On 03/03/2026 12:39, Nilay Shroff wrote:
Yes we got a reference to mpath_head while
allocating it but then these are two (alloc mpath_disk and
alloc mpath_head) disjoint operations. In that case, can't
we have both mpath_disk and mpath_head allocated under one
libmultipath API?
I would like to have something simpler (like mainline NVMe code), but
I have it this way because of SCSI, as above.
I understand the intended lifetime model due to SCSI, but the current
flow is somewhat confusing.
In nvme_mpath_alloc_disk(), mpath_disk and mpath_head are allocated
separately. However, during teardown, both objects are ultimately
released through mpath_free_disk(), which drops the reference to
mpath_head via mpath_put_head().
Since the allocation of mpath_disk and mpath_head happens independently,
it is not immediately obvious why their lifetime is tied together and
why they are not freed independently when the NVMe head node is removed.
This coupling makes the ownership and reference flow harder to reason
about.
Yes, and also having 2x separate structures bloats the code, as we are
continuously looking up one from another and so on. Only having a
mpath_head for a nvme_ns_head would be nice - I'll look at this approach
(again).
Additionally, I noticed that nvme_remove_head() has been removed in the
NVMe code that integrates with libmultipath. IMO, It might be clearer to
retain this function and make the teardown sequence explicit (after
removing mpath_put_head() from mpath_free_disk()).
For example:
nvme_remove_head():
mpath_unregister_disk(); /* removes mpath_disk and drops its ref */
mpath_put_head(); /* drops mpath_head reference */
nvme_put_ns_head(); /* drops NVMe namespace head reference */
Does the above example makes sense?
Yeah, something like that would be simpler. I just need to make it work
for scsi :)
My current implementation has it that the scsi_device manages the
mpath_head and the scsi_disk manages the mpath_disk.
Supporting a single structure for scsi is more complicated, as the
lifetime of the scsi_disk is not tied to that of the scsi_device, i.e.
we can do something like this:
echo "8:0:0:0" > /sys/bus/scsi/drivers/sd/unbind
Which removes the scsi_disk, but the scsi_device remains. So, for
multipath support, doing this would mean the that mpath_disk would be
removed (if the last path), but not the mpath_head.
Furthermore, the scsi_disk info is private to sd.c, so this would mean
that the mpath_disk could/should also be private. My point is that scsi
makes things more complicated.
Thanks for checking!
