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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 22, 2012 at 08:47:13AM +1100, NeilBrown wrote:
> On Wed, 21 Nov 2012 22:33:33 +0100 Jan Kara <jack@suse.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@oracle.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:

Maybe we should just fix this anyway?

I still have the patch that adds PG_stable (and changes the
wait_for_page_stable() test to use this flag instead of PG_writeback) kicking
around in my tree.  I wrote a patch to jbd that changes journal_do_submit_data
to set PG_stable, call clear_page_dirty_for_io(), and unsets the stable bit in
the end_io processing.

It seems to get rid of the checksum-after-write errors, though I'm not
convinced it's correct.  But, I'll send both patches along.

> > 
> > 	/*
> > 	 * 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 &&

test_opt(sb, DATA_FLAGS) != EXT3_MOUNT_WRITEBACK_DATA

since I bet data=journal mode is also borken wrt PageWriteback.

> > 	    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" ....

I'd rather just fix ext3. :)

(or remove it, since ext4 can handle ext3)

--D

> NeilBrown