[PATCH RFC 3/9] mm: add __GFP_PREZEROED flag and folio_test_clear_prezeroed()

Michael S. Tsirkin
2026-04-12T22:50:58Z

The previous patch skips zeroing in post_alloc_hook() when
__GFP_ZERO is used. However, several page allocation paths
zero pages via folio_zero_user() or clear_user_highpage() after
allocation, not via __GFP_ZERO.

Add __GFP_PREZEROED gfp flag that tells post_alloc_hook() to
preserve the MAGIC_PAGE_ZEROED sentinel in page->private so the
caller can detect pre-zeroed pages and skip its own zeroing.
Add folio_test_clear_prezeroed() helper to check and clear
the sentinel.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
---
include/linux/gfp_types.h | 5 +++++
include/linux/mm.h | 16 ++++++++++++++++
mm/page_alloc.c | 8 +++++++-
3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 6c75df30a281..903f87c7fec9 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -56,6 +56,7 @@ enum {
___GFP_NOLOCKDEP_BIT,
#endif
___GFP_NO_OBJ_EXT_BIT,
+ ___GFP_PREZEROED_BIT,
___GFP_LAST_BIT
};

@@ -97,6 +98,7 @@ enum {
#define ___GFP_NOLOCKDEP 0
#endif
#define ___GFP_NO_OBJ_EXT BIT(___GFP_NO_OBJ_EXT_BIT)
+#define ___GFP_PREZEROED BIT(___GFP_PREZEROED_BIT)

/*
* Physical address zone modifiers (see linux/mmzone.h - low four bits)
@@ -292,6 +294,9 @@ enum {
#define __GFP_SKIP_ZERO ((__force gfp_t)___GFP_SKIP_ZERO)
#define __GFP_SKIP_KASAN ((__force gfp_t)___GFP_SKIP_KASAN)

+/* Caller handles pre-zeroed pages; preserve MAGIC_PAGE_ZEROED in private */
+#define __GFP_PREZEROED ((__force gfp_t)___GFP_PREZEROED)
+
/* Disable lockdep for GFP context tracking */
#define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 59fc77c4c90e..caa1de31bbca 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4820,6 +4820,22 @@ static inline bool user_alloc_needs_zeroing(void)
*/
#define MAGIC_PAGE_ZEROED 0x5A45524FU /* ZERO */

+/**
+ * folio_test_clear_prezeroed - test and clear the pre-zeroed marker.
+ * @folio: the folio to test.
+ *
+ * Returns true if the folio was pre-zeroed by the host, and clears
+ * the marker. Callers can skip their own zeroing.
+ */
+static inline bool folio_test_clear_prezeroed(struct folio *folio)
+{
+ if (page_private(&folio->page) == MAGIC_PAGE_ZEROED) {
+ set_page_private(&folio->page, 0);
+ return true;
+ }
+ return false;
+}
+
int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status);
int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status);
int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index efb65eee826b..fba8321c45ed 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1867,7 +1867,13 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
bool zero_tags = init && (gfp_flags & __GFP_ZEROTAGS);
int i;

- set_page_private(page, 0);
+ /*
+ * If the page is pre-zeroed and the caller opted in via
+ * __GFP_PREZEROED, preserve the marker so the caller can
+ * skip its own zeroing. Otherwise always clear private.
+ */
+ if (!(prezeroed && (gfp_flags & __GFP_PREZEROED)))
+ set_page_private(page, 0);

/*
* If the page is pre-zeroed, skip memory initialization.
--
MST