[PATCH] kasan: Fix warnings caused by use of arch_enter_lazy_mmu_mode()

[PATCH] kasan: Fix warnings caused by use of arch_enter_lazy_mmu_mode()

[Balbir Singh]

commit c519c3c0a113 ("mm/kasan: avoid lazy MMU mode hazards") introduced
the use of arch_enter_lazy_mmu_mode(), which results in the compiler
complaining about "statement has no effect", when
__HAVE_ARCH_LAZY_MMU_MODE is not defined in include/linux/pgtable.h

The exact warning/error is:

In file included from ./include/linux/kasan.h:37,
                 from mm/kasan/shadow.c:14:
mm/kasan/shadow.c: In function ‘kasan_populate_vmalloc_pte’:
./include/linux/pgtable.h:247:41: error: statement with no effect [-Werror=unused-value]
  247 | #define arch_enter_lazy_mmu_mode()      (LAZY_MMU_DEFAULT)
      |                                         ^
mm/kasan/shadow.c:322:9: note: in expansion of macro ‘arch_enter_lazy_mmu_mode’
  322 |         arch_enter_lazy_mmu_mode();
      |         ^~~~~~~~~~~~~~~~~~~~~~~~

Fix the issue by explicitly casting the use of the function to void,
since the returned state is not forwarded/retained

Fixes: c519c3c0a113 ("mm/kasan: avoid lazy MMU mode hazards")
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
---
 mm/kasan/shadow.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index 60b1b72f5ce1..347e02a70892 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -319,7 +319,7 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	}
 	spin_unlock(&init_mm.page_table_lock);
 
-	arch_enter_lazy_mmu_mode();
+	(void)arch_enter_lazy_mmu_mode();
 
 	return 0;
 }
@@ -494,7 +494,7 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	if (likely(!none))
 		__free_page(pfn_to_page(pte_pfn(pte)));
 
-	arch_enter_lazy_mmu_mode();
+	(void)arch_enter_lazy_mmu_mode();
 
 	return 0;
 }
-- 
2.50.1

Re: [PATCH] kasan: Fix warnings caused by use of arch_enter_lazy_mmu_mode()

[Andrew Morton]

On Sat, 13 Sep 2025 09:55:15 +1000 Balbir Singh <balbirs@nvidia.com> wrote:

> commit c519c3c0a113 ("mm/kasan: avoid lazy MMU mode hazards") introduced
> the use of arch_enter_lazy_mmu_mode(), which results in the compiler
> complaining about "statement has no effect", when
> __HAVE_ARCH_LAZY_MMU_MODE is not defined in include/linux/pgtable.h
> 
> The exact warning/error is:
> 
> In file included from ./include/linux/kasan.h:37,
>                  from mm/kasan/shadow.c:14:
> mm/kasan/shadow.c: In function ‘kasan_populate_vmalloc_pte’:
> ./include/linux/pgtable.h:247:41: error: statement with no effect [-Werror=unused-value]
>   247 | #define arch_enter_lazy_mmu_mode()      (LAZY_MMU_DEFAULT)
>       |                                         ^
> mm/kasan/shadow.c:322:9: note: in expansion of macro ‘arch_enter_lazy_mmu_mode’
>   322 |         arch_enter_lazy_mmu_mode();
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~
> 
> Fix the issue by explicitly casting the use of the function to void,
> since the returned state is not forwarded/retained
> 
> ...
>
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -319,7 +319,7 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>  	}
>  	spin_unlock(&init_mm.page_table_lock);
>  
> -	arch_enter_lazy_mmu_mode();
> +	(void)arch_enter_lazy_mmu_mode();
>  
>  	return 0;
>  }
> @@ -494,7 +494,7 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>  	if (likely(!none))
>  		__free_page(pfn_to_page(pte_pfn(pte)));
>  
> -	arch_enter_lazy_mmu_mode();
> +	(void)arch_enter_lazy_mmu_mode();
>  
>  	return 0;

doh, I just figured out that my fix for your fix simply reverted your
fix!

I'll promote my cleanup into a hotfix:


From: Andrew Morton <akpm@linux-foundation.org>
Subject: include/linux/pgtable.h: convert arch_enter_lazy_mmu_mode() and friends to static inlines
Date: Sat Sep 13 05:03:39 PM PDT 2025

commit c519c3c0a113 ("mm/kasan: avoid lazy MMU mode hazards") introduced
the use of arch_enter_lazy_mmu_mode(), which results in the compiler
complaining about "statement has no effect", when
__HAVE_ARCH_LAZY_MMU_MODE is not defined in include/linux/pgtable.h

The exact warning/error is:

In file included from ./include/linux/kasan.h:37,
                 from mm/kasan/shadow.c:14:
mm/kasan/shadow.c: In function kasan_populate_vmalloc_pte:
./include/linux/pgtable.h:247:41: error: statement with no effect [-Werror=unused-value]
  247 | #define arch_enter_lazy_mmu_mode()      (LAZY_MMU_DEFAULT)
      |                                         ^
mm/kasan/shadow.c:322:9: note: in expansion of macro arch_enter_lazy_mmu_mode>   322 |         arch_enter_lazy_mmu_mode();
     |         ^~~~~~~~~~~~~~~~~~~~~~~~

switching these "functions" to static inlines fixes this up.

Fixes: c519c3c0a113 ("mm/kasan: avoid lazy MMU mode hazards") 
Reported-by: Balbir Singh <balbirs@nvidia.com>
Closes: https://lkml.kernel.org/r/20250912235515.367061-1-balbirs@nvidia.com
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/pgtable.h |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/linux/pgtable.h~include-linux-pgtableh-convert-arch_enter_lazy_mmu_mode-and-friends-to-static-inlines
+++ a/include/linux/pgtable.h
@@ -232,9 +232,9 @@ static inline int pmd_dirty(pmd_t pmd)
  * and the mode cannot be used in interrupt context.
  */
 #ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
-#define arch_enter_lazy_mmu_mode()	do {} while (0)
-#define arch_leave_lazy_mmu_mode()	do {} while (0)
-#define arch_flush_lazy_mmu_mode()	do {} while (0)
+static inline void arch_enter_lazy_mmu_mode(void) {}
+static inline void arch_leave_lazy_mmu_mode(void) {}
+static inline void arch_flush_lazy_mmu_mode(void) {}
 #endif
 
 #ifndef pte_batch_hint
_