[PATCH v10 10/37] mm: add folio_zero_user stub for configs without THP/HUGETLBFS

[Michael S. Tsirkin]

folio_zero_user() is defined in mm/memory.c under
CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS.  A subsequent patch
will call it from post_alloc_hook() for all user page zeroing, so
configs without THP or HUGETLBFS will need a stub.

Add a stub that uses clear_user_highpages() with aligned
addr_hint.

Without THP/HUGETLBFS, only order-0 user pages are allocated, so
the locality optimization in the real folio_zero_user() (zero near
the faulting address last) is not needed.
This also matches what vma_alloc_zeroed_movable_folio currently does.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh
---
 mm/folio_zero.h | 18 ++++++++++++++++++
 mm/page_alloc.c |  1 +
 2 files changed, 19 insertions(+)
 create mode 100644 mm/folio_zero.h

diff --git a/mm/folio_zero.h b/mm/folio_zero.h
new file mode 100644
index 000000000000..c135b3a34da8
--- /dev/null
+++ b/mm/folio_zero.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef MM_FOLIO_ZERO_H
+#define MM_FOLIO_ZERO_H
+
+#include <linux/highmem.h>
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
+void folio_zero_user(struct folio *folio, unsigned long addr_hint);
+#else
+static inline void folio_zero_user(struct folio *folio, unsigned long addr_hint)
+{
+	unsigned long base = ALIGN_DOWN(addr_hint, folio_size(folio));
+
+	clear_user_highpages(&folio->page, base, folio_nr_pages(folio));
+}
+#endif
+
+#endif /* MM_FOLIO_ZERO_H */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6d3f284c607d..0943ab724032 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -17,6 +17,7 @@
 #include <linux/stddef.h>
 #include <linux/mm.h>
 #include <linux/highmem.h>
+#include "folio_zero.h"
 #include <linux/interrupt.h>
 #include <linux/jiffies.h>
 #include <linux/compiler.h>
-- 
MST