[RFC PATCH] mm: avoid clearing 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 cleared in
post_alloc_hook() when init_on_alloc=1.

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

For PMD folios, folio_zero_user() is called to clear the folio again.
Fix it by calling folio_zero_user() only if init_on_alloc is set.
All arch are impacted.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 include/linux/highmem.h | 14 ++------------
 mm/huge_memory.c        |  4 +++-
 2 files changed, 5 insertions(+), 13 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 930a591b9b61..4b15224842e1 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -220,18 +220,8 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
  * Return: A folio containing one allocated and zeroed page or NULL if
  * we are out of memory.
  */
-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, false);
-	if (folio)
-		clear_user_highpage(&folio->page, vaddr);
-
-	return folio;
-}
+#define vma_alloc_zeroed_movable_folio(vma, vaddr) \
+	vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr, false)
 #endif
 
 static inline void clear_highpage(struct page *page)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a7b05f4c2a5e..ff746151896f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1177,7 +1177,9 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 		goto release;
 	}
 
-	folio_zero_user(folio, vmf->address);
+	if (!static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
+				&init_on_alloc))
+		folio_zero_user(folio, vmf->address);
 	/*
 	 * The memory barrier inside __folio_mark_uptodate makes sure that
 	 * folio_zero_user writes become visible before the set_pmd_at()
-- 
2.45.2