Re: [RFC PATCH v4 7/7] mm/demotion: Demote pages according to allocation fallback order

From: Aneesh Kumar K V
Date: Mon Jun 06 2022 - 04:04:34 EST

On 6/6/22 1:12 PM, Ying Huang wrote:
On Mon, 2022-06-06 at 11:51 +0530, Aneesh Kumar K.V wrote:
Ying Huang <ying.huang@xxxxxxxxx> writes:

.....


https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@xxxxxxxxx/

That is, something like below,

static struct page *alloc_demote_page(struct page *page, unsigned long node)
{
struct page *page;
nodemask_t allowed_mask;
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
};

page = alloc_migration_target(page, (unsigned long)&mtc);
if (page)
return page;

mtc.gfp_mask &= ~__GFP_THISNODE;
mtc.nmask = &allowed_mask;

return alloc_migration_target(page, (unsigned long)&mtc);
}

I skipped doing this in v5 because I was not sure this is really what we
want.

I think so. And this is the original behavior. We should keep the
original behavior as much as possible, then make changes if necessary.


That is the reason I split the new page allocation as a separate patch.
Previous discussion on this topic didn't conclude on whether we really
need to do the above or not
https://lore.kernel.org/lkml/CAAPL-u9endrWf_aOnPENDPdvT-2-YhCAeJ7ONGckGnXErTLOfQ@xxxxxxxxxxxxxx/

Please check the later email in the thread you referenced. Both Wei and
me agree that the use case needs to be supported. We just didn't reach
concensus about how to implement it. If you think Wei's solution is
better (referenced as below), you can try to do that too. Although I
think my proposed implementation is much simpler.

How about the below details

diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
index 79bd8d26feb2..cd6e71f702ad 100644
--- a/include/linux/memory-tiers.h
+++ b/include/linux/memory-tiers.h
@@ -21,6 +21,7 @@ void node_remove_from_memory_tier(int node);
 int node_get_memory_tier_id(int node);
 int node_set_memory_tier(int node, int tier);
 int node_reset_memory_tier(int node, int tier);
+void node_get_allowed_targets(int node, nodemask_t *targets);
 #else
 #define numa_demotion_enabled false
 static inline int next_demotion_node(int node)
@@ -28,6 +29,10 @@ static inline int next_demotion_node(int node)
  return NUMA_NO_NODE;
 }



+static inline void node_get_allowed_targets(int node, nodemask_t *targets)
+{
+ *targets = NODE_MASK_NONE;
+}
 #endif /* CONFIG_TIERED_MEMORY */



 #endif
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index b4e72b672d4d..592d939ec28d 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -18,6 +18,7 @@ struct memory_tier {



 struct demotion_nodes {
  nodemask_t preferred;
+ nodemask_t allowed;
 };



 #define to_memory_tier(device) container_of(device, struct memory_tier, dev)
@@ -378,6 +379,25 @@ int node_set_memory_tier(int node, int tier)
 }
 EXPORT_SYMBOL_GPL(node_set_memory_tier);



+void node_get_allowed_targets(int node, nodemask_t *targets)
+{
+ /*
+ * node_demotion[] is updated without excluding this
+ * function from running.
+ *
+ * If any node is moving to lower tiers then modifications
+ * in node_demotion[] are still valid for this node, if any
+ * node is moving to higher tier then moving node may be
+ * used once for demotion which should be ok so rcu should
+ * be enough here.
+ */
+ rcu_read_lock();
+
+ *targets = node_demotion[node].allowed;
+
+ rcu_read_unlock();
+}
+
 /**
  * next_demotion_node() - Get the next node in the demotion path
  * @node: The starting node to lookup the next node
@@ -437,8 +457,10 @@ static void __disable_all_migrate_targets(void)
 {
  int node;



