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SubjectRe: [RFC][PATCH 3/8] mm/vmscan: Attempt to migrate page in lieu of discard
On Tue, 30 Jun 2020, Yang Shi wrote:

> > > From: Dave Hansen <dave.hansen@linux.intel.com>
> > >
> > > If a memory node has a preferred migration path to demote cold pages,
> > > attempt to move those inactive pages to that migration node before
> > > reclaiming. This will better utilize available memory, provide a faster
> > > tier than swapping or discarding, and allow such pages to be reused
> > > immediately without IO to retrieve the data.
> > >
> > > When handling anonymous pages, this will be considered before swap if
> > > enabled. Should the demotion fail for any reason, the page reclaim
> > > will proceed as if the demotion feature was not enabled.
> > >
> > Thanks for sharing these patches and kick-starting the conversation, Dave.
> >
> > Could this cause us to break a user's mbind() or allow a user to
> > circumvent their cpuset.mems?
> >
> > Because we don't have a mapping of the page back to its allocation
> > context (or the process context in which it was allocated), it seems like
> > both are possible.
>
> Yes, this could break the memory placement policy enforced by mbind and
> cpuset. I discussed this with Michal on mailing list and tried to find a way
> to solve it, but unfortunately it seems not easy as what you mentioned above.
> The memory policy and cpuset is stored in task_struct rather than mm_struct.
> It is not easy to trace back to task_struct from page (owner field of
> mm_struct might be helpful, but it depends on CONFIG_MEMCG and is not
> preferred way).
>

Yeah, and Ying made a similar response to this message.

We can do this if we consider pmem not to be a separate memory tier from
the system perspective, however, but rather the socket perspective. In
other words, a node can only demote to a series of exclusive pmem ranges
and promote to the same series of ranges in reverse order. So DRAM node 0
can only demote to PMEM node 2 while DRAM node 1 can only demote to PMEM
node 3 -- a pmem range cannot be demoted to, or promoted from, more than
one DRAM node.

This naturally takes care of mbind() and cpuset.mems if we consider pmem
just to be slower volatile memory and we don't need to deal with the
latency concerns of cross socket migration. A user page will never be
demoted to a pmem range across the socket and will never be promoted to a
different DRAM node that it doesn't have access to.

That can work with the NUMA abstraction for pmem, but it could also
theoretically be a new memory zone instead. If all memory living on pmem
is migratable (the natural way that memory hotplug is done, so we can
offline), this zone would live above ZONE_MOVABLE. Zonelist ordering
would determine whether we can allocate directly from this memory based on
system config or a new gfp flag that could be set for users of a mempolicy
that allows allocations directly from pmem. If abstracted as a NUMA node
instead, interleave over nodes {0,2,3} or a cpuset.mems of {0,2,3} doesn't
make much sense.

Kswapd would need to be enlightened for proper pgdat and pmem balancing
but in theory it should be simpler because it only has its own node to
manage. Existing per-zone watermarks might be easy to use to fine tune
the policy from userspace: the scale factor determines how much memory we
try to keep free on DRAM for migration from pmem, for example. We also
wouldn't have to deal with node hotplug or updating of demotion/promotion
node chains.

Maybe the strongest advantage of the node abstraction is the ability to
use autonuma and migrate_pages()/move_pages() API for moving pages
explicitly? Mempolicies could be used for migration to "top-tier" memory,
i.e. ZONE_NORMAL or ZONE_MOVABLE, instead.

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