[dm-devel] dm-crypt: flush crypt_queue on suspend? on REQ_FLUSH?
dm-devel at lists.ewheeler.net
Fri Sep 2 22:13:47 UTC 2016
We have a KVM => dm-crypt => dm-thin stack in and a snapshot may have
partially completed queued IO out-of-order. EXT4 is giving errors like
this after mounting a snapshot, but only on files recently modified near
the snapshot time. This might imply out-of-order writes since,
presumably, the ext4 journal would handle deletions in journal order and
snapshots should be safe at any time:
kernel: EXT4-fs error (device dm-2): ext4_lookup:1441: inode #1196093: comm rsync: deleted inode referenced: 1188710
Notably, the ext4 error did not present prior to the snapshot. We have
rsync logs from hours before that didn't report this message, but all
later rsyncs do.
Perhaps related, or perhaps not, crypt_map() notes the following:
1914 * If bio is REQ_FLUSH or REQ_DISCARD, just bypass crypt queues.
1915 * - for REQ_FLUSH device-mapper core ensures that no IO is in-flight
1916 * - for REQ_DISCARD caller must use flush if IO ordering matters
However, crypt_map returns DM_MAPIO_SUBMITTED after calling
kcryptd_queue_crypt(io) which processes asynchronously on
cc->crypto_queue. In crypt_ctr(), alloc_workqueue sets max_active to
num_online_cpus() unless 'same_cpu_crypt' is set in the dm table (mine is
not), so if I understand correctly, completion could happen out of order
with CPUs >1.
Does the dm stack know that the bio (dm_crypt_io) hasn't been completed
even though it was put on a work queue? If so, I would like to understand
that dm bio-tracking mechanism, so please point me at documentation or
If not, then does crypt_map() need a workqueue_flush(cc->crypto_queue) on
I'm new to the dm-crypt code, so please educate me if I'm missing
Thank you for your help!
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