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 10:12:40 +0100
From:	Jan Kara <jack@...e.cz>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	NeilBrown <neilb@...e.de>, Jan Kara <jack@...e.cz>,
	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 21-11-12 17:47:55, Darrick J. Wong wrote:
> On Thu, Nov 22, 2012 at 08:47:13AM +1100, NeilBrown wrote:
> > 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:
> 
> 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.
  I'll check the patches. Fixing PageWriteback logic for ext3 is not easily
doable due to lock ranking constraints - PageWriteback has to be set under
PageLocked but that ranks above transaction start so kjournald cannot grab
page locks so it cannot set PageWriteback... And changing the lock ordering
is a major surgery.

What could be doable is waiting for buffer locks from ext3's ->write_begin
and ->page_mkwrite implementations in case stable writes are required. If
your approach with a separate page bit doesn't work out (and I have some
doubts about that as mm people are *really* thrifty with page bits).

> > > 	/*
> > > 	 * 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.
  It is broken wrt PageWriteback but it actually waits for buffer locks in
->write_begin() so at least write path should be properly protected. But
mmap is not handled properly there (although that wouldn't be that hard to
fix). So I agree the condition should rather be what you suggest.

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