- for_each_node_mask(node, node_states[N_MEMORY])
+ for_each_node_mask(node, node_states[N_MEMORY]) {
  node_demotion[node].preferred = NODE_MASK_NONE;
+ node_demotion[node].allowed = NODE_MASK_NONE;
+ }
 }



 static void disable_all_migrate_targets(void)
@@ -465,7 +487,7 @@ static void establish_migration_targets(void)
  struct demotion_nodes *nd;
  int target = NUMA_NO_NODE, node;
  int distance, best_distance;
- nodemask_t used;
+ nodemask_t used, allowed = NODE_MASK_NONE;



  if (!node_demotion)
  return;
@@ -511,6 +533,29 @@ static void establish_migration_targets(void)
  }
  } while (1);
  }
+ /*
+ * Now build the allowed mask for each node collecting node mask from
+ * all memory tier below it. This allows us to fallback demotion page
+ * allocation to a set of nodes that is closer the above selected
+ * perferred node.
+ */
+ list_for_each_entry(memtier, &memory_tiers, list)
+ nodes_or(allowed, allowed, memtier->nodelist);
+ /*
+ * Removes nodes not yet in N_MEMORY.
+ */
+ nodes_and(allowed, node_states[N_MEMORY], allowed);
+
+ list_for_each_entry(memtier, &memory_tiers, list) {
+ /*
+ * Keep removing current tier from allowed nodes,
+ * This will remove all nodes in current and above
+ * memory tier from the allowed mask.
+ */
+ nodes_andnot(allowed, allowed, memtier->nodelist);
+ for_each_node_mask(node, memtier->nodelist)
+ node_demotion[node].allowed = allowed;
+ }
 }



 /*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3a8f78277f99..b0792d838efb 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1460,19 +1460,32 @@ static void folio_check_dirty_writeback(struct folio *folio,
  mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
 }



-static struct page *alloc_demote_page(struct page *page, unsigned long node)
+static struct page *alloc_demote_page(struct page *page, unsigned long private)
 {
- 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
- };
+ struct page *target_page;
+ nodemask_t *allowed_mask;
+ struct migration_target_control *mtc;
+
+ mtc = (struct migration_target_control *)private;
+
+ allowed_mask = mtc->nmask;
+ /*
+ * make sure we allocate from the target node first also trying to
+ * reclaim pages from the target node via kswapd if we are low on
+ * free memory on target node. If we don't do this and if we have low
+ * free memory on the target memtier, we would start allocating pages
+ * from higher memory tiers without even forcing a demotion of cold
+ * pages from the target memtier. This can result in the kernel placing
+ * hotpages in higher memory tiers.
+ */
+ mtc->nmask = NULL;
+ mtc->gfp_mask |= __GFP_THISNODE;
+ target_page = alloc_migration_target(page, (unsigned long)&mtc);
+ if (target_page)
+ return target_page;
+
+ mtc->gfp_mask &= ~__GFP_THISNODE;
+ mtc->nmask = allowed_mask;



  return alloc_migration_target(page, (unsigned long)&mtc);
 }
@@ -1487,6 +1500,19 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
 {
  int target_nid = next_demotion_node(pgdat->node_id);
  unsigned int nr_succeeded;
+ nodemask_t allowed_mask;
+
+ 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_NOWARN |
+ __GFP_NOMEMALLOC | GFP_NOWAIT,
+ .nid = target_nid,
+ .nmask = &allowed_mask
+ };



  if (list_empty(demote_pages))
  return 0;
@@ -1494,10 +1520,12 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
  if (target_nid == NUMA_NO_NODE)
  return 0;



+ node_get_allowed_targets(pgdat->node_id, &allowed_mask);
+
  /* Demotion ignores all cpuset and mempolicy settings */
  migrate_pages(demote_pages, alloc_demote_page, NULL,
- target_nid, MIGRATE_ASYNC, MR_DEMOTION,
- &nr_succeeded);
+ (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
+ &nr_succeeded);

Firstly, it addressed my requirement, Thanks! And, I'd prefer to put
mtc definition in alloc_demote_page(). Because that makes all page
allocation logic in one function. Thus the readability of code is
better.

The challenge is in allowed_mask computation. That is based on the src_node and not target_node.

-aneesh