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@xxxxxxx> 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@xxxxxxxxxx>
> > > > > > ---
> > > > > > 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@xxxxxxx>
SUSE Labs, CR
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