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.
> >
> > /*
> > * 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
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