dm era: Recover committed writeset after crash

dm era: Recover committed writeset after crash

Following a system crash, dm-era fails to recover the committed writeset
for the current era, leading to lost writes. That is, we lose the
information about what blocks were written during the affected era.

dm-era assumes that the writeset of the current era is archived when the
device is suspended. So, when resuming the device, it just moves on to
the next era, ignoring the committed writeset.

This assumption holds when the device is properly shut down. But, when
the system crashes, the code that suspends the target never runs, so the
writeset for the current era is not archived.

There are three issues that cause the committed writeset to get lost:

1. dm-era doesn't load the committed writeset when opening the metadata
2. The code that resizes the metadata wipes the information about the
   committed writeset (assuming it was loaded at step 1)
3. era_preresume() starts a new era, without taking into account that
   the current era might not have been archived, due to a system crash.

To fix this:

1. Load the committed writeset when opening the metadata
2. Fix the code that resizes the metadata to make sure it doesn't wipe
   the loaded writeset
3. Fix era_preresume() to check for a loaded writeset and archive it,
   before starting a new era.

Fixes: b83b01ee643a86 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>