Re: [PATCH v2] hostmem: Honor multiple preferred nodes if possible

[David Hildenbrand]

On 15.12.22 10:55, Michal Privoznik wrote:
> 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>
> ---

[...]

> +#ifdef HAVE_NUMA_SET_PREFERRED_MANY
> +        if (mode == MPOL_PREFERRED && numa_has_preferred_many() > 0) {
> +            /*
> +             * Replace with MPOL_PREFERRED_MANY otherwise the mbind() below
> +             * silently picks the first node.
> +             */
> +            mode = MPOL_PREFERRED_MANY;
> +        }
> +#endif
> +


I was curios if we want to warn the user if "mode == 
MPOL_PREFERRED_MANY" and we were given more than one node.


Apart from that LGTM, thanks

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb