On Tue, Mar 16 2021 at 11:03pm -0400,
Ming Lei [off-list ref] wrote:

On Tue, Mar 16, 2021 at 07:35:44PM +0300, Sergei Shtepa wrote:

quoted

The 03/16/2021 11:09, Ming Lei wrote:

quoted

On Fri, Mar 12, 2021 at 06:44:54PM +0300, Sergei Shtepa wrote:

quoted

bdev_interposer allows to redirect bio requests to another devices.

Signed-off-by: Sergei Shtepa <redacted>

...

quoted

quoted

quoted

+
+int bdev_interposer_attach(struct block_device *original,
+			   struct block_device *interposer)
+{
+	int ret = 0;
+
+	if (WARN_ON(((!original) || (!interposer))))
+		return -EINVAL;
+	/*
+	 * interposer should be simple, no a multi-queue device
+	 */
+	if (!interposer->bd_disk->fops->submit_bio)
+		return -EINVAL;
+
+	if (WARN_ON(!blk_mq_is_queue_frozen(original->bd_disk->queue)))
+		return -EPERM;

The original request queue may become live now...

Yes.
I will remove the blk_mq_is_queue_frozen() function and use a different
approach.

Looks what attach and detach needs is that queue is kept as frozen state
instead of being froze simply at the beginning of the two functions, so
you can simply call freeze/unfreeze inside the two functions.

But what if 'original' isn't a MQ queue?  queue usage counter is just
grabed when calling ->submit_bio(), and queue freeze doesn't guarantee there
isn't any io activity, is that a problem for bdev_interposer use case?

Right, I raised the same concern here:
https://listman.redhat.com/archives/dm-devel/2021-March/msg00135.html
(toward bottom inlined after dm_disk_{freeze,unfreeze}

Anyway, this certainly needs to be addressed.

Mike