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(¤t->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.
> +
> + 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(¤t->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.
> +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.
> +{
> + int ret = 0;
> +
> + if (WARN_ON(((!original) || (!interposer))))
> + return -EINVAL;
No need for the inner two levels of braces.
> + * interposer should be simple, no a multi-queue device
> + */
> + if (!interposer->bd_disk->fops->submit_bio)
Please use queue_is_mq() instead.
> + 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?
> +struct bdev_interposer;
Not needed any more.
> +static inline bool bdev_has_interposer(struct block_device *bdev)
> +{
> + return (bdev->bd_interposer != NULL);
> +};
No need for the braces.
