[PATCH v5 32/36] mm/khugepaged: base private node collapse eligiblity on actor/cap bits

[Gregory Price <gourry@gourry.net>]

Collapse can move base pages across nodes.  The khugepaged daemon does
this transparently, while MADV_COLLAPSE is a userland construct.

Isolate private nodes from khugepaged exactly like ZONE_DEVICE.
Never collapse private node folios silently (the owner may not
support migration).

MADV_COLLAPSE is user-initiated, so it is allowed on a private node
opted into CAP_USER_NUMA, consistent with other userspace interfaces.

folio_allows_collapse() encodes this: false for khugepaged on a private
node, CAP_USER_NUMA for MADV_COLLAPSE.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
 mm/internal.h   | 26 ++++++++++++++++++++++++++
 mm/khugepaged.c | 16 +++++++++++++---
 2 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 9621eb127c28c..9dbc5752cd3fe 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -136,6 +136,32 @@ static inline bool folio_allows_numa_balance(struct folio *folio)
 	       node_allows_numa_balancing(folio_nid(folio));
 }
 
+/*
+ * folio_allows_collapse() - may collapse fold this folio into a THP?
+ * @is_khugepaged: true for the khugepaged, false for MADV_COLLAPSE.
+ *
+ * Collapse is a residency operation gated by the actor calling it:
+ *   - khugepaged never operates on private-node folios (like ZONE_DEVICE)
+ *   - MADV_COLLAPSE is gated by CAP_USER_NUMA
+ *
+ * Never true for ZONE_DEVICE.
+ */
+static inline bool folio_allows_collapse(struct folio *folio, bool is_khugepaged)
+{
+	int nid = folio_nid(folio);
+
+	if (folio_is_zone_device(folio))
+		return false;
+	if (is_khugepaged)
+		return !node_is_private(nid);
+	return node_allows_user_numa(nid);
+}
+
+static inline bool page_allows_collapse(struct page *page, bool is_khugepaged)
+{
+	return folio_allows_collapse(page_folio(page), is_khugepaged);
+}
+
 /*
  * folio_allows_longterm_pin() - may this folio be long-term GUP-pinned?
  *
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fb4378cc17b10..5f20839857738 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -700,7 +700,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			goto out;
 		}
 		page = vm_normal_page(vma, addr, pteval);
-		if (unlikely(!page) || unlikely(page_is_private_managed(page))) {
+		if (unlikely(!page) ||
+		    unlikely(!page_allows_collapse(page, cc->is_khugepaged))) {
 			result = SCAN_PAGE_NULL;
 			goto out;
 		}
@@ -1241,9 +1242,17 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
 		     GFP_TRANSHUGE);
 	int node = collapse_find_target_node(cc);
+	unsigned int aflags;
 	struct folio *folio;
+	bool allow;
 
-	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask, ALLOC_DEFAULT);
+	/* Private node access: khugepaged never, madvise with CAP_USER_NUMA */
+	allow = cc->is_khugepaged ? !node_is_private(node)
+				  : node_allows_user_numa(node);
+	aflags = (allow && node_is_private(node)) ?
+		ALLOC_ZONELIST_PRIVATE : ALLOC_DEFAULT;
+
+	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask, aflags);
 	if (!folio) {
 		*foliop = NULL;
 		if (is_pmd_order(order))
@@ -1687,7 +1696,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		}
 
 		page = vm_normal_page(vma, addr, pteval);
-		if (unlikely(!page) || unlikely(page_is_private_managed(page))) {
+		if (unlikely(!page) ||
+		    unlikely(!page_allows_collapse(page, cc->is_khugepaged))) {
 			result = SCAN_PAGE_NULL;
 			goto out_unmap;
 		}
-- 
2.53.0-Meta