If a memory-backend is configured with mode
HOST_MEM_POLICY_PREFERRED then
host_memory_backend_memory_complete() calls mbind() as:

  mbind(..., MPOL_PREFERRED, nodemask, ...);

Here, 'nodemask' is a bitmap of host NUMA nodes and corresponds
to the .host-nodes attribute. Therefore, there can be multiple
nodes specified. However, the documentation to MPOL_PREFERRED
says:

  MPOL_PREFERRED
    This mode sets the preferred node for allocation. ...
    If nodemask specifies more than one node ID, the first node
    in the mask will be selected as the preferred node.

Therefore, only the first node is honored and the rest is
silently ignored. Well, with recent changes to the kernel and
numactl we can do better.

The Linux kernel added in v5.15 via commit cfcaa66f8032
("mm/hugetlb: add support for mempolicy MPOL_PREFERRED_MANY")
support for MPOL_PREFERRED_MANY, which accepts multiple preferred
NUMA nodes instead.

Then, numa_has_preferred_many() API was introduced to numactl
(v2.0.15~26) allowing applications to query kernel support.

Wiring this all together, we can pass MPOL_PREFERRED_MANY to the
mbind() call instead and stop ignoring multiple nodes, silently.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <a0b4adce1af5bd2344c2218eb4a04b3ff7bcfdb4.1671097918.git.mprivozn@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>