[GIT PULL] errseq_t fix for v4.17

[Jeff Layton]

The following changes since commit 0644f186fc9d77bb5bd198369e59fb28927a3692:

  Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost (2018-04-26 16:36:11 -0700)

are available in the Git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git tags/errseq-v4.17

for you to fetch changes up to b4678df184b314a2bd47d2329feca2c2534aa12b:

  errseq: Always report a writeback error once (2018-04-27 08:51:26 -0400)

----------------------------------------------------------------
The PostgreSQL developers recently had a spirited discussion about the
writeback error handling in Linux, and reached out to us about a
behavoir change to the code that bit them when the errseq_t changes
were merged.

When we changed to using errseq_t for tracking writeback errors, we
lost the ability for an application to see a writeback error that
occurred before the open on which the fsync was issued. This was
problematic for PostgreSQL which offloads fsync calls to a completely
separate process from the DB writers.

This patch restores that ability. If the errseq_t value in the inode
does not have the SEEN flag set, then we just return 0 for the sample.
That ensures that any recorded error is always delivered at least once.

Note that we might still lose the error if the inode gets evicted from
the cache before anything can reopen it, but that was the case before
errseq_t was merged. At LSF/MM we had some discussion about keeping
inodes with unreported writeback errors around in the cache for longer
(possibly indefinitely), but that's really a separate problem.
----------------------------------------------------------------
Matthew Wilcox (1):
      errseq: Always report a writeback error once

 lib/errseq.c | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

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