Re: [dm-devel] Block regression since 3.1-rc3

[Mike Snitzer <snitzer redhat com>]

On Sat, Oct 08 2011 at  7:02am -0400,
Shaohua Li <shli kernel org> wrote:

> Looks the dm request based flush logic is broken.
> 
> saved_make_request_fn
>   __make_request
>     blk_insert_flush
> but blk_insert_flush doesn't put the original request to list, instead, the
> q->flush_rq is in list.
> then
> dm_request_fn
>   blk_peek_request
>     dm_prep_fn
>       clone_rq
>   map_request
>     blk_insert_cloned_request
> so q->flush_rq is cloned, and get dispatched. but we can't clone q->flush_rq
> and use it to do flush. map_request even could assign a different blockdev to
> the cloned request.

You haven't explained why cloning q->flush_rq is broken.  What is the
problem with map_request changing the blockdev?  For the purposes of
request-based DM the flush machinery has already managed the processing
of the flush at the higher level request_queue.

By the time request-based DM is cloning a flush request it really has no
need to reenter the flush machinery (even though Tejun wants it to --
but in practice it doesn't buy us anything because we never stack
request-based DM at the moment.  Instead it showcases how brittle this
path is).

> Clone q->flush_rq is absolutely wrong.

I'm still missing the _why_.

Taking a step back:

Unless others have an immediate ah-ha moment, I'd suggest we revert
commit 4853abaae7e4a2a (block: fix flush machinery for stacking drivers
with differring flush flags).  Whereby avoiding unnecessarily reentering
the flush machinery.

If commit ed8b752bccf256 (dm table: set flush capability based on
underlying devices) is in place the flush gets fed directly to
scsi_request_fn, which is fine because the request-based DM's
request_queue's flush_flags reflect the flush capabilities of the
underlying device(s).

We are then covered relative to the only request-based DM use-case
people care about (e.g. dm-multipath, which doesn't use stacked
request-based DM).

We can revisit upholding the purity of the flush machinery for stacked
devices in >= 3.2.