[PATCH -V10 0/9] Migrate Pages in lieu of discard

[PATCH -V10 0/9] Migrate Pages in lieu of discard

[Huang Ying]

The full series is also available here:

	https://github.com/hying-caritas/linux/tree/automigrate-20210715

The changes since the last post are as follows,

 * Reduce code duplication via move common demotion condition code
   into can_demote_anon_pages() per Wei's comments.

--

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.  Huang Ying has follow-on work which
repurposes autonuma to promote hot pages 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:

	http://lkml.kernel.org/r/20190124231441.37A4A305@viggo.jf.intel.com

We have tested the patchset with the postgresql and pgbench.  On a
2-socket server machine with DRAM and PMEM, the kernel with the
patchset can improve the score of pgbench up to 22.1% compared with
that of the DRAM only + disk case.  This comes from the reduced disk
read throughput (which reduces up to 70.8%).

== Open Issues ==

 * Memory policies and cpusets that, for instance, restrict allocations
   to DRAM can be demoted to PMEM whenever they opt in to this
   new mechanism.  A cgroup-level API to opt-in or opt-out of
   these migrations will likely be required as a follow-on.
 * Could be more aggressive about where anon LRU scanning occurs
   since it no longer necessarily involves I/O.  get_scan_count()
   for instance says: "If we have no swap space, do not bother
   scanning anon pages"

--

Changes since (automigrate-20210625):
 * Reduce code duplication via move common demotion condition code
   into can_demote_anon_pages() per Wei's comments.

Changes since (automigrate-20210618):
 * Squash the original 01/10 and 02/10 and move the RCU protection
   from the original 03/10 to the squashed 1/9.
 * Make the newly added migrate_pages() parameter optional per Oscar's
   comments.
 * Restore the original behavior of MADV_PAGEOUT per Zi's comments.
 * Guard next_demotion_node() with numa_demotion_enabled per Wei's
   comments.

Changes since (automigrate-20210331):
 * Change the page allocation flags per Michal's comments.
 * Change the user interface to enable the feature.

Changes since (automigrate-20210304):
 * Add ack/review tags
 * Remove duplicate synchronize_rcu() call

Changes since (automigrate-20210122):
 * move from GFP_HIGHUSER -> GFP_HIGHUSER_MOVABLE since pages *are*
   movable.
 * Separate out helpers that check for being able to relaim anonymous
   pages versus being able to meaningfully scan the anon LRU.

Changes since (automigrate-20200818):
 * Fall back to normal reclaim when demotion fails
 * Fix some compile issues, when page migration and NUMA are off

Changes since (automigrate-20201007):
 * separate out checks for "can scan anon LRU" from "can actually
   swap anon pages right now".  Previous series conflated them
   and may have been overly aggressive scanning LRU
 * add MR_DEMOTION to tracepoint header
 * remove unnecessary hugetlb page check

Changes since (https://lwn.net/Articles/824830/):
 * Use higher-level migrate_pages() API approach from Yang Shi's
   earlier patches.
 * made sure to actually check node_reclaim_mode's new bit
 * disabled migration entirely before introducing RECLAIM_MIGRATE
 * Replace GFP_NOWAIT with explicit __GFP_KSWAPD_RECLAIM and
   comment why we want that.
 * Comment on effects of that keep multiple source nodes from
   sharing target nodes

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: osalvador <osalvador@suse.de>
Cc: Wei Xu <weixugc@google.com>
Cc: Zi Yan <ziy@nvidia.com>

[PATCH -V10 1/9] mm/numa: automatically generate node migration order

[Huang Ying]

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

Prepare for the kernel to auto-migrate pages to other memory nodes
with a node migration table. This allows creating single migration
target for each NUMA node to enable the kernel to do NUMA page
migrations instead of simply discarding colder pages. A node with no
target is a "terminal node", so reclaim acts normally there.  The
migration target does not fundamentally _need_ to be a single node,
but this implementation starts there to limit complexity.

When memory fills up on a node, memory contents can be
automatically migrated to another node.  The biggest problems are
knowing when to migrate and to where the migration should be
targeted.

The most straightforward way to generate the "to where" list would
be to follow the page allocator fallback lists.  Those lists
already tell us if memory is full where to look next.  It would
also be logical to move memory in that order.

But, the allocator fallback lists have a fatal flaw: most nodes
appear in all the lists.  This would potentially lead to migration
cycles (A->B, B->A, A->B, ...).

Instead of using the allocator fallback lists directly, keep a
separate node migration ordering.  But, reuse the same data used
to generate page allocator fallback in the first place:
find_next_best_node().

This means that the firmware data used to populate node distances
essentially dictates the ordering for now.  It should also be
architecture-neutral since all NUMA architectures have a working
find_next_best_node().

RCU is used to allow lock-less read of node_demotion[] and prevent
demotion cycles been observed.  If multiple reads of node_demotion[]
are performed, a single rcu_read_lock() must be held over all reads to
ensure no cycles are observed.  Details are as follows.

=== What does RCU provide? ===

Imaginge a simple loop which walks down the demotion path looking
for the last node:

        terminal_node = start_node;
        while (node_demotion[terminal_node] != NUMA_NO_NODE) {
                terminal_node = node_demotion[terminal_node];
        }

The initial values are:

        node_demotion[0] = 1;
        node_demotion[1] = NUMA_NO_NODE;

and are updated to:

        node_demotion[0] = NUMA_NO_NODE;
        node_demotion[1] = 0;

What guarantees that the cycle is not observed:

        node_demotion[0] = 1;
        node_demotion[1] = 0;

and would loop forever?

With RCU, a rcu_read_lock/unlock() can be placed around the
loop.  Since the write side does a synchronize_rcu(), the loop
that observed the old contents is known to be complete before the
synchronize_rcu() has completed.

RCU, combined with disable_all_migrate_targets(), ensures that
the old migration state is not visible by the time
__set_migration_target_nodes() is called.

=== What does READ_ONCE() provide? ===

READ_ONCE() forbids the compiler from merging or reordering
successive reads of node_demotion[].  This ensures that any
updates are *eventually* observed.

Consider the above loop again.  The compiler could theoretically
read the entirety of node_demotion[] into local storage
(registers) and never go back to memory, and *permanently*
observe bad values for node_demotion[].

Note: RCU does not provide any universal compiler-ordering
guarantees:

	https://lore.kernel.org/lkml/20150921204327.GH4029@linux.vnet.ibm.com/

This code is unused for now.  It will be called later in the
series.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

--

Changes from 20210618:
 * Merge patches for data structure definition and initialization
 * Move RCU usage from the next patch in series per Zi's comments

Changes from 20210302:
 * Fix typo in node_demotion[] comment

Changes since 20200122:
 * Make node_demotion[] __read_mostly
 * Add big node_demotion[] comment

Changes in July 2020:
 - Remove loop from next_demotion_node() and get_online_mems().
   This means that the node returned by next_demotion_node()
   might now be offline, but the worst case is that the
   allocation fails.  That's fine since it is transient.
---
 mm/internal.h   |   5 ++
 mm/migrate.c    | 216 ++++++++++++++++++++++++++++++++++++++++++++++++
 mm/page_alloc.c |   2 +-
 3 files changed, 222 insertions(+), 1 deletion(-)

diff --git a/mm/internal.h b/mm/internal.h
index 31ff935b2547..0744b27e1ce9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -539,12 +539,17 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn,
 
 #ifdef CONFIG_NUMA
 extern int node_reclaim(struct pglist_data *, gfp_t, unsigned int);
+extern int find_next_best_node(int node, nodemask_t *used_node_mask);
 #else
 static inline int node_reclaim(struct pglist_data *pgdat, gfp_t mask,
 				unsigned int order)
 {
 	return NODE_RECLAIM_NOSCAN;
 }
+static inline int find_next_best_node(int node, nodemask_t *used_node_mask)
+{
+	return NUMA_NO_NODE;
+}
 #endif
 
 extern int hwpoison_filter(struct page *p);
diff --git a/mm/migrate.c b/mm/migrate.c
index 34a9ad3e0a4f..b7a40ab47648 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1099,6 +1099,80 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	return rc;
 }
 
