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Subject[RFC][PATCH 0/8] Migrate Pages in lieu of discard
I've been sitting on these for too long.  Tha main purpose of this
post is to have a public discussion with the other folks who are
interested in this functionalty and converge on a single

This set directly incorporates a statictics patch from Yang Shi and
also includes one to ensure good behavior with cgroup reclaim which
was very closely derived from this series:

Since the last post, the major changes are:
- Added patch to skip migration when doing cgroup reclaim
- Added stats patch from Yang Shi

The full series is also available here:


We're starting to see systems with more and more kinds of memory such
as Intel's implementation of persistent memory.

Let's say you have a system with some DRAM and some persistent memory.
Today, once DRAM fills up, reclaim will start and some of the DRAM
contents will be thrown out. Allocations will, at some point, start
falling over to the slower persistent memory.

That has two nasty properties. First, the newer allocations can end
up in the slower persistent memory. Second, reclaimed data in DRAM
are just discarded even if there are gobs of space in persistent
memory that could be used.

This set implements a solution to these problems. At the end of the
reclaim process in shrink_page_list() just before the last page
refcount is dropped, the page is migrated to persistent memory instead
of being dropped.

While I've talked about a DRAM/PMEM pairing, this approach would
function in any environment where memory tiers exist.

This is not perfect. It "strands" pages in slower memory and never
brings them back to fast DRAM. Other things need to be built to
promote hot pages back to DRAM.

This is part of a larger patch set. If you want to apply these or
play with them, I'd suggest using the tree from here. It includes
autonuma-based hot page promotion back to DRAM:

This is also all based on an upstream mechanism that allows
persistent memory to be onlined and used as if it were volatile:

Cc: Yang Shi <>
Cc: David Rientjes <>
Cc: Huang Ying <>
Cc: Dan Williams <>


Dave Hansen (5):
mm/numa: node demotion data structure and lookup
mm/vmscan: Attempt to migrate page in lieu of discard
mm/numa: automatically generate node migration order
mm/vmscan: never demote for memcg reclaim
mm/numa: new reclaim mode to enable reclaim-based migration

Keith Busch (2):
mm/migrate: Defer allocating new page until needed
mm/vmscan: Consider anonymous pages without swap

Yang Shi (1):
mm/vmscan: add page demotion counter

Documentation/admin-guide/sysctl/vm.rst | 9
include/linux/migrate.h | 6
include/linux/node.h | 9
include/linux/vm_event_item.h | 2
include/trace/events/migrate.h | 3
mm/debug.c | 1
mm/internal.h | 1
mm/migrate.c | 400 ++++++++++++++++++++++++++------
mm/page_alloc.c | 2
mm/vmscan.c | 88 ++++++-
mm/vmstat.c | 2
11 files changed, 439 insertions(+), 84 deletions(-)

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