linux-ext4 - Re: [PATCH 4/4] ext3: Warn if mounting rw on a disk requiring stable page writes

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Date:	Thu, 22 Nov 2012 08:47:13 +1100
From:	NeilBrown <neilb@...e.de>
To:	Jan Kara <jack@...e.cz>
Cc:	"Darrick J. Wong" <darrick.wong@...cle.com>, axboe@...nel.dk,
	lucho@...kov.net, ericvh@...il.com, tytso@....edu,
	rminnich@...dia.gov, viro@...iv.linux.org.uk,
	martin.petersen@...cle.com, david@...morbit.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
	v9fs-developer@...ts.sourceforge.net, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 4/4] ext3: Warn if mounting rw on a disk requiring
 stable page writes

On Wed, 21 Nov 2012 22:33:33 +0100 Jan Kara <jack@...e.cz> wrote:

> On Wed 21-11-12 13:13:19, Darrick J. Wong wrote:
> > On Wed, Nov 21, 2012 at 03:15:43AM +0100, Jan Kara wrote:
> > > On Tue 20-11-12 18:00:56, Darrick J. Wong wrote:
> > > > ext3 doesn't properly isolate pages from changes during writeback.  Since the
> > > > recommended fix is to use ext4, for now we'll just print a warning if the user
> > > > tries to mount in write mode.
> > > > 
> > > > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > > > ---
> > > >  fs/ext3/super.c |    8 ++++++++
> > > >  1 file changed, 8 insertions(+)
> > > > 
> > > > 
> > > > diff --git a/fs/ext3/super.c b/fs/ext3/super.c
> > > > index 5366393..5b3725d 100644
> > > > --- a/fs/ext3/super.c
> > > > +++ b/fs/ext3/super.c
> > > > @@ -1325,6 +1325,14 @@ static int ext3_setup_super(struct super_block *sb, struct ext3_super_block *es,
> > > >  			"forcing read-only mode");
> > > >  		res = MS_RDONLY;
> > > >  	}
> > > > +	if (!read_only &&
> > > > +	    queue_requires_stable_pages(bdev_get_queue(sb->s_bdev))) {
> > > > +		ext3_msg(sb, KERN_ERR,
> > > > +			"error: ext3 cannot safely write data to a disk "
> > > > +			"requiring stable pages writes; forcing read-only "
> > > > +			"mode.  Upgrading to ext4 is recommended.");
> > > > +		res = MS_RDONLY;
> > > > +	}
> > > >  	if (read_only)
> > > >  		return res;
> > > >  	if (!(sbi->s_mount_state & EXT3_VALID_FS))
> > >   Why this? ext3 should be fixed by your change to
> > > filemap_page_mkwrite()... Or does testing show otherwise?
> > 
> > Yes, it's still broken even with this new set of changes.  Now that I think
> > about it a little more, I recall that writeback mode was actually fine, so this
> > is a little harsh.
> > 
> > Hm... looking at the ordered code a little more, it looks like
> > ext3_ordered_write_end is calling journal_dirty_data_fn, which (I guess?) tries
> > to write mapped buffers back through the journal?  Taking it out seems to fix
> > ordered mode, though I have a suspicion that it might very well break ordered
> > mode too.
>   Oh, right. kjournald writing buffers directly (without setting
> PageWriteback) will break things. So please, change warning to:
> 
> 	/*
> 	 * In data=ordered mode, kjournald writes buffers without setting
> 	 * PageWriteback bit thus generic code does not properly wait for
> 	 * writeback of those buffers to finish.
> 	 */
> 	if (!read_only &&
> 	    test_opt(sb, DATA_FLAGS) == EXT3_MOUNT_ORDERED_DATA &&
> 	    queue_requires_stable_pages(bdev_get_queue(sb->s_bdev))) {
> 		ext3_msg(sb, KERN_ERR,
> 			"error: data=ordered mode does not support stable "
> 			"page writes required by the disk; forcing read-only "
> 			"mode. Upgrading to ext4 is recommended.");
> 		res = MS_RDONLY;
> 	}
> 
> then you need a similar check in ext3_remount() so that filesystem cannot
> be remounted read-write.
> 
> 								Honza

Given this restriction, there is no way that I can change md/raid5 to set the
"stable pages" flag and stop copying pages into the stripe-cache.  ext3 on
raid5 will be much too common to allow this breakage.

I would really like to be able to say "I prefer stable pages, but they aren't
a requirement as long as I know which is which" ....

NeilBrown

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