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

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Date:	Wed, 24 Feb 2010 17:56:46 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Jan Kara <jack@...e.cz>, 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 24-02-10 19:01:27, Dmitry Monakhov wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> >> The fact is that I've been able to reproduce the problem on LVM block
> >> devices, and sd* block devices so it's definitely not a loop device
> >> specific problem.
> >> 
> >> By the way, I tried several other things other than "echo s
> >> >/proc/sysrq_trigger" I tried multiple sync followed with a one minute
> >> "sleep",
> >> 
> >> "echo 3 >/proc/sys/vm/drop_caches" seems to lower the chances of "hash
> >> changes" but doesn't stops them.
> >   Strange. When I use sync(1) in your script and use /dev/sda5 instead of a
> > /dev/loop0, I cannot reproduce the problem (was running the script for
> > something like an hour).
> Theoretically some pages may exist after rw=>ro remount
> because of generic race between write/sync, And they will be written
> in by writepage if page already has buffers. This not happen in ext4
> because. Each time it try to perform writepages it try to start_journal
> and this result in EROFS.
> The race bug will be closed some day but new one may appear again.
  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. But by no means we should solve this VFS problem by spilling error
messages from the filesystem. Especially because block_write_full_page can
fail from a number of legal reasons (ENOSPC, EDQUOT, EIO) and we don't want
to pollute logs with such stuff.
  BTW: This isn't the race Camille could see because he did all the writes,
then sync and then remount-ro...

  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()?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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