Re: [PATCH RESEND v2 01/18] xfs: Fix multi-transaction larp replay

Alli
2022-08-16T20:41:33Z

On Mon, 2022-08-15 at 22:07 -0700, Darrick J. Wong wrote:
> On Tue, Aug 16, 2022 at 10:54:38AM +1000, Dave Chinner wrote:
> > 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!
> >
> > I think we can get away with this quite easily without breaking the
> > existing experimental code.
> >
> > struct xfs_attri_log_format {
> > uint16_t alfi_type; /* attri log item type */
> > uint16_t alfi_size; /* size of this item */
> > uint32_t __pad; /* pad to 64 bit aligned */
> > uint64_t alfi_id; /* attri identifier */
> > uint64_t alfi_ino; /* the inode for this attr
> > operation */
> > uint32_t alfi_op_flags; /* marks the op as a set or
> > remove */
> > uint32_t alfi_name_len; /* attr name length */
> > uint32_t alfi_value_len; /* attr value length */
> > uint32_t alfi_attr_filter;/* attr filter flags */
> > };
> >
> > We have a padding field in there that is currently all zeros. Let's
> > make that a count of the number of {name, value} tuples that are
> > appended to the format. i.e.
> >
> > struct xfs_attri_log_name {
> > uint32_t alfi_op_flags; /* marks the op as a set or
> > remove */
> > uint32_t alfi_name_len; /* attr name length */
> > uint32_t alfi_value_len; /* attr value length */
> > uint32_t alfi_attr_filter;/* attr filter flags */
> > };
> >
> > struct xfs_attri_log_format {
> > uint16_t alfi_type; /* attri log item type */
> > uint16_t alfi_size; /* size of this item */
> > uint8_t alfi_attr_cnt; /* count of name/val pairs
> > */
> > uint8_t __pad1; /* pad to 64 bit
> > aligned */
> > uint16_t __pad2; /* pad to 64 bit aligned */
> > uint64_t alfi_id; /* attri identifier */
> > uint64_t alfi_ino; /* the inode for this attr
> > operation */
> > struct xfs_attri_log_name alfi_attr[]; /* attrs to operate on
> > */
> > };
> >
> > Basically, the size and shape of the structure has not changed, and
> > if alfi_attr_cnt == 0 we just treat it as if alfi_attr_cnt == 1 as
> > the backwards compat code for the existing code.
> >
> > And then we just have as many followup regions for name/val pairs
> > as are defined by the alfi_attr_cnt and alfi_attr[] parts of the
> > structure. Each attr can have a different operation performed on
> > them, and they can have different filters applied so they can exist
> > in different namespaces, too.
> >
> > SO I don't think we need a new on-disk feature bit for this
> > enhancement - it definitely comes under the heading of "this stuff
> > is experimental, this is the sort of early structure revision that
> > EXPERIMENTAL is supposed to cover....
>
> You might even callit "alfi_extra_names" to avoid the "0 means 1"
> stuff.
> ;)
>
> --D

Oh, I just noticed these comments this morning when I sent out the new
attri/d patch. I'll add this changes to v2. Please let me know if
there's anything else you'd like me to change from the v1. Thx!

Allison

>
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com