+
+/*
+ * node_demotion[] example:
+ *
+ * Consider a system with two sockets.  Each socket has
+ * three classes of memory attached: fast, medium and slow.
+ * Each memory class is placed in its own NUMA node.  The
+ * CPUs are placed in the node with the "fast" memory.  The
+ * 6 NUMA nodes (0-5) might be split among the sockets like
+ * this:
+ *
+ *	Socket A: 0, 1, 2
+ *	Socket B: 3, 4, 5
+ *
+ * When Node 0 fills up, its memory should be migrated to
+ * Node 1.  When Node 1 fills up, it should be migrated to
+ * Node 2.  The migration path start on the nodes with the
+ * processors (since allocations default to this node) and
+ * fast memory, progress through medium and end with the
+ * slow memory:
+ *
+ *	0 -> 1 -> 2 -> stop
+ *	3 -> 4 -> 5 -> stop
+ *
+ * This is represented in the node_demotion[] like this:
+ *
+ *	{  1, // Node 0 migrates to 1
+ *	   2, // Node 1 migrates to 2
+ *	  -1, // Node 2 does not migrate
+ *	   4, // Node 3 migrates to 4
+ *	   5, // Node 4 migrates to 5
+ *	  -1} // Node 5 does not migrate
+ */
+
+/*
+ * Writes to this array occur without locking.  Cycles are
+ * not allowed: Node X demotes to Y which demotes to X...
+ *
+ * If multiple reads are performed, a single rcu_read_lock()
+ * must be held over all reads to ensure that no cycles are
+ * observed.
+ */
+static int node_demotion[MAX_NUMNODES] __read_mostly =
+	{[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
+
+/**
+ * next_demotion_node() - Get the next node in the demotion path
+ * @node: The starting node to lookup the next node
+ *
+ * @returns: node id for next memory node in the demotion path hierarchy
+ * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
+ * @node online or guarantee that it *continues* to be the next demotion
+ * target.
+ */
+int next_demotion_node(int node)
+{
+	int target;
+
+	/*
+	 * node_demotion[] is updated without excluding this
+	 * function from running.  RCU doesn't provide any
+	 * compiler barriers, so the READ_ONCE() is required
+	 * to avoid compiler reordering or read merging.
+	 *
+	 * Make sure to use RCU over entire code blocks if
+	 * node_demotion[] reads need to be consistent.
+	 */
+	rcu_read_lock();
+	target = READ_ONCE(node_demotion[node]);
+	rcu_read_unlock();
+
+	return target;
+}
+
 /*
  * Obtain the lock on page, remove all ptes and migrate the page
  * to the newly allocated page in newpage.
@@ -2982,3 +3056,145 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
 }
 EXPORT_SYMBOL(migrate_vma_finalize);
 #endif /* CONFIG_DEVICE_PRIVATE */
+
+/* Disable reclaim-based migration. */
+static void __disable_all_migrate_targets(void)
+{
+	int node;
+
+	for_each_online_node(node)
+		node_demotion[node] = NUMA_NO_NODE;
+}
+
+static void disable_all_migrate_targets(void)
+{
+	__disable_all_migrate_targets();
+
+	/*
+	 * Ensure that the "disable" is visible across the system.
+	 * Readers will see either a combination of before+disable
+	 * state or disable+after.  They will never see before and
+	 * after state together.
+	 *
+	 * The before+after state together might have cycles and
+	 * could cause readers to do things like loop until this
+	 * function finishes.  This ensures they can only see a
+	 * single "bad" read and would, for instance, only loop
+	 * once.
+	 */
+	synchronize_rcu();
+}
+
+/*
+ * Find an automatic demotion target for 'node'.
+ * Failing here is OK.  It might just indicate
+ * being at the end of a chain.
+ */
+static int establish_migrate_target(int node, nodemask_t *used)
+{
+	int migration_target;
+
+	/*
+	 * Can not set a migration target on a
+	 * node with it already set.
+	 *
+	 * No need for READ_ONCE() here since this
+	 * in the write path for node_demotion[].
+	 * This should be the only thread writing.
+	 */
+	if (node_demotion[node] != NUMA_NO_NODE)
+		return NUMA_NO_NODE;
+
+	migration_target = find_next_best_node(node, used);
+	if (migration_target == NUMA_NO_NODE)
+		return NUMA_NO_NODE;
+
+	node_demotion[node] = migration_target;
+
+	return migration_target;
+}
+
+/*
+ * When memory fills up on a node, memory contents can be
+ * automatically migrated to another node instead of
+ * discarded at reclaim.
+ *
+ * Establish a "migration path" which will start at nodes
+ * with CPUs and will follow the priorities used to build the
+ * page allocator zonelists.
+ *
+ * The difference here is that cycles must be avoided.  If
+ * node0 migrates to node1, then neither node1, nor anything
+ * node1 migrates to can migrate to node0.
+ *
+ * This function can run simultaneously with readers of
+ * node_demotion[].  However, it can not run simultaneously
+ * with itself.  Exclusion is provided by memory hotplug events
+ * being single-threaded.
+ */
+static void __set_migration_target_nodes(void)
+{
+	nodemask_t next_pass	= NODE_MASK_NONE;
+	nodemask_t this_pass	= NODE_MASK_NONE;
+	nodemask_t used_targets = NODE_MASK_NONE;
+	int node;
+
+	/*
+	 * Avoid any oddities like cycles that could occur
+	 * from changes in the topology.  This will leave
+	 * a momentary gap when migration is disabled.
+	 */
+	disable_all_migrate_targets();
+
+	/*
+	 * Allocations go close to CPUs, first.  Assume that
+	 * the migration path starts at the nodes with CPUs.
+	 */
+	next_pass = node_states[N_CPU];
+again:
+	this_pass = next_pass;
+	next_pass = NODE_MASK_NONE;
+	/*
+	 * To avoid cycles in the migration "graph", ensure
+	 * that migration sources are not future targets by
+	 * setting them in 'used_targets'.  Do this only
+	 * once per pass so that multiple source nodes can
+	 * share a target node.
+	 *
+	 * 'used_targets' will become unavailable in future
+	 * passes.  This limits some opportunities for
+	 * multiple source nodes to share a destination.
+	 */
+	nodes_or(used_targets, used_targets, this_pass);
+	for_each_node_mask(node, this_pass) {
+		int target_node = establish_migrate_target(node, &used_targets);
+
+		if (target_node == NUMA_NO_NODE)
+			continue;
+
+		/*
+		 * Visit targets from this pass in the next pass.
+		 * Eventually, every node will have been part of
+		 * a pass, and will become set in 'used_targets'.
+		 */
+		node_set(target_node, next_pass);
+	}
+	/*
+	 * 'next_pass' contains nodes which became migration
+	 * targets in this pass.  Make additional passes until
+	 * no more migrations targets are available.
+	 */
+	if (!nodes_empty(next_pass))
+		goto again;
+}
+
+/*
+ * For callers that do not hold get_online_mems() already.
+ */
+__maybe_unused // <- temporay to prevent warnings during bisects
+static void set_migration_target_nodes(void)
+{
+	get_online_mems();
+	__set_migration_target_nodes();
+	put_online_mems();
+}
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3b97e17806be..cdb741399886 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6139,7 +6139,7 @@ static int node_load[MAX_NUMNODES];
  *
  * Return: node id of the found node or %NUMA_NO_NODE if no node is found.
  */
-static int find_next_best_node(int node, nodemask_t *used_node_mask)
+int find_next_best_node(int node, nodemask_t *used_node_mask)
 {
 	int n, val;
 	int min_val = INT_MAX;
-- 
2.30.2

[PATCH -V10 2/9] mm/migrate: update node demotion order on hotplug events

[Huang Ying]

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

Reclaim-based migration is attempting to optimize data placement in
memory based on the system topology.  If the system changes, so must
the migration ordering.

The implementation is conceptually simple and entirely unoptimized.
On any memory or CPU hotplug events, assume that a node was added or
removed and recalculate all migration targets.  This ensures that the
node_demotion[] array is always ready to be used in case the new
reclaim mode is enabled.

This recalculation is far from optimal, most glaringly that it does
not even attempt to figure out the hotplug event would have some
*actual* effect on the demotion order.  But, given the expected
paucity of hotplug events, this should be fine.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: osalvador <osalvador@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

--

Changes since 20210618:
 * moved RCU part to the prev patch in series.

