Re: dm: Respect REQ_NOWAIT bios

[Mike Snitzer <snitzer@kernel.org>]

On Tue, Oct 24 2023 at  3:18P -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Tue, 24 Oct 2023, Mike Snitzer wrote:
> 
> > For the benefit of others, since this patch was posted to the old
> > dm-devel list, here is the original patch proposal:
> > https://patchwork.kernel.org/project/dm-devel/patch/15ca26cc-174a-d4e8-9780-d09f8e5a6ea5@redhat.com/
> > 
> > On Tue, Oct 03 2023 at 11:30P -0400,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > Hi
> > > 
> > > Here I'm sending that patch for REQ_NOWAIT for review.
> > > 
> > > It is not tested, except for some trivial tests that involve logical 
> > > volume activation.
> > 
> > At a minimum we need to test with the simple test code Jens provided
> > in commit a9ce385344f9 ("dm: don't attempt to queue IO under RCU protection")
> 
> The question is - how do we simulate the memory allocation failures? Do we 
> need to add some test harness that will randomly return NULL to these 
> allocations? Or will we use the kernel fault-injection framework?

Will think about it (and do research).  Maybe others have suggestions?

> > Below you'll see I insulated dm_split_and_process_bio() from ever
> > checking ci.nowait_failed -- prefer methods like __send_empty_flush
> > that are called from dm_split_and_process_bio() return blk_status_t
> > (like __process_abnormal_io and __split_and_process_bio do).
> > 
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index d6fbbaa7600b..2a9ff269c28b 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -572,18 +572,16 @@ static void dm_end_io_acct(struct dm_io *io)
> >  	dm_io_acct(io, true);
> >  }
> >  
> > -static struct dm_io *alloc_io(struct clone_info *ci, struct mapped_device *md, struct bio *bio)
> > +static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
> >  {
> >  	struct dm_io *io;
> >  	struct dm_target_io *tio;
> >  	struct bio *clone;
> >  
> > -	if (unlikely(ci->is_nowait)) {
> > +	if (unlikely(bio->bi_opf & REQ_NOWAIT)) {
> >  		clone = bio_alloc_clone(NULL, bio, GFP_NOWAIT, &md->mempools->io_bs);
> > -		if (!clone) {
> > -			ci->nowait_failed = true;
> > -			return NULL;
> > -		}
> > +		if (!clone)
> > +			return PTR_ERR(-EAGAIN);
> 
> :s/PTR_ERR/ERR_PTR/

Ha, not sure how I inverted it.. but thanks.

> If -EAGAIN is the only possible error code, should we return NULL instead?

Yeah, thought about that.  Think returning NULL is fine and if/when
other reasons for failure possible we can extend as needed.

> > @@ -1813,17 +1815,17 @@ static void dm_split_and_process_bio(struct mapped_device *md,
> >  			return;
> >  	}
> >  
> > -	init_clone_info(&ci, md, map, bio, is_abnormal);
> > -	if (unlikely(ci.nowait_failed)) {
> > -		error = BLK_STS_AGAIN;
> > -		goto out;
> > +	io = alloc_io(md, bio);
> > +	if (unlikely(IS_ERR(io) && ERR_PTR(io) == -EAGAIN)) {
> :s/ERR_PTR/PTR_ERR/

Yes.

> > +		/* Unable to do anything without dm_io. */
> > +		bio_wouldblock_error(bio);
> > +		return;
> 
> I would use "if (unlikely(IS_ERR(io)) {
> 		bio->bi_status = errno_to_blk_status(PTR_ERR(io));
> 		bio_set_flag(bio, BIO_QUIET);
> 		bio_endio(bio);
> 		return;",
> so that all possible error codes (that could be added in the future) are 
> propageted.
> 
> Or, if you change alloc_io to return NULL, you can leave 
> bio_wouldblock_error here.

Yeah, will just return NULL and stick with bio_wouldblock_error() for
now.

Thanks,
Mike