hv_balloon: Fallback to generic_online_page() for non-HV hot added mem

hv_balloon: Fallback to generic_online_page() for non-HV hot added mem

The Hyper-V balloon driver installs a custom callback for handling page
onlining operations performed by the memory hotplug subsystem. This
custom callback is global, and overrides the default callback
(generic_online_page) that Linux otherwise uses. The custom callback
properly handles memory that is hot-added by the balloon driver as part
of a Hyper-V hot-add region.

But memory can also be hot-added directly by a device driver for a vPCI
device, particularly GPUs. In such a case, the custom callback installed by
the balloon driver runs, but won't find the page in its hot-add region list
and doesn't online it, which could cause driver initialization failures.

Fix this by having the balloon custom callback run generic_online_page()
when the page isn't part of a Hyper-V hot-add region, thereby doing the
default Linux behavior. This allows device driver hot-adds to work
properly. Similar cases are handled the same way in the virtio-mem driver.

Suggested-by: Vikram Sethi <vsethi@nvidia.com>
Tested-by: Michael Frohlich <mfrohlich@microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250107180918.1053933-1-jacob.pan@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250107180918.1053933-1-jacob.pan@linux.microsoft.com>