[RFC PATCH v4 20/27] mm/gup: NP_OPS_LONGTERM_PIN - private node longterm pin support

[Gregory Price]

Private node folios should not be longterm-pinnable by default.
A pinned folio is frozen in place, no migration, compaction, or
reclaim, so the service loses control for the duration of the pin.

Some services may depend on hot-unplugability and must disallow
longterm pinning.  Others (accelerators with shared CPU-device state)
need pinning to work.

Add NP_OPS_LONGTERM_PIN flag for services to opt in with. Hook into
folio_is_longterm_pinnable() in mm.h, which all GUP callers
out-of-line helper, node_private_allows_longterm_pin(),  called
only for N_MEMORY_PRIVATE nodes.

Without the flag: folio_is_longterm_pinnable() returns false, migration
fails (no __GFP_PRIVATE in GFP mask) and pin_user_pages(FOLL_LONGTERM)
returns -ENOMEM.

With the flag: pin succeeds and the folio stays on the private node.

Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
 drivers/base/node.c          | 15 +++++++++++++++
 include/linux/mm.h           | 22 ++++++++++++++++++++++
 include/linux/node_private.h |  2 ++
 3 files changed, 39 insertions(+)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index da523aca18fa..5d2487fd54f4 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -866,6 +866,21 @@ void register_memory_blocks_under_node_hotplug(int nid, unsigned long start_pfn,
 static DEFINE_MUTEX(node_private_lock);
 static bool node_private_initialized;
 
+/**
+ * node_private_allows_longterm_pin - Check if a private node allows longterm pinning
+ * @nid: Node identifier
+ *
+ * Out-of-line helper for folio_is_longterm_pinnable() since mm.h cannot
+ * include node_private.h (circular dependency).
+ *
+ * Returns true if the node has NP_OPS_LONGTERM_PIN set.
+ */
+bool node_private_allows_longterm_pin(int nid)
+{
+	return node_private_has_flag(nid, NP_OPS_LONGTERM_PIN);
+}
+EXPORT_SYMBOL_GPL(node_private_allows_longterm_pin);
+
 /**
  * node_private_register - Register a private node
  * @nid: Node identifier
diff --git a/include/linux/mm.h b/include/linux/mm.h
index fb1819ad42c3..9088fd08aeb9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2192,6 +2192,13 @@ static inline bool is_zero_folio(const struct folio *folio)
 
 /* MIGRATE_CMA and ZONE_MOVABLE do not allow pin folios */
 #ifdef CONFIG_MIGRATION
+
+#ifdef CONFIG_NUMA
+bool node_private_allows_longterm_pin(int nid);
+#else
+static inline bool node_private_allows_longterm_pin(int nid) { return false; }
+#endif
+
 static inline bool folio_is_longterm_pinnable(struct folio *folio)
 {
 #ifdef CONFIG_CMA
@@ -2215,6 +2222,21 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
 	if (folio_is_fsdax(folio))
 		return false;
 
+	/*
+	 * Private node folios are not longterm pinnable by default.
+	 * Services that support pinning opt in via NP_OPS_LONGTERM_PIN.
+	 * node_private_allows_longterm_pin() is out-of-line because
+	 * node_private.h includes mm.h (circular dependency).
+	 *
+	 * Guarded by CONFIG_NUMA because on !CONFIG_NUMA the single-node
+	 * node_state() stub returns true for node 0, which would make
+	 * all folios non-pinnable via the false-returning stub.
+	 */
+#ifdef CONFIG_NUMA
+	if (node_state(folio_nid(folio), N_MEMORY_PRIVATE))
+		return node_private_allows_longterm_pin(folio_nid(folio));
+#endif
+
 	/* Otherwise, non-movable zone folios can be pinned. */
 	return !folio_is_zone_movable(folio);
 
diff --git a/include/linux/node_private.h b/include/linux/node_private.h
index fe0336773ddb..7a7438fb9eda 100644
--- a/include/linux/node_private.h
+++ b/include/linux/node_private.h
@@ -144,6 +144,8 @@ struct node_private_ops {
 #define NP_OPS_NUMA_BALANCING		BIT(5)
 /* Allow compaction to run on the node.  Service must start kcompactd. */
 #define NP_OPS_COMPACTION		BIT(6)
+/* Allow longterm DMA pinning (RDMA, VFIO, etc.) of folios on this node */
+#define NP_OPS_LONGTERM_PIN		BIT(7)
 
 /* Private node is OOM-eligible: reclaim can run and pages can be demoted here */
 #define NP_OPS_OOM_ELIGIBLE		(NP_OPS_RECLAIM | NP_OPS_DEMOTION)
-- 
2.53.0