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SubjectRe: [RFC PATCH v4 7/7] mm/demotion: Demote pages according to allocation fallback order
Date
Ying Huang <ying.huang@intel.com> writes:

.....

> > > >
>> > > > https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@intel.com/
>> > > >
>> > > > 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@mail.gmail.com/
>
> 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);

if (current_is_kswapd())
__count_vm_events(PGDEMOTE_KSWAPD, nr_succeeded);
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