Changes since 20210302:
 * remove duplicate synchronize_rcu()
---
 mm/migrate.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 89 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index b7a40ab47648..a40c391f9ca7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -49,6 +49,7 @@
 #include <linux/sched/mm.h>
 #include <linux/ptrace.h>
 #include <linux/oom.h>
+#include <linux/memory.h>
 
 #include <asm/tlbflush.h>
 
@@ -3057,6 +3058,7 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
 EXPORT_SYMBOL(migrate_vma_finalize);
 #endif /* CONFIG_DEVICE_PRIVATE */
 
+#if defined(CONFIG_MEMORY_HOTPLUG)
 /* Disable reclaim-based migration. */
 static void __disable_all_migrate_targets(void)
 {
@@ -3191,10 +3193,96 @@ static void __set_migration_target_nodes(void)
 /*
  * For callers that do not hold get_online_mems() already.
  */
-__maybe_unused // <- temporay to prevent warnings during bisects
 static void set_migration_target_nodes(void)
 {
 	get_online_mems();
 	__set_migration_target_nodes();
 	put_online_mems();
 }
+
+/*
+ * React to hotplug events that might affect the migration targets
+ * like events that online or offline NUMA nodes.
+ *
+ * The ordering is also currently dependent on which nodes have
+ * CPUs.  That means we need CPU on/offline notification too.
+ */
+static int migration_online_cpu(unsigned int cpu)
+{
+	set_migration_target_nodes();
+	return 0;
+}
+
+static int migration_offline_cpu(unsigned int cpu)
+{
+	set_migration_target_nodes();
+	return 0;
+}
+
+/*
+ * This leaves migrate-on-reclaim transiently disabled between
+ * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
+ * whether reclaim-based migration is enabled or not, which
+ * ensures that the user can turn reclaim-based migration at
+ * any time without needing to recalculate migration targets.
+ *
+ * These callbacks already hold get_online_mems().  That is why
+ * __set_migration_target_nodes() can be used as opposed to
+ * set_migration_target_nodes().
+ */
+static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
+						 unsigned long action, void *arg)
+{
+	switch (action) {
+	case MEM_GOING_OFFLINE:
+		/*
+		 * Make sure there are not transient states where
+		 * an offline node is a migration target.  This
+		 * will leave migration disabled until the offline
+		 * completes and the MEM_OFFLINE case below runs.
+		 */
+		disable_all_migrate_targets();
+		break;
+	case MEM_OFFLINE:
+	case MEM_ONLINE:
+		/*
+		 * Recalculate the target nodes once the node
+		 * reaches its final state (online or offline).
+		 */
+		__set_migration_target_nodes();
+		break;
+	case MEM_CANCEL_OFFLINE:
+		/*
+		 * MEM_GOING_OFFLINE disabled all the migration
+		 * targets.  Reenable them.
+		 */
+		__set_migration_target_nodes();
+		break;
+	case MEM_GOING_ONLINE:
+	case MEM_CANCEL_ONLINE:
+		break;
+	}
+
+	return notifier_from_errno(0);
+}
+
+static int __init migrate_on_reclaim_init(void)
+{
+	int ret;
+
+	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "migrate on reclaim",
+				migration_online_cpu,
+				migration_offline_cpu);
+	/*
+	 * In the unlikely case that this fails, the automatic
+	 * migration targets may become suboptimal for nodes
+	 * where N_CPU changes.  With such a small impact in a
+	 * rare case, do not bother trying to do anything special.
+	 */
+	WARN_ON(ret < 0);
+
+	hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
+	return 0;
+}
+late_initcall(migrate_on_reclaim_init);
+#endif /* CONFIG_MEMORY_HOTPLUG */
-- 
2.30.2

[PATCH -V10 3/9] mm/migrate: enable returning precise migrate_pages() success count

[Huang Ying]

From: Yang Shi <yang.shi@linux.alibaba.com>

Under normal circumstances, migrate_pages() returns the number of
pages migrated.  In error conditions, it returns an error code.  When
returning an error code, there is no way to know how many pages were
migrated or not migrated.

Make migrate_pages() return how many pages are demoted successfully
for all cases, including when encountering errors.  Page reclaim
behavior will depend on this in subsequent patches.

Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: osalvador <osalvador@suse.de> [optional parameter]
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

--

Note: Yang Shi originally wrote the patch, thus the SoB.  There was
also a Reviewed-by provided since there were some modifications made
to this after the original work.

Changes since 20210618:
 * Reword some bits of the changelog/subject
 * Allow callers to pass a NULL 'ret_succeeded'.  This ensures that
   callers don't have to declare and pass a dummy variable.

Changes since 20210302:
 * Fix definition of CONFIG_MIGRATION=n stub migrate_pages().  Its
   parameters were wrong, but oddly enough did not generate any
   compile errors.

Changes since 20200122:
 * Fix migrate_pages() to manipulate nr_succeeded *value*
   rather than the pointer.
---
 include/linux/migrate.h |  5 +++--
 mm/compaction.c         |  2 +-
 mm/gup.c                |  2 +-
 mm/memory-failure.c     |  2 +-
 mm/memory_hotplug.c     |  2 +-
 mm/mempolicy.c          |  4 ++--
 mm/migrate.c            | 11 ++++++++---
 mm/page_alloc.c         |  2 +-
 8 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 23dadf7aeba8..8ab88d46318e 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -41,7 +41,8 @@ extern int migrate_page(struct address_space *mapping,
 			struct page *newpage, struct page *page,
 			enum migrate_mode mode);
 extern int migrate_pages(struct list_head *l, new_page_t new, free_page_t free,
-		unsigned long private, enum migrate_mode mode, int reason);
+		unsigned long private, enum migrate_mode mode, int reason,
+		unsigned int *ret_succeeded);
 extern struct page *alloc_migration_target(struct page *page, unsigned long private);
 extern int isolate_movable_page(struct page *page, isolate_mode_t mode);
 
@@ -56,7 +57,7 @@ extern int migrate_page_move_mapping(struct address_space *mapping,
 static inline void putback_movable_pages(struct list_head *l) {}
 static inline int migrate_pages(struct list_head *l, new_page_t new,
 		free_page_t free, unsigned long private, enum migrate_mode mode,
-		int reason)
+		int reason, unsigned int *ret_succeeded)
 	{ return -ENOSYS; }
 static inline struct page *alloc_migration_target(struct page *page,
 		unsigned long private)
diff --git a/mm/compaction.c b/mm/compaction.c
index 621508e0ecd5..61fb64f47a06 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2398,7 +2398,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 
 		err = migrate_pages(&cc->migratepages, compaction_alloc,
 				compaction_free, (unsigned long)cc, cc->mode,
-				MR_COMPACTION);
+				MR_COMPACTION, NULL);
 
 		trace_mm_compaction_migratepages(cc->nr_migratepages, err,
 							&cc->migratepages);
diff --git a/mm/gup.c b/mm/gup.c
index 42b8b1fa6521..c4441fc4cfba 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1772,7 +1772,7 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
 	if (!list_empty(&movable_page_list)) {
 		ret = migrate_pages(&movable_page_list, alloc_migration_target,
 				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
-				    MR_LONGTERM_PIN);
+				    MR_LONGTERM_PIN, NULL);
 		if (ret && !list_empty(&movable_page_list))
 			putback_movable_pages(&movable_page_list);
 	}
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index eefd823deb67..3eed65e56f93 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2093,7 +2093,7 @@ static int __soft_offline_page(struct page *page)
 
 	if (isolate_page(hpage, &pagelist)) {
 		ret = migrate_pages(&pagelist, alloc_migration_target, NULL,
-			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_FAILURE);
+			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_FAILURE, NULL);
 		if (!ret) {
 			bool release = !huge;
 
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 8cb75b26ea4f..0c46458a3402 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1469,7 +1469,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 		if (nodes_empty(nmask))
 			node_set(mtc.nid, nmask);
 		ret = migrate_pages(&source, alloc_migration_target, NULL,
-			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
+			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG, NULL);
 		if (ret) {
 			list_for_each_entry(page, &source, lru) {
 				if (__ratelimit(&migrate_rs)) {
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e32360e90274..939eabcaf488 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1084,7 +1084,7 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
 
 	if (!list_empty(&pagelist)) {
 		err = migrate_pages(&pagelist, alloc_migration_target, NULL,
-				(unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL);
+				(unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL, NULL);
 		if (err)
 			putback_movable_pages(&pagelist);
 	}
@@ -1338,7 +1338,7 @@ static long do_mbind(unsigned long start, unsigned long len,
 		if (!list_empty(&pagelist)) {
 			WARN_ON_ONCE(flags & MPOL_MF_LAZY);
 			nr_failed = migrate_pages(&pagelist, new_page, NULL,
-				start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
+				start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND, NULL);
 			if (nr_failed)
 				putback_movable_pages(&pagelist);
 		}
diff --git a/mm/migrate.c b/mm/migrate.c
index a40c391f9ca7..35d34ef837ed 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1429,6 +1429,8 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  * @mode:		The migration mode that specifies the constraints for
  *			page migration, if any.
  * @reason:		The reason for page migration.
+ * @ret_succeeded:	Set to the number of pages migrated successfully if
+ *			the caller passes a non-NULL pointer.
  *
  * The function returns after 10 attempts or if no pages are movable any more
  * because the list has become empty or no retryable pages exist any more.
@@ -1439,7 +1441,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  */
 int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		free_page_t put_new_page, unsigned long private,
-		enum migrate_mode mode, int reason)
+		enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
 {
 	int retry = 1;
 	int thp_retry = 1;
@@ -1594,6 +1596,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	if (!swapwrite)
 		current->flags &= ~PF_SWAPWRITE;
 
