[PATCH 0/3] btrfs: scrub: enhance freezing and signal handling

[Qu Wenruo <wqu@suse.com>]

It's a long known bug that when scrub/dev-replace is running, power
management suspension will time out and fail.

After more debugging and helps from Askar Safin, it turns out there are
at least 3 components involved:

- Process freezing
  This is at the preparation for suspension, which requires all user
  space processes (and some kthreads) to be frozen, which requires the
  process return to user space.

  Thus if the process (normally btrfs command) is falling into a long
  running ioctl (like scrub/dev-replace) it will not be frozen thus
  breaking the pm suspension.

  This mean paused scrub is not feasible, as paused scrub will still
  make the ioctl executing process trapped inside kernel space.

- Filesystem freezing
  It's an optional behavior during pm suspension, previously I submitted
  one patch detecting such situation, and so far it works as expected.
  But this fs freezing is only optional, not yet default behavior of pm
  suspension.

- Systemd slice freezing
  This is the most complex part that I have not yet fully pinned down.

To address the first 2 problems, the series will:

- Add extra cancel/pause/removed bg checks for raid56 parity stripes
  Mostly to reduce delay for RAID56 cases, and make the behavior more
  consistent.

- Cancel the scrub if the fs/process is being frozen

- Cancel the scrub if there is a pending signal
  For the systemd slice situation I have no idea how slice freezing is
  done, but at least we should check pending signals (not only fatal
  ones), which will align the behavior to relocation.

Qu Wenruo (3):
  btrfs: scrub: add cancel/pause/removed bg checks for raid56 parity
    stripes
  btrfs: scrub: cancel the run if the process or fs is being frozen
  btrfs: scrub: cancel the run if there is a pending signal

 fs/btrfs/scrub.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

-- 
2.51.0