On Tue 03-08-21 13:59:18, Feng Tang wrote:

From: Dave Hansen <dave.hansen@linux.intel.com>

The NUMA APIs currently allow passing in a "preferred node" as a
single bit set in a nodemask.  If more than one bit it set, bits
after the first are ignored.

This single node is generally OK for location-based NUMA where
memory being allocated will eventually be operated on by a single
CPU.  However, in systems with multiple memory types, folks want
to target a *type* of memory instead of a location.  For instance,
someone might want some high-bandwidth memory but do not care about
the CPU next to which it is allocated.  Or, they want a cheap,
high capacity allocation and want to target all NUMA nodes which
have persistent memory in volatile mode.  In both of these cases,
the application wants to target a *set* of nodes, but does not
want strict MPOL_BIND behavior as that could lead to OOM killer or
SIGSEGV.

So add MPOL_PREFERRED_MANY policy to support the multiple preferred
nodes requirement. This is not a pie-in-the-sky dream for an API.
This was a response to a specific ask of more than one group at Intel.
Specifically:

1. There are existing libraries that target memory types such as
   https://github.com/memkind/memkind.  These are known to suffer
   from SIGSEGV's when memory is low on targeted memory "kinds" that
   span more than one node.  The MCDRAM on a Xeon Phi in "Cluster on
   Die" mode is an example of this.
2. Volatile-use persistent memory users want to have a memory policy
   which is targeted at either "cheap and slow" (PMEM) or "expensive and
   fast" (DRAM).  However, they do not want to experience allocation
   failures when the targeted type is unavailable.
3. Allocate-then-run.  Generally, we let the process scheduler decide
   on which physical CPU to run a task.  That location provides a
   default allocation policy, and memory availability is not generally
   considered when placing tasks.  For situations where memory is
   valuable and constrained, some users want to allocate memory first,
   *then* allocate close compute resources to the allocation.  This is
   the reverse of the normal (CPU) model.  Accelerators such as GPUs
   that operate on core-mm-managed memory are interested in this model.

A check is added in sanitize_mpol_flags() to not permit 'prefer_many'
policy to be used for now, and will be removed in later patch after all
implementations for 'prefer_many' are ready, as suggested by Michal Hocko.

[Michal Hocko: suggest to refine policy_node/policy_nodemask handling]
Link: https://lore.kernel.org/r/20200630212517.308045-4-ben.widawsky@intel.com (local)
Co-developed-by: Ben Widawsky <redacted>
Signed-off-by: Ben Widawsky <redacted>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Feng Tang <redacted>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!
-- 
Michal Hocko
SUSE Labs