+	if (ret_succeeded)
+		*ret_succeeded = nr_succeeded;
+
 	return rc;
 }
 
@@ -1663,7 +1668,7 @@ static int do_move_pages_to_node(struct mm_struct *mm,
 	};
 
 	err = migrate_pages(pagelist, alloc_migration_target, NULL,
-			(unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL);
+		(unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL, NULL);
 	if (err)
 		putback_movable_pages(pagelist);
 	return err;
@@ -2178,7 +2183,7 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
 
 	list_add(&page->lru, &migratepages);
 	nr_remaining = migrate_pages(&migratepages, *new, NULL, node,
-				     MIGRATE_ASYNC, MR_NUMA_MISPLACED);
+				     MIGRATE_ASYNC, MR_NUMA_MISPLACED, NULL);
 	if (nr_remaining) {
 		if (!list_empty(&migratepages)) {
 			list_del(&page->lru);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cdb741399886..82ba531ac524 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8960,7 +8960,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 		cc->nr_migratepages -= nr_reclaimed;
 
 		ret = migrate_pages(&cc->migratepages, alloc_migration_target,
-				NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
+			NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE, NULL);
 
 		/*
 		 * On -ENOMEM, migrate_pages() bails out right away. It is pointless
-- 
2.30.2

[PATCH -V10 4/9] mm/migrate: demote pages during reclaim

[Huang Ying]

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

This is mostly derived from a patch from Yang Shi:

	https://lore.kernel.org/linux-mm/1560468577-101178-10-git-send-email-yang.shi@linux.alibaba.com/

Add code to the reclaim path (shrink_page_list()) to "demote" data
to another NUMA node instead of discarding the data.  This always
avoids the cost of I/O needed to read the page back in and sometimes
avoids the writeout cost when the page is dirty.

A second pass through shrink_page_list() will be made if any demotions
fail.  This essentially falls back to normal reclaim behavior in the
case that demotions fail.  Previous versions of this patch may have
simply failed to reclaim pages which were eligible for demotion but
were unable to be demoted in practice.

For some cases, for example, MADV_PAGEOUT, the pages are always
discarded instead of demoted to follow the kernel API definition.
Because MADV_PAGEOUT is defined as freeing specified pages regardless
in which tier they are.

Note: This just adds the start of infrastructure for migration. It is
actually disabled next to the FIXME in migrate_demote_page_ok().

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Wei Xu <weixugc@google.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>

--
changes from 20210625:
 * Move common demotion condition into can_demote_anon_pages() and
   call it out of loop in shrink_page_list() per Wei's comments.

changes from 20210618:
 * Fix some typos in comments and patch description.
 * Fix MADV_PAGEOUT behavior per Zi's comments.

changes from 20210122:
 * move from GFP_HIGHUSER -> GFP_HIGHUSER_MOVABLE (Ying)

changes from 202010:
 * add MR_NUMA_MISPLACED to trace MIGRATE_REASON define
 * make migrate_demote_page_ok() static, remove 'sc' arg until
   later patch
 * remove unnecessary alloc_demote_page() hugetlb warning
 * Simplify alloc_demote_page() gfp mask.  Depend on
   __GFP_NORETRY to make it lightweight instead of fancier
   stuff like leaving out __GFP_IO/FS.
 * Allocate migration page with alloc_migration_target()
   instead of allocating directly.
changes from 20200730:
 * Add another pass through shrink_page_list() when demotion
   fails.
changes from 20210302:
 * Use __GFP_THISNODE and revise the comment explaining the
   GFP mask constructionn
---
 include/linux/migrate.h        |  9 ++++
 include/trace/events/migrate.h |  3 +-
 mm/vmscan.c                    | 85 ++++++++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 1 deletion(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 8ab88d46318e..326250996b4e 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -28,6 +28,7 @@ enum migrate_reason {
 	MR_NUMA_MISPLACED,
 	MR_CONTIG_RANGE,
 	MR_LONGTERM_PIN,
+	MR_DEMOTION,
 	MR_TYPES
 };
 
@@ -167,6 +168,14 @@ struct migrate_vma {
 int migrate_vma_setup(struct migrate_vma *args);
 void migrate_vma_pages(struct migrate_vma *migrate);
 void migrate_vma_finalize(struct migrate_vma *migrate);
+int next_demotion_node(int node);
+
+#else /* CONFIG_MIGRATION disabled: */
+
+static inline int next_demotion_node(int node)
+{
+	return NUMA_NO_NODE;
+}
 
 #endif /* CONFIG_MIGRATION */
 
diff --git a/include/trace/events/migrate.h b/include/trace/events/migrate.h
index 9fb2a3bbcdfb..779f3fad9ecd 100644
--- a/include/trace/events/migrate.h
+++ b/include/trace/events/migrate.h
@@ -21,7 +21,8 @@
 	EM( MR_MEMPOLICY_MBIND,	"mempolicy_mbind")		\
 	EM( MR_NUMA_MISPLACED,	"numa_misplaced")		\
 	EM( MR_CONTIG_RANGE,	"contig_range")			\
-	EMe(MR_LONGTERM_PIN,	"longterm_pin")
+	EM( MR_LONGTERM_PIN,	"longterm_pin")			\
+	EMe(MR_DEMOTION,	"demotion")
 
 /*
  * First define the enums in the above macros to be exported to userspace
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4620df62f0ff..6ca6baa09c81 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -41,6 +41,7 @@
 #include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/memcontrol.h>
+#include <linux/migrate.h>
 #include <linux/delayacct.h>
 #include <linux/sysctl.h>
 #include <linux/oom.h>
@@ -118,6 +119,9 @@ struct scan_control {
 	/* The file pages on the current node are dangerously low */
 	unsigned int file_is_tiny:1;
 
+	/* Always discard instead of demoting to lower tier memory */
+	unsigned int no_demotion:1;
+
 	/* Allocation order */
 	s8 order;
 
@@ -515,6 +519,17 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 	return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
 }
 
+static bool can_demote_anon_pages(int nid, struct scan_control *sc)
+{
+	if (sc->no_demotion)
+		return false;
+	if (next_demotion_node(nid) == NUMA_NO_NODE)
+		return false;
+
+	// FIXME: actually enable this later in the series
+	return false;
+}
+
 /*
  * This misses isolated pages which are not accounted for to save counters.
  * As the data only determines if reclaim or compaction continues, it is
@@ -1261,6 +1276,49 @@ static void page_check_dirty_writeback(struct page *page,
 		mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
 }
 
+static struct page *alloc_demote_page(struct page *page, unsigned long node)
+{
+	struct migration_target_control mtc = {
+		/*
+		 * Allocate from 'node', or fail quickly and quietly.
+		 * When this happens, 'page' will likely just be discarded
+		 * instead of migrated.
+		 */
+		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
+			    __GFP_THISNODE  | __GFP_NOWARN |
+			    __GFP_NOMEMALLOC | GFP_NOWAIT,
+		.nid = node
+	};
+
+	return alloc_migration_target(page, (unsigned long)&mtc);
+}
+
+/*
+ * Take pages on @demote_list and attempt to demote them to
+ * another node.  Pages which are not demoted are left on
+ * @demote_pages.
+ */
+static unsigned int demote_page_list(struct list_head *demote_pages,
+				     struct pglist_data *pgdat)
+{
+	int target_nid = next_demotion_node(pgdat->node_id);
+	unsigned int nr_succeeded;
+	int err;
+
+	if (list_empty(demote_pages))
+		return 0;
+
+	if (target_nid == NUMA_NO_NODE)
+		return 0;
+
+	/* Demotion ignores all cpuset and mempolicy settings */
+	err = migrate_pages(demote_pages, alloc_demote_page, NULL,
+			    target_nid, MIGRATE_ASYNC, MR_DEMOTION,
+			    &nr_succeeded);
+
+	return nr_succeeded;
+}
+
 /*
  * shrink_page_list() returns the number of reclaimed pages
  */
@@ -1272,12 +1330,16 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 {
 	LIST_HEAD(ret_pages);
 	LIST_HEAD(free_pages);
+	LIST_HEAD(demote_pages);
 	unsigned int nr_reclaimed = 0;
 	unsigned int pgactivate = 0;
+	bool do_demote_pass;
 
 	memset(stat, 0, sizeof(*stat));
 	cond_resched();
+	do_demote_pass = can_demote_anon_pages(pgdat->node_id, sc);
 
+retry:
 	while (!list_empty(page_list)) {
 		struct address_space *mapping;
 		struct page *page;
@@ -1426,6 +1488,17 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 			; /* try to reclaim the page below */
 		}
 
