[PATCH v5 18/36] mm/gup: disallow longterm pin of private node folios

[Gregory Price]

FOLL_LONGTERM on a N_MEMORY_PRIVATE node folio is wrong both ways the
GUP longterm path resolves it:

   a ZONE_NORMAL folio would be pinned in place
   a ZONE_MOVABLE folio would be migrated off first

A private node folio should do neither - the pin should fail in place.

Add folio_longterm_pin_forbidden() (true for any private node folio) and
reject such a pin in check_and_migrate_movable_pages_or_folios() before
any isolation, so nothing is migrated.

The gup-fast path defers a private folio to the slow path via
folio_allows_longterm_pin().

Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
 mm/gup.c      | 27 +++++++++++++++++++++++++--
 mm/internal.h | 25 +++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 1d32e9a3dc79c..a7d4de223785c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -547,10 +547,10 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
 	/*
 	 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
 	 * right zone, so fail and let the caller fall back to the slow
-	 * path.
+	 * path.  Fail for private-node folios here so slow path rejects.
 	 */
 	if (unlikely((flags & FOLL_LONGTERM) &&
-		     !folio_is_longterm_pinnable(folio))) {
+		     !folio_allows_longterm_pin(folio))) {
 		folio_put_refs(folio, refs);
 		return NULL;
 	}
@@ -2389,12 +2389,35 @@ migrate_longterm_unpinnable_folios(struct list_head *movable_folio_list,
 	return ret;
 }
 
+/*
+ * True if any folio sits on a private node whose folios may not be longterm
+ * pinned.  Such folios can neither be pinned nor migrated, so the whole pin
+ * must fail before migration.  Checked before any isolation occurs.
+ */
+static bool pofs_has_ltpin_forbidden(struct pages_or_folios *pofs)
+{
+	struct folio *folio;
+	long i = 0;
+
+	for (folio = pofs_get_folio(pofs, i); folio;
+	     folio = pofs_next_folio(folio, pofs, &i)) {
+		if (folio_longterm_pin_forbidden(folio))
+			return true;
+	}
+	return false;
+}
+
 static long
 check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
 {
 	LIST_HEAD(movable_folio_list);
 	unsigned long collected;
 
+	if (pofs_has_ltpin_forbidden(pofs)) {
+		pofs_unpin(pofs);
+		return -EFAULT;
+	}
+
 	collected = collect_longterm_unpinnable_folios(&movable_folio_list,
 						       pofs);
 	if (!collected)
diff --git a/mm/internal.h b/mm/internal.h
index cb9f4a8342e32..85c460296cea1 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -110,6 +110,31 @@ static inline bool page_is_private_managed(struct page *page)
 	return folio_is_private_managed(page_folio(page));
 }
 
+/*
+ * folio_allows_longterm_pin() - may this folio be long-term GUP-pinned?
+ *
+ * checks folio_is_longterm_pinnable() rules plus private node permissions.
+ * private node permission is checked here to resolve circular header
+ * dependencies in folio_is_longterm_pinnable.
+ */
+static inline bool folio_allows_longterm_pin(struct folio *folio)
+{
+	return folio_is_longterm_pinnable(folio) &&
+	       !folio_is_private_node(folio);
+}
+
+/*
+ * folio_longterm_pin_forbidden() - must a longterm pin of this folio fail
+ * outright (neither pinned in place nor migrated off the node)?
+ *
+ * True for any folio on a private node: such memory can be neither pinned
+ * nor migrated, so the pin must be rejected with the folio left in place.
+ */
+static inline bool folio_longterm_pin_forbidden(struct folio *folio)
+{
+	return folio_is_private_node(folio);
+}
+
 /*
  * Maintains state across a page table move. The operation assumes both source
  * and destination VMAs already exist and are specified by the user.
-- 
2.53.0-Meta