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  1. Feb 11, 2021
    • Nikos Tsironis's avatar
      dm era: Update in-core bitset after committing the metadata · 2099b145
      Nikos Tsironis authored 
      

In case of a system crash, dm-era might fail to mark blocks as written
in its metadata, although the corresponding writes to these blocks were
passed down to the origin device and completed successfully.

Consider the following sequence of events:

1. We write to a block that has not been yet written in the current era
2. era_map() checks the in-core bitmap for the current era and sees
   that the block is not marked as written.
3. The write is deferred for submission after the metadata have been
   updated and committed.
4. The worker thread processes the deferred write
   (process_deferred_bios()) and marks the block as written in the
   in-core bitmap, **before** committing the metadata.
5. The worker thread starts committing the metadata.
6. We do more writes that map to the same block as the write of step (1)
7. era_map() checks the in-core bitmap and sees that the block is marked
   as written, **although the metadata have not been committed yet**.
8. These writes are passed down to the origin device immediately and the
   device reports them as completed.
9. The system crashes, e.g., power failure, before the commit from step
   (5) finishes.

When the system recovers and we query the dm-era target for the list of
written blocks it doesn't report the aforementioned block as written,
although the writes of step (6) completed successfully.

The issue is that era_map() decides whether to defer or not a write
based on non committed information. The root cause of the bug is that we
update the in-core bitmap, **before** committing the metadata.

Fix this by updating the in-core bitmap **after** successfully
committing the metadata.

Fixes: eec40579 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: default avatarNikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      2099b145
    • Nikos Tsironis's avatar
      dm era: Recover committed writeset after crash · de89afc1
      Nikos Tsironis authored 
      

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: eec40579 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: default avatarNikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      de89afc1
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