Re: [PATCH 17/17] xfs: ATTR_REPLACE algorithm with LARP enabled needs rework

Dave Chinner
2022-05-08T23:25:11Z

On Mon, May 09, 2022 at 07:50:23AM +1000, Dave Chinner wrote:
> On Fri, May 06, 2022 at 11:01:23PM -0700, Alli wrote:
> > On Fri, 2022-05-06 at 19:45 +1000, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > >
> > > We can't use the same algorithm for replacing an existing attribute
> > > when logging attributes. The existing algorithm is essentially:
> > >
> > > 1. create new attr w/ INCOMPLETE
> > > 2. atomically flip INCOMPLETE flags between old + new attribute
> > > 3. remove old attr which is marked w/ INCOMPLETE
> > >
> > > This algorithm guarantees that we see either the old or new
> > > attribute, and if we fail after the atomic flag flip, we don't have
> > > to recover the removal of the old attr because we never see
> > > INCOMPLETE attributes in lookups.
> > >
> > > For logged attributes, however, this does not work. The logged
> > > attribute intents do not track the work that has been done as the
> > > transaction rolls, and hence the only recovery mechanism we have is
> > > "run the replace operation from scratch".
> > >
> > > This is further exacerbated by the attempt to avoid needing the
> > > INCOMPLETE flag to create an atomic swap. This means we can create
> > > a second active attribute of the same name before we remove the
> > > original. If we fail at any point after the create but before the
> > > removal has completed, we end up with duplicate attributes in
> > > the attr btree and recovery only tries to replace one of them.
> > >
> > > There are several other failure modes where we can leave partially
> > > allocated remote attributes that expose stale data, partially free
> > > remote attributes that enable UAF based stale data exposure, etc.
> > >
> > > TO fix this, we need a different algorithm for replace operations
> > > when LARP is enabled. Luckily, it's not that complex if we take the
> > > right first step. That is, the first thing we log is the attri
> > > intent with the new name/value pair and mark the old attr as
> > > INCOMPLETE in the same transaction.
> > >
> > > From there, we then remove the old attr and keep relogging the
> > > new name/value in the intent, such that we always know that we have
> > > to create the new attr in recovery. Once the old attr is removed,
> > > we then run a normal ATTR_CREATE operation relogging the intent as
> > > we go. If the new attr is local, then it gets created in a single
> > > atomic transaction that also logs the final intent done. If the new
> > > attr is remote, the we set INCOMPLETE on the new attr while we
> > > allocate and set the remote value, and then we clear the INCOMPLETE
> > > flag at in the last transaction taht logs the final intent done.
> > >
> > > If we fail at any point in this algorithm, log recovery will always
> > > see the same state on disk: the new name/value in the intent, and
> > > either an INCOMPLETE attr or no attr in the attr btree. If we find
> > > an INCOMPLETE attr, we run the full replace starting with removing
> > > the INCOMPLETE attr. If we don't find it, then we simply create the
> > > new attr.
> > >
> > > Notably, recovery of a failed create that has an INCOMPLETE flag set
> > > is now the same - we start with the lookup of the INCOMPLETE attr,
> > > and if that exists then we do the full replace recovery process,
> > > otherwise we just create the new attr.
> > >
> > > Hence changing the way we do the replace operation when LARP is
> > > enabled allows us to use the same log recovery algorithm for both
> > > the ATTR_CREATE and ATTR_REPLACE operations. This is also the same
> > > algorithm we use for runtime ATTR_REPLACE operations (except for the
> > > step setting up the initial conditions).
> > >
> > > The result is that:
> > >
> > > - ATTR_CREATE uses the same algorithm regardless of whether LARP is
> > > enabled or not
> > > - ATTR_REPLACE with larp=0 is identical to the old algorithm
> > > - ATTR_REPLACE with larp=1 runs an unmodified attr removal algorithm
> > > from the larp=0 code and then runs the unmodified ATTR_CREATE
> > > code.
> > > - log recovery when larp=1 runs the same ATTR_REPLACE algorithm as
> > > it uses at runtime.
> > >
> > > Because the state machine is now quite clean, changing the algorithm
> > > is really just a case of changing the initial state and how the
> > > states link together for the ATTR_REPLACE case. Hence it's not a
> > > huge amoutn of code for what is a fairly substantial rework
> > > of the attr logging and recovery algorithm....
> > >
> > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > This looks mostly good, though when I run it with the new tests, I seem
> > to run into a failed filesystem check at the end. "bad attribute count
> > 0 in attr block 0". I suspect we still dont have the removal of the
> > fork quite right. It sounds like you're still working with things
> > though, I'll keep looking too.
>
> Yeah, that's a strange one. I didn't get it with the based branch
> testing (based on 5.18-rc+for-next) but over the weekend where it
> got merged with 5.18-rc5 it appears that the error has manifested
> in several test runs. I'll dig into it this morning.

Ok, the bug is in the intorduction of the remove path to the
_set_iter() state machine - the node attr remove path doesn't
collapse empty leaf blocks back down to shortform. That removes the
fork if there are not attrs left at all.

That needs to be fixed in the patch that introduces this, and it
that will cascade into needing to rebase subsequent patches, so I
might just resend the lot.

Hmmm - xfs_attr3_leaf_verify() doesn't verify that the attr count is
non-zero in a leaf block. That would have could this before it was
written to disk. We should probably fix that, seeing as repair
complains about it...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com