On Thu, Aug 11, 2022 at 06:55:16PM -0700, Alli wrote: > On Wed, 2022-08-10 at 16:12 +1000, Dave Chinner wrote: > > On Tue, Aug 09, 2022 at 10:01:49PM -0700, Alli wrote: > > > On Wed, 2022-08-10 at 11:58 +1000, Dave Chinner wrote: > > > > On Tue, Aug 09, 2022 at 09:52:55AM -0700, Darrick J. Wong wrote: > > > > > On Thu, Aug 04, 2022 at 12:39:56PM -0700, Allison Henderson > > > > > wrote: > > > > > > Recent parent pointer testing has exposed a bug in the > > > > > > underlying > > > > > > attr replay. A multi transaction replay currently performs a > > > > > > single step of the replay, then deferrs the rest if there is > > > > > > more > > > > > > to do. > > > > > > > > Yup. > > > > > > > > > > This causes race conditions with other attr replays that > > > > > > might be recovered before the remaining deferred work has had > > > > > > a > > > > > > chance to finish. > > > > > > > > What other attr replays are we racing against? There can only be > > > > one incomplete attr item intent/done chain per inode present in > > > > log > > > > recovery, right? > > > No, a rename queues up a set and remove before committing the > > > transaction. One for the new parent pointer, and another to remove > > > the > > > old one. > > > > Ah. That really needs to be described in the commit message - > > changing from "single intent chain per object" to "multiple > > concurrent independent and unserialised intent chains per object" is > > a pretty important design rule change... > > > > The whole point of intents is to allow complex, multi-stage > > operations on a single object to be sequenced in a tightly > > controlled manner. They weren't intended to be run as concurrent > > lines of modification on single items; if you need to do two > > modifications on an object, the intent chain ties the two > > modifications together into a single whole. > > > > One of the reasons I rewrote the attr state machine for LARP was to > > enable new multiple attr operation chains to be easily build from > > the entry points the state machien provides. Parent attr rename > > needs a new intent chain to be built, not run multiple independent > > intent chains for each modification. > > > > > It cant be an attr replace because technically the names are > > > different. > > > > I disagree - we have all the pieces we need in the state machine > > already, we just need to define separate attr names for the > > remove and insert steps in the attr intent. > > > > That is, the "replace" operation we execute when an attr set > > overwrites the value is "technically" a "replace value" operation, > > but we actually implement it as a "replace entire attribute" > > operation. > > > > Without LARP, we do that overwrite in independent steps via an > > intermediate INCOMPLETE state to allow two xattrs of the same name > > to exist in the attr tree at the same time. IOWs, the attr value > > overwrite is effectively a "set-swap-remove" operation on two > > entirely independent xattrs, ensuring that if we crash we always > > have either the old or new xattr visible. > > > > With LARP, we can remove the original attr first, thereby avoiding > > the need for two versions of the xattr to exist in the tree in the > > first place. However, we have to do these two operations as a pair > > of linked independent operations. The intent chain provides the > > linking, and requires us to log the name and the value of the attr > > that we are overwriting in the intent. Hence we can always recover > > the modification to completion no matter where in the operation we > > fail. > > > > When it comes to a parent attr rename operation, we are effectively > > doing two linked operations - remove the old attr, set the new attr > > - on different attributes. Implementation wise, it is exactly the > > same sequence as a "replace value" operation, except for the fact > > that the new attr we add has a different name. > > > > Hence the only real difference between the existing "attr replace" > > and the intent chain we need for "parent attr rename" is that we > > have to log two attr names instead of one. > > To be clear, this would imply expanding xfs_attri_log_format to have > another alfi_new_name_len feild and another iovec for the attr intent > right? Does that cause issues to change the on disk log layout after > the original has merged? Or is that ok for things that are still > experimental? Thanks! XFS_SB_FEAT_INCOMPAT_PARENT should protect against that, since the userspace xattr api does not support replacing an attr's name, only the value. --D > Allison > > > Basically, we have a new > > XFS_ATTRI_OP_FLAGS... type for this, and that's what tells us that > > we are operating on two different attributes instead of just one. > > > > The recovery operation becomes slightly different - we have to run a > > remove on the old, then a replace on the new - so there a little bit > > of new code needed to manage that in the state machine. > > > > These, however, are just small tweaks on the existing replace attr > > operation, and there should be little difference in performance or > > overhead between a "replace value" and a "replace entire xattr" > > operation as they are largely the same runtime operation for LARP. > > > > > So the recovered set grows the leaf, and returns the egain, then > > > rest > > > gets capture committed. Next up is the recovered remove which > > > pulls > > > out the fork, which causes problems when the rest of the set > > > operation > > > resumes as a deferred operation. > > > > Yup, and all this goes away when we build the right intent chain for > > replacing a parent attr rename.... > > > > Cheers, > > > > Dave. >
