block device I/O,
> in case you have a database running and doing direct I/O to a block
> device.  (Or again, we could simply not support that case; how many
> people will be running running a database accessing a block deivce on
> their laptop?)

block device I/O uses generic_file*whateveriscurrenthere*_write, which
checks for the freeze flag, so the infrastructure for that is there
aswell.