Re: [PATCH] mm: optimize once judgment with clang

[Mike Rapoport <rppt@kernel.org>]

On Sat, Mar 07, 2026 at 02:54:04PM +0800, Xie Yuanbin wrote:
> commit 242b872239f6a7deacbc ("include/linux/once_lite.h: fix judgment in
> WARN_ONCE with clang") helps optimize performance and size under the
> clang compiler, but the modification is not complete.

How much does it actually optimize for size?
Note that performance is really not critical here because we are already
dealing with slow path of debug code.
 
> Port the modification to WARN_ON_ONCE_GFP(), VM_WARN_ON_ONCE_PAGE(),
> VM_WARN_ON_ONCE_FOLIO(), VM_WARN_ON_ONCE_MM() and VM_WARN_ON_ONCE_VMA().
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Xie Yuanbin <qq570070308@gmail.com>
> ---
>  include/linux/mmdebug.h | 8 ++++----
>  mm/internal.h           | 3 ++-
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
> index ab60ffba08f5..a167c5aa525e 100644
> --- a/include/linux/mmdebug.h
> +++ b/include/linux/mmdebug.h
> @@ -60,7 +60,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
>  	static bool __section(".data..once") __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
> -	if (unlikely(__ret_warn_once && !__warned)) {			\
> +	if (unlikely(__ret_warn_once) && unlikely(!__warned)) {		\
>  		dump_page(page, "VM_WARN_ON_ONCE_PAGE(" __stringify(cond)")");\
>  		__warned = true;					\
>  		WARN_ON(1);						\
> @@ -80,7 +80,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
>  	static bool __section(".data..once") __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
> -	if (unlikely(__ret_warn_once && !__warned)) {			\
> +	if (unlikely(__ret_warn_once) && unlikely(!__warned)) {		\
>  		dump_page(&folio->page, "VM_WARN_ON_ONCE_FOLIO(" __stringify(cond)")");\
>  		__warned = true;					\
>  		WARN_ON(1);						\
> @@ -91,7 +91,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
>  	static bool __section(".data..once") __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
> -	if (unlikely(__ret_warn_once && !__warned)) {			\
> +	if (unlikely(__ret_warn_once) && unlikely(!__warned)) {		\
>  		dump_mm(mm);						\
>  		__warned = true;					\
>  		WARN_ON(1);						\
> @@ -102,7 +102,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
>  	static bool __section(".data..once") __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
> -	if (unlikely(__ret_warn_once && !__warned)) {			\
> +	if (unlikely(__ret_warn_once) && unlikely(!__warned)) {		\
>  		dump_vma(vma);						\
>  		__warned = true;					\
>  		WARN_ON(1);						\
> diff --git a/mm/internal.h b/mm/internal.h
> index 6e1162e13289..52367f52d623 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -93,7 +93,8 @@ struct pagetable_move_control {
>  	static bool __section(".data..once") __warned;			\
>  	int __ret_warn_once = !!(cond);					\
>  									\
> -	if (unlikely(!(gfp & __GFP_NOWARN) && __ret_warn_once && !__warned)) { \
> +	if (unlikely(__ret_warn_once) && !(gfp & __GFP_NOWARN) &&	\
> +	    unlikely(!__warned)) {					\
>  		__warned = true;					\
>  		WARN_ON(1);						\
>  	}								\
> -- 
> 2.51.0
> 

-- 
Sincerely yours,
Mike.