[dm-devel] fold direct_make_requst into generic_make_request
Mike Snitzer
snitzer at redhat.com
Tue Feb 27 00:58:53 UTC 2018
On Mon, Feb 26 2018 at 7:13pm -0500,
Mike Snitzer <snitzer at redhat.com> wrote:
> On Mon, Feb 26 2018 at 5:56pm -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
>
> > The block layer provides a function direct_make_request - but it doesn't
> > provide any test whether this function may be used or not.
> >
> > Device mapper currently uses a dirty trick - it compares the result of
> > bdevname against the string "nvme" to test if direct_make_request can be
> > used.
>
> dm's DM_TYPE_NVME_BIO_BASED is backed by that crude "nvme" check _but_
> it is a means to an end. Alternative would be to have NVMe (and other
> devices) set something like QUEUE_FLAG_NO_PARTIAL_COMPLETION or
> something -- and then DM would need to verify all underlying devices set
> that flag.
>
> > The information whether the driver will or won't recurse can be easily
> > obtained in generic_make_request (by comparing make_request_fn - or
> > alternatively, we could introduce a queue flag for this), so my suggestion
> > is to just delete direct_make_request and fold this "norecurse"
> > optimization directly into generic_make_request This will allow us to
> > simplify device mapper paths and get rid of device name matching.
>
> Sorry, Nack from me as implemented. Please see below.
As we discussed, but for the benefit of others: I was mistaken.. I
reviewed your proposal too quickly.
> > Index: linux-2.6/block/blk-core.c
> > ===================================================================
> > --- linux-2.6.orig/block/blk-core.c 2018-02-26 20:37:19.088499000 +0100
> > +++ linux-2.6/block/blk-core.c 2018-02-26 20:43:57.839999000 +0100
> > @@ -2265,6 +2265,8 @@ end_io:
> > return false;
> > }
> >
> > +blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio);
> > +
> > /**
> > * generic_make_request - hand a buffer to its device driver for I/O
> > * @bio: The bio describing the location in memory and on the device.
> > @@ -2300,10 +2302,14 @@ blk_qc_t generic_make_request(struct bio
> > */
> > struct bio_list bio_list_on_stack[2];
> > blk_qc_t ret = BLK_QC_T_NONE;
> > + bool may_recurse;
> >
> > if (!generic_make_request_checks(bio))
> > goto out;
> >
> > + may_recurse = bio->bi_disk->queue->make_request_fn != blk_queue_bio &&
> > + bio->bi_disk->queue->make_request_fn != blk_mq_make_request;
> > +
>
> This does _not_ allow dm's DM_TYPE_NVME_BIO_BASED to make use of your
> direct_make_request() equivalent that you've encoded into
> generic_make_request().
>
> There is no easy way to _know_, at runtime, that an arbitrary queue
> (e.g. stacked DM device) does _not_ split IO (therefore not needing to
> recurse).
>
> Same can be said of NVMe given NVMe multipath sets its make_request_fn
> to nvme_ns_head_make_request().
>
> So for both cases that don't need to recurse you're setting @may_recurse
> to true.
Both DM multipath and NVMe native multipath will change the bio->bi_disk
to the underlying NVMe device (which has make_request_fn set to
blk_mq_make_request). SO: @may_recurse is set to false for both cases.
Sorry for tainting your work, I retract my Nack. Avoiding the need for
drivers to reason about whether it best to call direct_make_request() vs
generic_make_request() is a welcome advance. I think you plan on
submitting a v2 that avoids the make_request_fn check. I'll review that
much more carefully before commenting ;)
Thanks,
Mike
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