+		/*
+		 * Before reclaiming the page, try to relocate
+		 * its contents to another node.
+		 */
+		if (do_demote_pass &&
+		    (thp_migration_supported() || !PageTransHuge(page))) {
+			list_add(&page->lru, &demote_pages);
+			unlock_page(page);
+			continue;
+		}
+
 		/*
 		 * Anonymous process memory has backing store?
 		 * Try to allocate it some swap space here.
@@ -1677,6 +1750,17 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		list_add(&page->lru, &ret_pages);
 		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
 	}
+	/* 'page_list' is always empty here */
+
+	/* Migrate pages selected for demotion */
+	nr_reclaimed += demote_page_list(&demote_pages, pgdat);
+	/* Pages that could not be demoted are still in @demote_pages */
+	if (!list_empty(&demote_pages)) {
+		/* Pages which failed to demoted go back on @page_list for retry: */
+		list_splice_init(&demote_pages, page_list);
+		do_demote_pass = false;
+		goto retry;
+	}
 
 	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
 
@@ -2324,6 +2408,7 @@ unsigned long reclaim_pages(struct list_head *page_list)
 		.may_writepage = 1,
 		.may_unmap = 1,
 		.may_swap = 1,
+		.no_demotion = 1,
 	};
 
 	noreclaim_flag = memalloc_noreclaim_save();
-- 
2.30.2

[PATCH -V10 5/9] mm/vmscan: add page demotion counter

[Huang Ying]

From: Yang Shi <yang.shi@linux.alibaba.com>

Account the number of demoted pages.

Add pgdemote_kswapd and pgdemote_direct VM counters showed in
/proc/vmstat.

[ daveh:
   - __count_vm_events() a bit, and made them look at the THP
     size directly rather than getting data from migrate_pages()
]

Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Wei Xu <weixugc@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: osalvador <osalvador@suse.de>

--

Changes since 202010:
 * remove unused scan-control 'demoted' field
---
 include/linux/vm_event_item.h | 2 ++
 mm/vmscan.c                   | 5 +++++
 mm/vmstat.c                   | 2 ++
 3 files changed, 9 insertions(+)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index ae0dd1948c2b..a185cc75ff52 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -33,6 +33,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		PGREUSE,
 		PGSTEAL_KSWAPD,
 		PGSTEAL_DIRECT,
+		PGDEMOTE_KSWAPD,
+		PGDEMOTE_DIRECT,
 		PGSCAN_KSWAPD,
 		PGSCAN_DIRECT,
 		PGSCAN_DIRECT_THROTTLE,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6ca6baa09c81..35913e35369d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1316,6 +1316,11 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
 			    target_nid, MIGRATE_ASYNC, MR_DEMOTION,
 			    &nr_succeeded);
 
+	if (current_is_kswapd())
+		__count_vm_events(PGDEMOTE_KSWAPD, nr_succeeded);
+	else
+		__count_vm_events(PGDEMOTE_DIRECT, nr_succeeded);
+
 	return nr_succeeded;
 }
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index b0534e068166..ec5a2e789dd2 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1217,6 +1217,8 @@ const char * const vmstat_text[] = {
 	"pgreuse",
 	"pgsteal_kswapd",
 	"pgsteal_direct",
+	"pgdemote_kswapd",
+	"pgdemote_direct",
 	"pgscan_kswapd",
 	"pgscan_direct",
 	"pgscan_direct_throttle",
-- 
2.30.2

[PATCH -V10 6/9] mm/vmscan: add helper for querying ability to age anonymous pages

[Huang Ying]

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

Anonymous pages are kept on their own LRU(s).  These lists could
theoretically always be scanned and maintained.  But, without swap,
there is currently nothing the kernel can *do* with the results of a
scanned, sorted LRU for anonymous pages.

A check for '!total_swap_pages' currently serves as a valid check as
to whether anonymous LRUs should be maintained.  However, another
method will be added shortly: page demotion.

Abstract out the 'total_swap_pages' checks into a helper, give it a
logically significant name, and check for the possibility of page
demotion.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Greg Thelen <gthelen@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: osalvador <osalvador@suse.de>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

Changes since 20210625:
 * Rename function to be more consistent with other similar functions.

Changes since 20210618:
 * Rename function per Oscar's comments.
 * Change parameter per Wei's comments.
 * Make the function static.
 * Consider whether demotion is disabled.
---
 mm/vmscan.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35913e35369d..302f8d6e7b65 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2724,6 +2724,21 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	}
 }
 
+/*
+ * Anonymous LRU management is a waste if there is
+ * ultimately no way to reclaim the memory.
+ */
+static bool can_age_anon_pages(struct pglist_data *pgdat,
+			       struct scan_control *sc)
+{
+	/* Aging the anon LRU is valuable if swap is present: */
+	if (total_swap_pages > 0)
+		return true;
+
+	/* Also valuable if anon pages can be demoted: */
+	return can_demote_anon_pages(pgdat->node_id, sc);
+}
+
 static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 {
 	unsigned long nr[NR_LRU_LISTS];
@@ -2833,7 +2848,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	 * Even if we did not try to evict anon pages at all, we want to
 	 * rebalance the anon lru active/inactive ratio.
 	 */
-	if (total_swap_pages && inactive_is_low(lruvec, LRU_INACTIVE_ANON))
+	if (can_age_anon_pages(lruvec_pgdat(lruvec), sc) &&
+	    inactive_is_low(lruvec, LRU_INACTIVE_ANON))
 		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
 				   sc, LRU_ACTIVE_ANON);
 }
@@ -3662,7 +3678,7 @@ static void age_active_anon(struct pglist_data *pgdat,
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 
-	if (!total_swap_pages)
+	if (!can_age_anon_pages(pgdat, sc))
 		return;
 
 	lruvec = mem_cgroup_lruvec(NULL, pgdat);
-- 
2.30.2

[PATCH -V10 7/9] mm/vmscan: Consider anonymous pages without swap

[Huang Ying]

From: Keith Busch <kbusch@kernel.org>

Reclaim anonymous pages if a migration path is available now that
demotion provides a non-swap recourse for reclaiming anon pages.

Note that this check is subtly different from the can_age_anon_pages()
checks.  This mechanism checks whether a specific page in a specific
context can actually be reclaimed, given current swap space and
cgroup limits.

can_age_anon_pages() is a much simpler and more preliminary check
which just says whether there is a possibility of future reclaim.

Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Wei Xu <weixugc@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

--

Changes since 20210618:
 * Consider whether demotion is disabled

Changes from Dave 202010:
 * remove 'total_swap_pages' modification

Changes from Dave 202006:
 * rename reclaim_anon_pages()->can_reclaim_anon_pages()

Note: Keith's Intel SoB is commented out because he is no
longer at Intel and his @intel.com mail will bounce.
---
 mm/vmscan.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 302f8d6e7b65..0f9be998230f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -521,7 +521,7 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 
 static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 {
-	if (sc->no_demotion)
+	if (sc && sc->no_demotion)
 		return false;
 	if (next_demotion_node(nid) == NUMA_NO_NODE)
 		return false;
@@ -530,6 +530,31 @@ static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 	return false;
 }
 
+static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
+					  int nid,
+					  struct scan_control *sc)
+{
+	if (memcg == NULL) {
+		/*
+		 * For non-memcg reclaim, is there
+		 * space in any swap device?
+		 */
+		if (get_nr_swap_pages() > 0)
+			return true;
+	} else {
+		/* Is the memcg below its swap limit? */
+		if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
+			return true;
+	}
+
+	/*
+	 * The page can not be swapped.
+	 *
+	 * Can it be reclaimed from this node via demotion?
+	 */
+	return can_demote_anon_pages(nid, sc);
+}
+
 /*
  * This misses isolated pages which are not accounted for to save counters.
  * As the data only determines if reclaim or compaction continues, it is
@@ -541,7 +566,7 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
 
 	nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
 		zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
-	if (get_nr_swap_pages() > 0)
+	if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
 		nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
 			zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
 
@@ -2539,6 +2564,7 @@ enum scan_balance {
 static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 			   unsigned long *nr)
 {
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	unsigned long anon_cost, file_cost, total_cost;
 	int swappiness = mem_cgroup_swappiness(memcg);
@@ -2549,7 +2575,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	enum lru_list lru;
 
 	/* If we have no swap space, do not bother scanning anon pages. */
-	if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
+	if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
 		scan_balance = SCAN_FILE;
 		goto out;
 	}
@@ -2919,7 +2945,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
 	 */
 	pages_for_compaction = compact_gap(sc->order);
 	inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
-	if (get_nr_swap_pages() > 0)
+	if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
 		inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
 
 	return inactive_lru_pages > pages_for_compaction;
-- 
2.30.2

[PATCH -V10 8/9] mm/vmscan: never demote for memcg reclaim

[Huang Ying]

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

Global reclaim aims to reduce the amount of memory used on
a given node or set of nodes.  Migrating pages to another
node serves this purpose.

memcg reclaim is different.  Its goal is to reduce the
total memory consumption of the entire memcg, across all
nodes.  Migration does not assist memcg reclaim because
it just moves page contents between nodes rather than
actually reducing memory consumption.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: Yang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: osalvador <osalvador@suse.de>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

--
changes from 20210618:
 * Avoid to scan anon lists for demotion for cgroup reclaim.
---
 mm/vmscan.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0f9be998230f..b697f1a6108c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -521,8 +521,13 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 
 static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 {
-	if (sc && sc->no_demotion)
-		return false;
+	if (sc) {
+		if (sc->no_demotion)
+			return false;
+		/* It is pointless to do demotion in memcg reclaim */
+		if (cgroup_reclaim(sc))
+			return false;
+	}
 	if (next_demotion_node(nid) == NUMA_NO_NODE)
 		return false;
 
-- 
2.30.2

[PATCH -V10 9/9] mm/migrate: add sysfs interface to enable reclaim migration

[Huang Ying]

Some method is obviously needed to enable reclaim-based migration.

