Re: [PATCH 1/4] mm/mprotect: encourage inlining with __always_inline

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/19/26 19:31, Pedro Falcato wrote:
> Encourage the compiler to inline batch PTE logic and resolve constant
> branches by adding __always_inline strategically.
> 
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> ---
>  mm/mprotect.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 9681f055b9fc..1bd0d4aa07c2 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -103,7 +103,7 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	return can_change_shared_pte_writable(vma, pte);
>  }
>  
> -static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
> +static __always_inline int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
>  				    pte_t pte, int max_nr_ptes, fpb_t flags)
>  {
>  	/* No underlying folio, so cannot batch */
> @@ -117,9 +117,9 @@ static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
>  }
>  
>  /* Set nr_ptes number of ptes, starting from idx */
> -static void prot_commit_flush_ptes(struct vm_area_struct *vma, unsigned long addr,
> -		pte_t *ptep, pte_t oldpte, pte_t ptent, int nr_ptes,
> -		int idx, bool set_write, struct mmu_gather *tlb)
> +static __always_inline void prot_commit_flush_ptes(struct vm_area_struct *vma,
> +		unsigned long addr, pte_t *ptep, pte_t oldpte, pte_t ptent,
> +		int nr_ptes, int idx, bool set_write, struct mmu_gather *tlb)
>  {
>  	/*
>  	 * Advance the position in the batch by idx; note that if idx > 0,
> @@ -169,7 +169,7 @@ static int page_anon_exclusive_sub_batch(int start_idx, int max_len,
>   * pte of the batch. Therefore, we must individually check all pages and
>   * retrieve sub-batches.
>   */
> -static void commit_anon_folio_batch(struct vm_area_struct *vma,
> +static __always_inline void commit_anon_folio_batch(struct vm_area_struct *vma,
>  		struct folio *folio, struct page *first_page, unsigned long addr, pte_t *ptep,
>  		pte_t oldpte, pte_t ptent, int nr_ptes, struct mmu_gather *tlb)
>  {

From my micro-optimization work on zapping and fork, I learned that
these batching functions are best optimized for order-0 page by
explicitly calling them from the code with "nr_ptes == 1" and then
force-inlining them. nr_ptes and all loops will essentially be optimized
out.

With no such explicit constants, is there really a real benefit to be
had here?

-- 
Cheers,

David