[PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

[Zi Yan]

Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and
init_on_free=1 boot options") forces allocated page to be zeroed in
post_alloc_hook() when init_on_alloc=1.

For order-0 folios, if arch does not define
vma_alloc_zeroed_movable_folio(), the default implementation again zeros
the page return from the buddy allocator. So the page is zeroed twice.
Fix it by passing __GFP_ZERO instead to avoid double page zeroing.
At the moment, s390,arm64,x86,alpha,m68k are not impacted since they
define their own vma_alloc_zeroed_movable_folio().

For >0 order folios (mTHP and PMD THP), folio_zero_user() is called to
zero the folio again. Fix it by calling folio_zero_user() only if
init_on_alloc is set. All arch are impacted.

Added alloc_zeroed() helper to encapsulate the init_on_alloc check.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 include/linux/highmem.h | 8 +-------
 mm/huge_memory.c        | 3 ++-
 mm/internal.h           | 6 ++++++
 mm/memory.c             | 3 ++-
 4 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index bec9bd715acf..6e452bd8e7e3 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -224,13 +224,7 @@ static inline
 struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
 				   unsigned long vaddr)
 {
-	struct folio *folio;
-
-	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
-	if (folio)
-		clear_user_highpage(&folio->page, vaddr);
-
-	return folio;
+	return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr);
 }
 #endif
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 82f464865570..5dcbea96edb7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1176,7 +1176,8 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma,
 	}
 	folio_throttle_swaprate(folio, gfp);
 
-	folio_zero_user(folio, addr);
+	if (!alloc_zeroed())
+		folio_zero_user(folio, addr);
 	/*
 	 * The memory barrier inside __folio_mark_uptodate makes sure that
 	 * folio_zero_user writes become visible before the set_pmd_at()
diff --git a/mm/internal.h b/mm/internal.h
index 906da6280c2d..508f7802dd2b 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1233,6 +1233,12 @@ void touch_pud(struct vm_area_struct *vma, unsigned long addr,
 void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
 	       pmd_t *pmd, bool write);
 
+static inline bool alloc_zeroed(void)
+{
+	return static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
+			&init_on_alloc);
+}
+
 enum {
 	/* mark page accessed */
 	FOLL_TOUCH = 1 << 16,
diff --git a/mm/memory.c b/mm/memory.c
index c67359ddb61a..88252f0e06d0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4719,7 +4719,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
 				goto next;
 			}
 			folio_throttle_swaprate(folio, gfp);
-			folio_zero_user(folio, vmf->address);
+			if (!alloc_zeroed())
+				folio_zero_user(folio, vmf->address);
 			return folio;
 		}
 next:
-- 
2.45.2