Just like traditional autonuma, there will be some workloads that
will benefit like workloads with more "static" configurations where
hot pages stay hot and cold pages stay cold.  If pages come and go
from the hot and cold sets, the benefits of this approach will be
more limited.

The benefits are truly workload-based and *not* hardware-based.
We do not believe that there is a viable threshold where certain
hardware configurations should have this mechanism enabled while
others do not.

To be conservative, earlier work defaulted to disable reclaim-
based migration and did not include a mechanism to enable it.
This proposes add a new sysfs file

  /sys/kernel/mm/numa/demotion_enabled

as a method to enable it.

We are open to any alternative that allows end users to enable
this mechanism or disable it if workload harm is detected (just
like traditional autonuma).

Once this is enabled page demotion may move data to a NUMA node
that does not fall into the cpuset of the allocating process.
This could be construed to violate the guarantees of cpusets.
However, since this is an opt-in mechanism, the assumption is
that anyone enabling it is content to relax the guarantees.

Originally-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>

Changes since 20210618:
 * Guard next_demotion_node() with numa_demotion_enabled if necessary
   per Wei's comments.

Changes since 20210331:
 * Use sysfs interface separated from the zone_reclaim sysctl.

Changes since 20210304:
 * Add Documentation/ material about relaxing cpuset constraints

Changes since 20200122:
 * Changelog material about relaxing cpuset constraints
---
 .../ABI/testing/sysfs-kernel-mm-numa          | 24 ++++++++
 include/linux/mempolicy.h                     |  4 ++
 mm/mempolicy.c                                | 61 +++++++++++++++++++
 mm/vmscan.c                                   |  5 +-
 4 files changed, 92 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-numa

diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-numa b/Documentation/ABI/testing/sysfs-kernel-mm-numa
new file mode 100644
index 000000000000..77e559d4ed80
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-numa
@@ -0,0 +1,24 @@
+What:		/sys/kernel/mm/numa/
+Date:		June 2021
+Contact:	Linux memory management mailing list <linux-mm@kvack.org>
+Description:	Interface for NUMA
+
+What:		/sys/kernel/mm/numa/demotion_enabled
+Date:		June 2021
+Contact:	Linux memory management mailing list <linux-mm@kvack.org>
+Description:	Enable/disable demoting pages during reclaim
+
+		Page migration during reclaim is intended for systems
+		with tiered memory configurations.  These systems have
+		multiple types of memory with varied performance
+		characteristics instead of plain NUMA systems where
+		the same kind of memory is found at varied distances.
+		Allowing page migration during reclaim enables these
+		systems to migrate pages from fast tiers to slow tiers
+		when the fast tier is under pressure.  This migration
+		is performed before swap.  It may move data to a NUMA
+		node that does not fall into the cpuset of the
+		allocating process which might be construed to violate
+		the guarantees of cpusets.  This should not be enabled
+		on systems which need strict cpuset location
+		guarantees.
diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index 0aaf91b496e2..4ca025e2a77e 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -184,6 +184,8 @@ extern bool vma_migratable(struct vm_area_struct *vma);
 extern int mpol_misplaced(struct page *, struct vm_area_struct *, unsigned long);
 extern void mpol_put_task_policy(struct task_struct *);
 
+extern bool numa_demotion_enabled;
+
 #else
 
 struct mempolicy {};
@@ -292,5 +294,7 @@ static inline nodemask_t *policy_nodemask_current(gfp_t gfp)
 {
 	return NULL;
 }
+
+#define numa_demotion_enabled	false
 #endif /* CONFIG_NUMA */
 #endif
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 939eabcaf488..e675bfb856da 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -3021,3 +3021,64 @@ void mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
 		p += scnprintf(p, buffer + maxlen - p, ":%*pbl",
 			       nodemask_pr_args(&nodes));
 }
+
+bool numa_demotion_enabled = false;
+
+#ifdef CONFIG_SYSFS
+static ssize_t numa_demotion_enabled_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%s\n",
+			  numa_demotion_enabled? "true" : "false");
+}
+
+static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
+					   struct kobj_attribute *attr,
+					   const char *buf, size_t count)
+{
+	if (!strncmp(buf, "true", 4) || !strncmp(buf, "1", 1))
+		numa_demotion_enabled = true;
+	else if (!strncmp(buf, "false", 5) || !strncmp(buf, "0", 1))
+		numa_demotion_enabled = false;
+	else
+		return -EINVAL;
+
+	return count;
+}
+
+static struct kobj_attribute numa_demotion_enabled_attr =
+	__ATTR(demotion_enabled, 0644, numa_demotion_enabled_show,
+	       numa_demotion_enabled_store);
+
+static struct attribute *numa_attrs[] = {
+	&numa_demotion_enabled_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group numa_attr_group = {
+	.attrs = numa_attrs,
+};
+
+static int __init numa_init_sysfs(void)
+{
+	int err;
+	struct kobject *numa_kobj;
+
+	numa_kobj = kobject_create_and_add("numa", mm_kobj);
+	if (!numa_kobj) {
+		pr_err("failed to create numa kobject\n");
+		return -ENOMEM;
+	}
+	err = sysfs_create_group(numa_kobj, &numa_attr_group);
+	if (err) {
+		pr_err("failed to register numa group\n");
+		goto delete_obj;
+	}
+	return 0;
+
+delete_obj:
+	kobject_put(numa_kobj);
+	return err;
+}
+subsys_initcall(numa_init_sysfs);
+#endif
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b697f1a6108c..1afbbd7e853a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -521,6 +521,8 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 
 static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 {
+	if (!numa_demotion_enabled)
+		return false;
 	if (sc) {
 		if (sc->no_demotion)
 			return false;
@@ -531,8 +533,7 @@ static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 	if (next_demotion_node(nid) == NUMA_NO_NODE)
 		return false;
 
-	// FIXME: actually enable this later in the series
-	return false;
+	return true;
 }
 
 static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
-- 
2.30.2

Re: [PATCH -V10 1/9] mm/numa: automatically generate node migration order

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 5105 bytes --]

On 15 Jul 2021, at 1:51, Huang Ying wrote:

> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Prepare for the kernel to auto-migrate pages to other memory nodes
> with a node migration table. This allows creating single migration
> target for each NUMA node to enable the kernel to do NUMA page
> migrations instead of simply discarding colder pages. A node with no
> target is a "terminal node", so reclaim acts normally there.  The
> migration target does not fundamentally _need_ to be a single node,
> but this implementation starts there to limit complexity.
>
> When memory fills up on a node, memory contents can be
> automatically migrated to another node.  The biggest problems are
> knowing when to migrate and to where the migration should be
> targeted.
>
> The most straightforward way to generate the "to where" list would
> be to follow the page allocator fallback lists.  Those lists
> already tell us if memory is full where to look next.  It would
> also be logical to move memory in that order.
>
> But, the allocator fallback lists have a fatal flaw: most nodes
> appear in all the lists.  This would potentially lead to migration
> cycles (A->B, B->A, A->B, ...).
>
> Instead of using the allocator fallback lists directly, keep a
> separate node migration ordering.  But, reuse the same data used
> to generate page allocator fallback in the first place:
> find_next_best_node().
>
> This means that the firmware data used to populate node distances
> essentially dictates the ordering for now.  It should also be
> architecture-neutral since all NUMA architectures have a working
> find_next_best_node().
>
> RCU is used to allow lock-less read of node_demotion[] and prevent
> demotion cycles been observed.  If multiple reads of node_demotion[]
> are performed, a single rcu_read_lock() must be held over all reads to
> ensure no cycles are observed.  Details are as follows.
>
> === What does RCU provide? ===
>
> Imaginge a simple loop which walks down the demotion path looking

