tree c9d928197d0839a942ea12f613eedc5d5b503d06
parent 642db2207df3cc092c85e9a7dc0f46b929ee223c
author Darrick J. Wong <darrick.wong@oracle.com> 1593134336 -0700
committer Darrick J. Wong <darrick.wong@oracle.com> 1602352674 -0700

xfs: periodically relog deferred intent items

There's a subtle design flaw in the deferred log item code that can lead
to pinning the log tail.  Taking up the defer ops chain examples from
the previous commit, we can get trapped in sequences like this:

Caller hands us a transaction t0 with D0-D3 attached.  The defer ops
chain will look like the following if the transaction rolls succeed:

t1: D0(t0), D1(t0), D2(t0), D3(t0)
t2: d4(t1), d5(t1), D1(t0), D2(t0), D3(t0)
t3: d5(t1), D1(t0), D2(t0), D3(t0)
...
t9: d9(t7), D3(t0)
t10: D3(t0)
t11: d10(t10), d11(t10)
t12: d11(t10)

In transaction 9, we finish d9 and try to roll to t10 while holding onto
an intent item for D3 that we logged in t0.

The previous commit changed the order in which we place new defer ops in
the defer ops processing chain to reduce the maximum chain length.  Now
make xfs_defer_finish_noroll capable of relogging the entire chain
periodically so that we can always move the log tail forward.  Most
chains will never get relogged, except for operations that generate very
long chains (large extents containing many blocks with different sharing
levels) or are on filesystems with small logs and a lot of ongoing
metadata updates.

Callers are now required to ensure that the transaction reservation is
large enough to handle logging done items and new intent items for the
maximum possible chain length.  Most callers are careful to keep the
chain lengths low, so the overhead should be minimal.

The decision to relog an intent item is made based on two factors: (a)
whether the intent was logged in a previous checkpoint, and (b) if the
log has hit the 75% full threshold.  There's no point in relogging an
intent into the same checkpoint, and there's no need to relog if there's
plenty of free space in the log.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>