[dm-devel] block: properly handle flush/fua requests in blk_insert_cloned_request

[Mike Snitzer]

On Tue, Aug 09 2011 at 12:13pm -0400,
Tejun Heo <tj at kernel.org> wrote:

> Hello,
> 
> On Tue, Aug 09, 2011 at 11:53:51AM -0400, Jeff Moyer wrote:
> > Tejun Heo <tj at kernel.org> writes:
> > > I'm a bit confused.  We still need ELEVATOR_INSERT_FLUSH fix for
> > > insertion paths, right?  Or is blk_insert_cloned_request() supposed to
> > > used only by request based dm which lives under the elevator?  If so,
> > > it would be great to make that explicit in the comment.  Maybe just
> > > renaming it to blk_insert_dm_cloned_request() would be better as it
> > > wouldn't be safe for other cases anyway.
> > 
> > request-based dm is the only caller at present.  I'm not a fan of
> > renaming the function, but I'm more than willing to comment it.
> 
> I'm still confused and don't think the patch is correct (you can't
> turn off REQ_FUA without decomposing it to data + post flush).
> 
> Going through flush machinery twice is okay and I think is the right
> thing to do.  At the upper queue, the request is decomposed to member
> requests.  After decomposition, it's either REQ_FLUSH w/o data or data
> request w/ or w/o REQ_FUA.  When the decomposed request reaches lower
> queue, the lower queue will then either short-circuit it, execute
> as-is or decompose data w/ REQ_FUA into data + REQ_FLUSH sequence.
> 
> AFAICS, the breakages are...
> 
> * ELEVATOR_INSERT_FLUSH not used properly from insert paths.
> 
> * Short circuit not kicking in for the dm requests. (the above and the
>   policy patch should solve this, right?)
> 
> * BUG(!rq->bio || ...) in blk_insert_flush().  I think we can lift
>   this restriction for empty REQ_FLUSH but also dm can just send down
>   requests with empty bio.

[cc'ing dm-devel]

All of these issues have come to light because DM was not setting
flush_flags based on the underlying device(s).  Now fixed in v3.1-rc1:
ed8b752 dm table: set flush capability based on underlying devices

Given that commit, and that request-based DM is beneath the elevator, it
seems any additional effort to have DM flushes re-enter the flush
machinary is unnecessary.

We expect:
1) flushes to have gone through the flush machinary
2) no FLUSH/FUA should be entering underlying queues if not supported

I think it best to just document the expectation that any FLUSH/FUA
request that enters blk_insert_cloned_request() will already match the
queue that the request is being sent to.  One way to document it is to
change Jeff's flag striping in to pure BUG_ON()s, e.g.:

---
 block/blk-core.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index b627558..201bb27 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1710,6 +1710,14 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
 	    should_fail_request(&rq->rq_disk->part0, blk_rq_bytes(rq)))
 		return -EIO;
 
+	/*
+	 * All FLUSH/FUA requests are expected to have gone through the
+	 * flush machinary.  If a request's cmd_flags doesn't match the
+	 * flush_flags of the underlying request_queue it is a bug.
+	 */
+	BUG_ON((rq->cmd_flags & REQ_FLUSH) && !(q->flush_flags & REQ_FLUSH));
+	BUG_ON((rq->cmd_flags & REQ_FUA) && !(q->flush_flags & REQ_FUA));
+
 	spin_lock_irqsave(q->queue_lock, flags);
 
 	/*