On Wed, 2021-11-17 at 11:22 +1100, Dave Chinner wrote:

On Tue, Nov 16, 2021 at 10:59:05AM -0500, Brian Foster wrote:

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On Tue, Nov 16, 2021 at 02:01:20PM +1100, Dave Chinner wrote:

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On Tue, Nov 16, 2021 at 09:03:31AM +0800, Ian Kent wrote:

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On Tue, 2021-11-16 at 09:24 +1100, Dave Chinner wrote:

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If it isn't safe for ext4 to do that, then we have a general
pathwalk problem, not an XFS issue. But, as you say, it is
safe
to do this zeroing, so the fix to xfs_ifree() is to zero the
link buffer instead of freeing it, just like ext4 does.

As a side issue, we really don't want to move what XFS does
in
.destroy_inode to .free_inode because that then means we need
to
add synchronise_rcu() calls everywhere in XFS that might need
to
wait on inodes being inactivated and/or reclaimed. And
because
inode reclaim uses lockless rcu lookups, there's substantial
danger of adding rcu callback related deadlocks to XFS here.
That's just not a direction we should be moving in.

Another reason I decided to use the ECHILD return instead is
that
I thought synchronise_rcu() might add an unexpected delay.

It depends where you put the synchronise_rcu() call. :)

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Since synchronise_rcu() will only wait for processes that
currently have the rcu read lock do you think that could
actually
be a problem in this code path?

No, I don't think it will.  The inode recycle case in XFS inode
lookup can trigger in two cases:

1. VFS cache eviction followed by immediate lookup
2. Inode has been unlinked and evicted, then free and reallocated
by
the filesytsem.

In case #1, that's a cold cache lookup and hence delays are
acceptible (e.g. a slightly longer delay might result in having
to
fetch the inode from disk again). Calling synchronise_rcu() in
this
case is not going to be any different from having to fetch the
inode
from disk...

In case #2, there's a *lot* of CPU work being done to modify
metadata (inode btree updates, etc), and so the operations can
block
on journal space, metadata IO, etc. Delays are acceptible, and
could
be in the order of hundreds of milliseconds if the transaction
subsystem is bottlenecked. waiting for an RCU grace period when
we
reallocate an indoe immediately after freeing it isn't a big
deal.

IOWs, if synchronize_rcu() turns out to be a problem, we can
optimise that separately - we need to correct the inode reuse
behaviour w.r.t. VFS RCU expectations, then we can optimise the
result if there are perf problems stemming from correct
behaviour.

FWIW, with a fairly crude test on a high cpu count system, it's not
that
difficult to reproduce an observable degradation in inode
allocation
rate with a synchronous grace period in the inode reuse path,
caused
purely by a lookup heavy workload on a completely separate
filesystem.

The following is a 5m snapshot of the iget stats from a filesystem
doing
allocs/frees with an external/heavy lookup workload (which not
included
in the stats), with and without a sync grace period wait in the
reuse
path:

baseline:       ig 1337026 1331541 4 5485 0 5541 1337026
sync_rcu_test:  ig 2955 2588 0 367 0 383 2955

The alloc/free part of the workload is a single threaded
create/unlink in a tight loop, yes?

This smells like a side effect of agressive reallocation of
just-freed XFS_IRECLAIMABLE inodes from the finobt that haven't had
their unlink state written back to disk yet. i.e. this is a corner
case in #2 above where a small set of inodes is being repeated
allocated and freed by userspace and hence being agressively reused
and never needing to wait for IO. i.e. a tempfile workload
optimisation...

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I think this is kind of the nature of RCU and why I'm not sure it's
a
great idea to rely on update side synchronization in a codepath
that
might want to scale/perform in certain workloads.

The problem here is not update side synchronisation. Root cause is
aggressive reallocation of recently freed VFS inodes via physical
inode allocation algorithms. Unfortunately, the RCU grace period
requirements of the VFS inode life cycle dictate that we can't
aggressively re-allocate and reuse freed inodes like this. i.e.
reallocation of a just-freed inode also has to wait for an RCU grace
period to pass before the in memory inode can be re-instantiated as
a newly allocated inode.

