Re: [dm-devel] dm-crypt: flush crypt_queue on suspend? on REQ_FLUSH?

On Fri, 2 Sep 2016, Eric Wheeler wrote:

We have a KVM => dm-crypt => dm-thin stack 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 
[since] it was put on a work queue [and then a write_thread]?  If so, I 
would like to understand that dm bio-tracking mechanism, so please point 
me at documentation or code.

If not, then does crypt_map() need a workqueue_flush(cc->crypto_queue) on 
REQ_FLUSH?  

I'm new to the dm-crypt code, so please educate me if I'm missing 
something here.

On second look, simply adding workqueue_flush isn't enough since I've not 
set 'submit_from_crypt_cpus'; it's the cc->write_thread's job to get the 
bio's submitted via kcryptd_io_write() after pulling them from a red-black 
tree that kcryptd_crypt_write_io_submit() inserted into.  Is the red-black 
tree guaranteed to pop off in order, or at least to flush in order when a 
REQ_FLUSH comes through?

If not, then I think this means I need to set both 'same_cpu_crypt' 
(DM_CRYPT_SAME_CPU) and 'submit_from_crypt_cpus' (DM_CRYPT_NO_OFFLOAD) to 
guarantee ordering by restricting the workqueue to serial work 
(DM_CRYPT_SAME_CPU) and force immediate generic_make_request inside 
kcryptd_crypt_write_io_submit (DM_CRYPT_NO_OFFLOAD) instead of queuing to 
the cc->write_thread.

Your thoughts?

-Eric

 

Thank you for your help!

--
Eric Wheeler

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