On Thu, Nov 22, 2012 at 10:12:40AM +0100, Jan Kara wrote:
> 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 <j...@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.w...@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.
> 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.
Hm. In journal mode, write_begin calls do_journal_get_write_access on each
buffer for a given page, and in turn, jbd's do_get_write_access calls
lock_buffer. Is that what you're referring to by "actually waits for buffer
locks"? I'm wondering how that helps us, since afaict PG_writeback doesn't get
set in that path, and I think it's a little early to be setting PG_writeback
anyway.
If the page has to be locked before the transaction starts, how much of a
problem is it to set PG_writeback? Even though that seems a bit early to be
doing that?
Just for fun, I tried porting ext4_page_mkwrite into ext3 (removing all the
parts that don't exist in ext3) so that do_journal_get_write_access would also
get called here, but it didn't seem to fix journal mode.
--D
>
> Honza
> --
> Jan Kara <j...@suse.cz>
> SUSE Labs, CR
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