[LSF/MM TOPIC] improving writeback error handling

[Jeff Layton <jlayton@kernel.org>]

I'd like to have a discussion on how to improve the handling of errors
that occur during writeback. I think there are 4 main issues that would
be helpful to cover:

1) In v4.16, we added the errseq mechanism to the kernel to make
writeback error reporting more reliable. That has helped some use cases,
but there are some applications (e.g. PostgreSQL) that were relying on
the ability to see writeback errors that occurred before the file
description existed. Do we need to tweak this mechanism to help those
use cases, or would that do more harm than good?

2) Most filesystems report data wb errors on all fds that were open at
the time of the error now, but metadata writeback can also fail, and
those don't get reported in the same way so far. Should we extend those
semantics to metadata writeback? How do we get there if so?

3) The question of what to do with pages in the pagecache that fail
writeback is still unresolved. Most filesystems just clear the dirty bit
and and carry on, but some invalidate them or just clear the uptodate
bit. This sort of inconsistency (and lack of documentation) is
problematic and has led to applications assuming behavior that doesn't
exist. I believe we need to declare an "ideal behavior" for Linux
filesystems in this regard, add VM/FS helpers to make it easier for
filesystems to conform to that behavior, and document it well. The big
question is : what sort of behavior makes the most sense here?

4) syncfs doesn't currently report an error when a single inode fails
writeback, only when syncing out the block device. Should it report
errors in that case as well?

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>