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Re: [RFC 00/17] RFC parent inode pointers.

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC 00/17] RFC parent inode pointers.
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Mon, 27 Jan 2014 13:41:47 -0600
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140118031247.GE18112@dastard>
References: <20140115220012.624438534@xxxxxxx> <20140116055607.GR3431@dastard> <52D99FD2.6000601@xxxxxxx> <20140118031247.GE18112@dastard>
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On 01/17/14 21:12, Dave Chinner wrote:

<massive delete we can go over it point by point if necessary, but let
 us start here>.

1) Yep, the parent inode generation number is needed. I thought I said
   it was, bad on me if I did not. It was an RFC and I was too lazy to
   go back and add it in.

2) Add the filename to EA. Not a fan, but I will ask but if DMF needs it
   for performance then it has to be done. My point was this assumes
   that we can keep all the links' EA entries inline in the inode. A
   couple 255 character files or several links of modest sized filenames
   would negate that assumption. I tried to minimize the EA entries to
   keep them inline in the inode. I will talk to the DMF group.

3) There is a unlink/link race because the directory and EA changes
   are done without a common lock. I hit this in testing.

     Assume the sequence was something like *:
     ln a filename1 (EA saved to inode)
     rm filename1
     ln a filename1 (EA not saved because it is a duplicate)
                    (rm EA operation happens and removes the only PIP
     rm filename1 (no EA entry error)

* My speculation from counters and my testing.

      i) Why not add the lock to keep the directory/EA changes in sync?
     ii) 2009 code required duplicate EA entries to compensate.
          A) Required a counter/inode to make every link unique.
             Granted this counter could be in a inode field.
          B) Required a EA walk to find one of the duplicates entries
             for the remove.
             i) Mark no likey, much bitching and moaning...
          C) More below.

Mark, don't get me wrong - the 2009 patchset is not perfect and it's
not finished and it simply reflects what we knew at the time. When I
refer to that patch, I'm comparing the architecture and design of
the different parent pointer approaches, not the implementation.
The design has to be sound before I care about the implementation
and quality of the code.  If we can't agree on basic architecture
and design points, then we are most definitely not going to agree on
the implementation.

Right now, the design of the proposed patchset does not address
the critical problem of identifier uniqueness and ignores the
bulk-lookup performance requirements that we know about. Addressing
those are going to require a change of on-disk attribute format in
that patch set and that invalidates the in-inode-core optimisations
that have been made. IOWs, we need to solve the problem first, then

So, what do we need in the parent pointer attribute to solve all the
known problems? The implementation will flow cleanly from what we
can store on disk, and we know that we need at least these things to
solve all the known issues:

        * parent inode number and generation (unique identifier)


        * link disambiguation (unlink/link race detection)

why allow a unlink/link race?

        * filename (for bulk lookup performance)

So the question is how to implement the link disambiguation
efficiently. That is currently implemented in the 2009 patchset with
a the monotonic increasing counter that is appended to the attribute
name. Do we even need a generation count, or is there some other
info we can use that uniquely identifies a dirent?

While the diroffset of a filename is not unique enough to identify
the child, I think the {diroffset,filename,child_inode} tuple is
sufficient. That is, if the diroffset gets reused and points to a
different filename, we can detect that from the contents of EA and
abort. If a link of the same name is created, then we can check
whether it points at the same inode. If it does, then we just don't
care that there was a race because our current pointer is still
valid. And we don't need to store the child inode number in the EA -
we already have that in the child struct xfs_inode structure. That
verification can even be done in userspace.

Hence I think we've already got all the info we need if we make a
hybrid format from the two approaches:

        name=parent_inode,gen,diroffset value=filename

The inode/gen gives all the information we need to reliably identify
the parent without requiring child->parent lock ordering, and allows
userspace to do pathname component level reconstruction without the
kernel ever needing to verify the parent itself as part of ioctl

And finally, by using the diroffset in the EA name, we have a method of
knowing the exact parent pointer EA we need to modify/remove in
rename/unlink without an unbound searching.

I think that solves all the architectural issues that we know
about with both implemenations.



Thinking out loud:

EA names have to be unique.
A link/unlink/link EA sequence would have to do a EA RENAME (overwrite
 the duplicate EA with new name).
Have to do either:
 Do a EA lookup and compare before remove.
 Add a new EA command that removes a name/value pair.

Not sure if this would work on more than one unlink/link race and seems
like this would still not work if filename of the 2 links are
the same.

Leaving a known race makes me a bit queezy. My internal version uses
locks, but I were clear that you did not like the locks and so they were
not included in the RFC.

  ---- small hypothetical digression ----

If we could use the inode fields for a PIP entry (no filename
in the EA requirement), Olaf Weber came up with a clever PIP entry
EA swizzle that would leave all the PIP inserts/deletes to be done to
the incore inode fields at the same time as the directory operation.
It requires the offset(s) be looked up before the directory
insert/deletes. Pretty much academic if we cannot use in the inode


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