CVE-2026-64244

In the Linux kernel, the following vulnerability has been resolved:

drivers/base/memory: set mem->altmap after successful device registration

If _addmemoryblock() fails at xastore() (under memory pressure for example), deviceunregister() is called, which eventually triggers memoryblockrelease() with mem->altmap still set, causing a WARNON(mem->altmap). This was triggered by modifying virtio-mem driver.

Fix this by delaying the assignment of mem->altmap until after __addmemoryblock() has succeeded.