[dm-devel] [PATCH v7 2/3] block: add bdev_interposer

[Sergei Shtepa]

The 03/14/2021 12:28, Christoph Hellwig wrote:
> On Fri, Mar 12, 2021 at 06:44:54PM +0300, Sergei Shtepa wrote:
> > bdev_interposer allows to redirect bio requests to another devices.
> 
> I think this warrants a somewhat more detailed description.
> 
> The code itself looks pretty good to me now, a bunch of nitpicks and
> a question below:
> 
> > +static noinline blk_qc_t submit_bio_interposed(struct bio *bio)
> > +{
> > +	blk_qc_t ret = BLK_QC_T_NONE;
> > +	struct bio_list bio_list[2] = { };
> > +	struct gendisk *orig_disk;
> > +
> > +	if (current->bio_list) {
> > +		bio_list_add(&current->bio_list[0], bio);
> > +		return BLK_QC_T_NONE;
> > +	}
> 
> I don't think this case can ever happen:
> 
>  - current->bio_list != NULL means a ->submit_bio or blk_mq_submit_bio
>    is active.  But if this device is being interposed this means the
>    interposer recurses into itself, which should never happen.  So
>    I think we'll want a WARN_ON_ONCE here as a debug check instead.

Yes, it is.
Completely remove this check or add "BUG_ON(current->bio_list);" for
an emergency?

> 
> > +
> > +	orig_disk = bio->bi_bdev->bd_disk;
> > +	if (unlikely(bio_queue_enter(bio)))
> > +		return BLK_QC_T_NONE;
> > +
> > +	current->bio_list = bio_list;
> > +
> > +	do {
> > +		struct block_device *interposer = bio->bi_bdev->bd_interposer;
> > +
> > +		if (unlikely(!interposer)) {
> > +			/* interposer was removed */
> > +			bio_list_add(&current->bio_list[0], bio);
> > +			break;
> > +		}
> > +		/* assign bio to interposer device */
> > +		bio_set_dev(bio, interposer);
> > +		bio_set_flag(bio, BIO_INTERPOSED);
> 
> Reassigning the bi_bdev here means the original source is lost by the
> time we reach the interposer.  This initially seemed a little limiting,
> but I guess the interposer driver can just record that information
> locally, so we should be fine.  The big upside of this is that no
> extra argument to submit_bio_checks, which means less changes to the
> normal fast path, so if this works for everyone that is a nice
> improvement over my draft.
> 
> > +
> > +		if (!submit_bio_checks(bio))
> > +			break;
> > +		/*
> > +		 * Because the current->bio_list is initialized,
> > +		 * the submit_bio callback will always return BLK_QC_T_NONE.
> > +		 */
> > +		interposer->bd_disk->fops->submit_bio(bio);
> > +	} while (false);
> 
> I find the do { ... } while (false) idiom here a little strange.  Normal
> kernel style would be a goto done instead of the breaks.
> 

Ok. I'll use the normal kernel style.

> > +int bdev_interposer_attach(struct block_device *original,
> > +			   struct block_device *interposer)
> 
> A kerneldoc comment for bdev_interposer_attach (and
> bdev_interposer_detach) would be nice to explain the API a little more.
> 

Yes, I should add kerneldoc comments.

> > +{
> > +	int ret = 0;
> > +
> > +	if (WARN_ON(((!original) || (!interposer))))
> > +		return -EINVAL;
> 
> No need for the inner two levels of braces.

Ok.

> 
> > +	 * interposer should be simple, no a multi-queue device
> > +	 */
> > +	if (!interposer->bd_disk->fops->submit_bio)
> 
> Please use queue_is_mq() instead.

Ok.

> 
> > +	if (bdev_has_interposer(original))
> > +		ret = -EBUSY;
> > +	else {
> > +		original->bd_interposer = bdgrab(interposer);
> 
> Just thinking out a loud:  what happens if the interposed device
> goes away?  Shouldn't we at very least also make sure this
> gabs another refererence on bdev as well?

If the original device is removed from the system, the interposer device
will be permanently occupied. I need to add an interposer release when
deleting a block device.

> 
> > +struct bdev_interposer;
> 
> Not needed any more.

Yes.

> 
> > +static inline bool bdev_has_interposer(struct block_device *bdev)
> > +{
> > +	return (bdev->bd_interposer != NULL);
> > +};
> 
> No need for the braces.

Ok.

-- 
Sergei Shtepa
Veeam Software developer.