s/Imaginge/Imagine

> for the last node:
>
>         terminal_node = start_node;
>         while (node_demotion[terminal_node] != NUMA_NO_NODE) {
>                 terminal_node = node_demotion[terminal_node];
>         }
>
> The initial values are:
>
>         node_demotion[0] = 1;
>         node_demotion[1] = NUMA_NO_NODE;
>
> and are updated to:
>
>         node_demotion[0] = NUMA_NO_NODE;
>         node_demotion[1] = 0;
>
> What guarantees that the cycle is not observed:
>
>         node_demotion[0] = 1;
>         node_demotion[1] = 0;
>
> and would loop forever?
>
> With RCU, a rcu_read_lock/unlock() can be placed around the
> loop.  Since the write side does a synchronize_rcu(), the loop
> that observed the old contents is known to be complete before the
> synchronize_rcu() has completed.
>
> RCU, combined with disable_all_migrate_targets(), ensures that
> the old migration state is not visible by the time
> __set_migration_target_nodes() is called.
>
> === What does READ_ONCE() provide? ===
>
> READ_ONCE() forbids the compiler from merging or reordering
> successive reads of node_demotion[].  This ensures that any
> updates are *eventually* observed.
>
> Consider the above loop again.  The compiler could theoretically
> read the entirety of node_demotion[] into local storage
> (registers) and never go back to memory, and *permanently*
> observe bad values for node_demotion[].
>
> Note: RCU does not provide any universal compiler-ordering
> guarantees:
>
> 	https://lore.kernel.org/lkml/20150921204327.GH4029@linux.vnet.ibm.com/
>
> This code is unused for now.  It will be called later in the
> series.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
>
> --
>
> Changes from 20210618:
>  * Merge patches for data structure definition and initialization
>  * Move RCU usage from the next patch in series per Zi's comments
>
> Changes from 20210302:
>  * Fix typo in node_demotion[] comment
>
> Changes since 20200122:
>  * Make node_demotion[] __read_mostly
>  * Add big node_demotion[] comment
>
> Changes in July 2020:
>  - Remove loop from next_demotion_node() and get_online_mems().
>    This means that the node returned by next_demotion_node()
>    might now be offline, but the worst case is that the
>    allocation fails.  That's fine since it is transient.
> ---
>  mm/internal.h   |   5 ++
>  mm/migrate.c    | 216 ++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c |   2 +-
>  3 files changed, 222 insertions(+), 1 deletion(-)

LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>


—
Best Regards,
Yan, Zi

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Re: [PATCH -V10 2/9] mm/migrate: update node demotion order on hotplug events

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 1659 bytes --]

On 15 Jul 2021, at 1:51, Huang Ying wrote:

> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Reclaim-based migration is attempting to optimize data placement in
> memory based on the system topology.  If the system changes, so must
> the migration ordering.
>
> The implementation is conceptually simple and entirely unoptimized.
> On any memory or CPU hotplug events, assume that a node was added or
> removed and recalculate all migration targets.  This ensures that the
> node_demotion[] array is always ready to be used in case the new
> reclaim mode is enabled.
>
> This recalculation is far from optimal, most glaringly that it does
> not even attempt to figure out the hotplug event would have some
> *actual* effect on the demotion order.  But, given the expected
> paucity of hotplug events, this should be fine.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: osalvador <osalvador@suse.de>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
>
> --
>
> Changes since 20210618:
>  * moved RCU part to the prev patch in series.
>
> Changes since 20210302:
>  * remove duplicate synchronize_rcu()
> ---
>  mm/migrate.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 89 insertions(+), 1 deletion(-)

LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>


—
Best Regards,
Yan, Zi

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Re: [PATCH -V10 3/9] mm/migrate: enable returning precise migrate_pages() success count

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 2176 bytes --]

On 15 Jul 2021, at 1:51, Huang Ying wrote:

> From: Yang Shi <yang.shi@linux.alibaba.com>
>
> Under normal circumstances, migrate_pages() returns the number of
> pages migrated.  In error conditions, it returns an error code.  When
> returning an error code, there is no way to know how many pages were
> migrated or not migrated.
>
> Make migrate_pages() return how many pages are demoted successfully
> for all cases, including when encountering errors.  Page reclaim
> behavior will depend on this in subsequent patches.
>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Suggested-by: osalvador <osalvador@suse.de> [optional parameter]
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
>
> --
>
> Note: Yang Shi originally wrote the patch, thus the SoB.  There was
> also a Reviewed-by provided since there were some modifications made
> to this after the original work.
>
> Changes since 20210618:
>  * Reword some bits of the changelog/subject
>  * Allow callers to pass a NULL 'ret_succeeded'.  This ensures that
>    callers don't have to declare and pass a dummy variable.
>
> Changes since 20210302:
>  * Fix definition of CONFIG_MIGRATION=n stub migrate_pages().  Its
>    parameters were wrong, but oddly enough did not generate any
>    compile errors.
>
> Changes since 20200122:
>  * Fix migrate_pages() to manipulate nr_succeeded *value*
>    rather than the pointer.
> ---
>  include/linux/migrate.h |  5 +++--
>  mm/compaction.c         |  2 +-
>  mm/gup.c                |  2 +-
>  mm/memory-failure.c     |  2 +-
>  mm/memory_hotplug.c     |  2 +-
>  mm/mempolicy.c          |  4 ++--
>  mm/migrate.c            | 11 ++++++++---
>  mm/page_alloc.c         |  2 +-
>  8 files changed, 18 insertions(+), 12 deletions(-)
>
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

—
Best Regards,
Yan, Zi

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]

Re: [PATCH -V10 0/9] Migrate Pages in lieu of discard

[Andrew Morton]

On Thu, 15 Jul 2021 13:51:36 +0800 Huang Ying <ying.huang@intel.com> wrote:

> The full series is also available here:
> 
> 	https://github.com/hying-caritas/linux/tree/automigrate-20210715
> 
> The changes since the last post are as follows,
> 
>  * Reduce code duplication via move common demotion condition code
>    into can_demote_anon_pages() per Wei's comments.
> 
> --
> 
> 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.  Huang Ying has follow-on work which
> repurposes autonuma to promote hot pages 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:
> 
> 	http://lkml.kernel.org/r/20190124231441.37A4A305@viggo.jf.intel.com
> 
> We have tested the patchset with the postgresql and pgbench.  On a
> 2-socket server machine with DRAM and PMEM, the kernel with the
> patchset can improve the score of pgbench up to 22.1% compared with
> that of the DRAM only + disk case.  This comes from the reduced disk
> read throughput (which reduces up to 70.8%).

The [0/n] description talks a lot about PMEM, but the patches
themselves are all about NUMA nodes.  I assume that what ties this
together is that the PMEM tends to be organized as a NUMA node on its
own, and that by enabling migrate-to-remote-node-during-reclaim, we get
this PMEM behaviour as a desired side-effect?

IOW, perhaps this [0/n] description could explain the linkage between
PMEM and NUMA nodes more explicitly.


Secondly...  at some point it just won't make sense to migrate pages to
a remote node because that remote node isn't remote enough.  I mean, if
accessing the remote node is 2% slower than the local node, it's best
to just reclaim the page rather than going to the effort of migrating
it around?  And I assume the patchset doesn't (yet) attempt to make this
judgement?


Thirdly, the final patch which makes the feature off-by-default is a
concern.  I expect this will cause the new feature to have much less
testing (functional and performance).  We could make it default-on for
now, then flip that to default-off after 5.14-rc5 (for example).  That
will help, but is a bit lame.  Is it not possible for the kernel to
look at the overall system layout and make an educated guess as to
whether this system will benefit?

And I see this, from the [9/9] changelog:

: The benefits are truly workload-based and *not* hardware-based.  We do
: not believe that there is a viable threshold where certain hardware
: configurations should have this mechanism enabled while others do not.

so...  why not simply enable the thing for all systems and be done with
it?  What sort of downsides are you anticipating?  We could of course
include an emergency-off knob, but hoping that it won't need to be
used.

Finally, having a feature which is good for some workloads, bad for
others and which contains a system-wide enable knob is really quite
sad.  It requires a lot of work from each and every operator in the
world!  They must experimentally run their workloads both with and
without, and determine which is best.  And they should rerun this
testing periodically as userspace and the kernel evolve, to determine
whether their earlier experimenting holds true.  And what happens if
workload #1 wins and workload #2 loses?

