Re: [PATCH 1/2] cxl/region: Timeout auto region assembly waiting for endpoints

[<dan.j.williams@intel.com>]

Alison Schofield wrote:
> Currently, if not all expected endpoints arrive for an auto-created
> region, the region remains registered but disabled indefinitely. The
> region continues to reserve its memory resource, preventing DAX from
> registering the memory, and provides no indication that endpoints
> failed to arrive (useful for non-DAX configurations).

Does it block DAX? The HPA reservation does not mark the resource busy,
so DAX should still be able to operate. It might make a mess of resource
tree, but not block unless we are in this case of the boundaries
misaligning. I would rather rethink the insert_resource() in
__construct_region() if it is indeed a blocking problem. It is already
the case that an insert_resource() failure is not fatal to region
creation.

> Start a 30 second timeout when the first endpoint attaches. If the
> remaining endpoints have not attached before the timeout expires,
> abort region assembly and unregister the region.

The time to give up on regions present at boot should be at the end of
wait_for_device_probe(). There is no point waiting any longer for a
device that was alive a few seconds ago according to the BIOS.

> Cancel the timeout when all expected endpoints attach or the region is
> unregistered for any reason.

Setting aside the above, this looks like policy, and every time I see
policy the first question is "can userspace do it?". It would be
straightforward for userspace to kick a 30 second watchdog upon each
region KOBJ_ADD event. Each time that fires go cleanup partially
assembled regions.

For example there is no automatic cleanup of partially assembled RAID
arrays. So, precedent leans towards letting userspace decide what
happens when composite devices fail assembly.