(Hmmmm - I wonder if of the other filesystems might have similar
problems with physical inode reallocation inside a RCU grace period?
i.e. without inode instance re-use, the VFS could potentially see
multiple in-memory instances of the same physical inode at the same
time.)

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I'm not totally sure
if this will be a problem for real users running real workloads or
not,
or if this can be easily mitigated, whether it's all rcu or a
cascading
effect, etc. This is just a quick test so that all probably
requires
more test and analysis to discern.

This looks like a similar problem to what busy extents address - we
can't reuse a newly freed extent until the transaction containing
the EFI/EFD hit stable storage (and the discard operation on the
range is complete). Hence while a newly freed extent is
marked free in the allocbt, they can't be reused until they are
released from the busy extent tree.

I can think of several ways to address this, but let me think on it
a bit more.  I suspect there's a trick we can use to avoid needing
synchronise_rcu() completely by using the spare radix tree tag and
rcu grace period state checks with get_state_synchronize_rcu() and
poll_state_synchronize_rcu() to clear the radix tree tags via a
periodic radix tree tag walk (i.e. allocation side polling for "can
we use this inode" rather than waiting for the grace period to
expire once an inode has been selected and allocated.)

The synchronise_rcu() seems like it's too broad a brush.

It sounds like there are relatively simple ways to avoid the link
path race which I won't go into again but there's still a chance
inode re-use can cause confusion if done at the wrong time.

So it sounds like per-object (inode) granularity is needed for the
wait and that means answering the question "how do we know when it's
ok to re-use the inode" when we come to alloc the inode and want to
re-use one.

I don't know the answer to that question but introducing an XFS flag
to indicate the inode is in transition (or altering the meaning of an
an existing one) so we know to wait should be straight forward.

Perhaps the start of the rcu grace period for the object is a
suitable beginning then we just need to know if the grace period
for the object has expired to complete the wait ... possibly via
an xfs owned rcu call back on free to update the xfs flags ...

But I'm just thinking out loud here ...

There'd be a need to know when not to wait at all too ... mmm.

Ian

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Sorry, I don't understand what you mean by the root cause not
being identified?

The whole approach of "we don't know how to fix the inode reuse
case
so disable it" implies that nobody has understood where in the
reuse
case the problem lies. i.e. "inode reuse" by itself is not the
root
cause of the problem.

I don't think anybody suggested to disable inode reuse.

Nobody did, so that's not what I was refering to. I was refering to
the patches for and comments advocating disabling .get_link for RCU
pathwalk because of the apparently unsolved problems stemming from
inode reuse...

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The root cause is "allowing an inode to be reused without waiting
for an RCU grace period to expire". This might seem pedantic, but
"without waiting for an rcu grace period to expire" is the
important
part of the problem (i.e. the bug), not the "allowing an inode to
be
reused" bit.

Once the RCU part of the problem is pointed out, the solution
becomes obvious. As nobody had seen the obvious (wait for an RCU
grace period when recycling an inode) it stands to reason that
nobody really understood what the root cause of the inode reuse
problem.

The synchronize_rcu() approach was one of the first options
discussed in
the bug report once a reproducer was available.

What bug report would that be? :/

It's not one that I've read, and I don't recall seeing a pointer to
it anywhere in the path posting. IOWs, whatever discussion happened
in a private distro bug report can't be assumed as "general
knowledge" in an upstream discussion...

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AIUI, this is not currently a reproducible problem even before
patch 1,
which reduces the race window even further. Given that and the nak
on
the current patch (the justification for which I don't really
understand), I'm starting to agree with Ian's earlier statement
that
perhaps it is best to separate this one so we can (hopefully) move
patch
1 along on its own merit..

*nod*

The problem seems pretty rare, the pathwalk patch makes it
even rarer, so I think they can be separated just fine.

Cheers,

Dave.