Re: [patch] block: fix flush machinery for stacking drivers with differring flush flags

[Jeff Moyer <jmoyer@redhat.com>]

Tejun Heo <tj@kernel.org> writes:

>> -#define REQ_CLONE_MASK		REQ_COMMON_MASK
>> +/*
>> + * Cloned requests are inserted into the elevator via blk_insert_cloned_request.
>> + * Because the flush flags exported by the request-based dm target may in
>> + * theory be different from the flush flags of the underlying request_queue,
>> + * we need to pass along information regarding whether a particular request
>> + * is part of a flush sequence.  This is primarily used to complete I/Os early
>> + * that would otherwise not be necessary (such as an empty flush for a request
>> + * queue that does not support flush).  In such a case, the end_io path for
>> + * the request would try to account the I/O instead of ignoring it, resulting
>> + * in a null pointer dereference.
>> + */
>> +#define REQ_CLONE_MASK		(REQ_COMMON_MASK | REQ_FLUSH_SEQ)
>
> I'm probably missing something, but why do we still need to copy
> REQ_FLUSH_SEQ?  Why doesn't the following work?
>
> * dm driver always advertises REQ_FLUSH|FUA like other stacking
>   drivers.
>
> * blk-flush for the dm, decomposes flushes to FLUSH + FUA write and
>   send it down.
>
> * dm driver clones the requests and send them down to each member
>   queue.
>
> * blk-flush on member queue, handles FLUSH as FLUSH and decomposes FUA
>   write as necessary.
>
> What am I missing?  Why does end_io path still matter when it goes
> through blk-flush on the member device too?

You're missing the I/O completion of an empty flush trying to do I/O
accounting, and oopsing, as shown in the stack trace I provided before.

We could avoid passing REQ_FLUSH_SEQ, and then set it when completing an
empty flush, but I thought that was even worse.  Or, maybe we could
clear REQ_IO_STAT when completing such requests.

-Jeff