And of course, many operators simply won't do all of this and they'll
run slower, or will miss out of benefits.  What can we do to relieve
our users of all of this?

Re: [PATCH -V10 0/9] Migrate Pages in lieu of discard

[Dave Hansen]

On 7/15/21 12:38 PM, Andrew Morton wrote:
> On Thu, 15 Jul 2021 13:51:36 +0800 Huang Ying <ying.huang@intel.com> wrote:
>> We have tested the patchset with the postgresql and pgbench.  On a
>> 2-socket server machine with DRAM and PMEM, the kernel with the
>> patchset can improve the score of pgbench up to 22.1% compared with
>> that of the DRAM only + disk case.  This comes from the reduced disk
>> read throughput (which reduces up to 70.8%).
> 
> The [0/n] description talks a lot about PMEM, but the patches
> themselves are all about NUMA nodes.  I assume that what ties this
> together is that the PMEM tends to be organized as a NUMA node on its
> own, and that by enabling migrate-to-remote-node-during-reclaim, we get
> this PMEM behaviour as a desired side-effect?

Yes, an assumption of this whole thing is that there are uniform
performance capabilities within a NUMA node.  ACPI systems that
implement the HMAT table require that each proximity domain (PXM) have
these uniform capabilities.  This works out great since PXMs are so
closely translated to NUMA nodes.

> IOW, perhaps this [0/n] description could explain the linkage between
> PMEM and NUMA nodes more explicitly.

Sure, we can do that.

> Secondly...  at some point it just won't make sense to migrate pages to
> a remote node because that remote node isn't remote enough.  I mean, if
> accessing the remote node is 2% slower than the local node, it's best
> to just reclaim the page rather than going to the effort of migrating
> it around?

One thing to keep in mind is that these migrations are *cheap*.  Because
of the point where the migration is done near the end of the reclaim
process the page should be unmapped.  The expensive part like TLB
shootdowns don't need to happen.

But, yes, you're right.  There are going to be situations where it would
have been better to reclaim the page.

> And I assume the patchset doesn't (yet) attempt to make this judgement?

Correct.  The assumption is that, on average, having something in slower
memory is better than having it out of memory.

> Thirdly, the final patch which makes the feature off-by-default is a
> concern.  I expect this will cause the new feature to have much less
> testing (functional and performance).  We could make it default-on for
> now, then flip that to default-off after 5.14-rc5 (for example).  That
> will help, but is a bit lame.  Is it not possible for the kernel to
> look at the overall system layout and make an educated guess as to
> whether this system will benefit?

Unfortunately, no.  It's highly workload dependent.  I see it a lot like
autonuma or khugepaged/THP: there are going to be some big upsides, but
we're under no illusions that the wins will be universal.

I'd actually be quite happy to flip it to be default-on all the time.
But, I think I'll be a lot more confident about it once we have the
"promotion" side of the equation merged as well:

	https://lwn.net/Articles/835402/

> And I see this, from the [9/9] changelog:
> 
> : The benefits are truly workload-based and *not* hardware-based.  We do
> : not believe that there is a viable threshold where certain hardware
> : configurations should have this mechanism enabled while others do not.
> 
> so...  why not simply enable the thing for all systems and be done with
> it?  What sort of downsides are you anticipating?  We could of course
> include an emergency-off knob, but hoping that it won't need to be
> used.

The worst case scenario is that an important workload starts up, eats a
bunch of RAM and then goes idle, maybe during off-peak workload hours.
During the night, a bunch of batch jobs kick in and evict some of that
workload's data to PMEM.  The workload spools up again the next morning
and is hampered by the slower PMEM when there's lots of DRAM available.

This will be helped by the autonuma-based promotion once it's in place.

> Finally, having a feature which is good for some workloads, bad for
> others and which contains a system-wide enable knob is really quite
> sad.  It requires a lot of work from each and every operator in the
> world!  They must experimentally run their workloads both with and
> without, and determine which is best.  And they should rerun this
> testing periodically as userspace and the kernel evolve, to determine
> whether their earlier experimenting holds true.  And what happens if
> workload #1 wins and workload #2 loses?
> 
> And of course, many operators simply won't do all of this and they'll
> run slower, or will miss out of benefits.  What can we do to relieve
> our users of all of this?

I think this is a much bigger question than this patch set.  It's
basically the tale of /proc/sys/vm/* or /sys/kernel/mm.  Some workloads
win with transparent_hugepage/enabled=always, some lose.  Some win with
zone_reclaim_mode=7, some prefer zone_reclaim_mode=0.

The end game for auto-migration is to on by default, wherever these
hardware configurations show up.  Just like THP, I expect this to be
broadly useful.  But, given the breadth of our user base, I also expect
it to be nasty for a minority of users.  They'll need a way to turn it
off, and hopefully they'll also provide feedback so that, if possible,
we can improve the default behavior and bring them back into the fold.

Re: [PATCH -V10 0/9] Migrate Pages in lieu of discard

[Matthew Wilcox]

On Thu, Jul 15, 2021 at 02:42:07PM -0700, Dave Hansen wrote:
> The end game for auto-migration is to on by default, wherever these
> hardware configurations show up.  Just like THP, I expect this to be
> broadly useful.  But, given the breadth of our user base, I also expect
> it to be nasty for a minority of users.  They'll need a way to turn it
> off, and hopefully they'll also provide feedback so that, if possible,
> we can improve the default behavior and bring them back into the fold.

... or if the THP experience is anything to go by, they continue to
shout loudly about how borken it is for a decade ;-(

Re: [PATCH -V10 0/9] Migrate Pages in lieu of discard

[Huang, Ying]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Thu, 15 Jul 2021 13:51:36 +0800 Huang Ying <ying.huang@intel.com> wrote:
>
> The [0/n] description talks a lot about PMEM, but the patches
> themselves are all about NUMA nodes.  I assume that what ties this
> together is that the PMEM tends to be organized as a NUMA node on its
> own, and that by enabling migrate-to-remote-node-during-reclaim, we get
> this PMEM behaviour as a desired side-effect?
>
> IOW, perhaps this [0/n] description could explain the linkage between
> PMEM and NUMA nodes more explicitly.

Hi, Andrew,

I have added some words in the [0/9] description to link PMEM and NUMA
nodes.  The updated description is as below.  Can you take a look at it?

Best Regards,
Huang, Ying

--------------------------8<-----------------------------------

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 patchset 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.  Huang Ying has follow-on work which
repurposes autonuma to promote hot pages 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:

	http://lkml.kernel.org/r/20190124231441.37A4A305@viggo.jf.intel.com

With that, the DRAM and PMEM in each socket will be represented as 2
separate NUMA nodes, with the CPUs sit in the DRAM node.  So the
general inter-NUMA demotion mechanism introduced in the patchset can
migrate the cold DRAM pages to the PMEM node.

We have tested the patchset with the postgresql and pgbench.  On a
2-socket server machine with DRAM and PMEM, the kernel with the
patchset can improve the score of pgbench up to 22.1% compared with
that of the DRAM only + disk case.  This comes from the reduced disk
read throughput (which reduces up to 70.8%).

== Open Issues ==

 * Memory policies and cpusets that, for instance, restrict allocations
   to DRAM can be demoted to PMEM whenever they opt in to this
   new mechanism.  A cgroup-level API to opt-in or opt-out of
   these migrations will likely be required as a follow-on.
 * Could be more aggressive about where anon LRU scanning occurs
   since it no longer necessarily involves I/O.  get_scan_count()
   for instance says: "If we have no swap space, do not bother
   scanning anon pages"

Re: [PATCH -V10 0/9] Migrate Pages in lieu of discard

[Andrew Morton]

On Fri, 16 Jul 2021 11:32:09 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Thu, 15 Jul 2021 13:51:36 +0800 Huang Ying <ying.huang@intel.com> wrote:
> >
> > The [0/n] description talks a lot about PMEM, but the patches
> > themselves are all about NUMA nodes.  I assume that what ties this
> > together is that the PMEM tends to be organized as a NUMA node on its
> > own, and that by enabling migrate-to-remote-node-during-reclaim, we get
> > this PMEM behaviour as a desired side-effect?
> >
> > IOW, perhaps this [0/n] description could explain the linkage between
> > PMEM and NUMA nodes more explicitly.
> 
> Hi, Andrew,
> 
> I have added some words in the [0/9] description to link PMEM and NUMA
> nodes.  The updated description is as below.  Can you take a look at it?
> 
> ...
>
> With that, the DRAM and PMEM in each socket will be represented as 2
> separate NUMA nodes, with the CPUs sit in the DRAM node.  So the
> general inter-NUMA demotion mechanism introduced in the patchset can
> migrate the cold DRAM pages to the PMEM node.
> 

Bingo, thanks.