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

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

On 3/20/26 10:59, Pedro Falcato wrote:
> On Thu, Mar 19, 2026 at 10:28:47PM +0100, David Hildenbrand (Arm) wrote:
>> 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?
> 
> Per my measurements, I could measure a real speedup here. Of course things
> may heavily depend on the microarchitecture you use. I want to note that
> these three functions are part of the hot loop and thus we definitely want
> them inlined. Particularly if we start special-casing stuff. You can cut
> down _a lot_ of code if you simply tell it "yeah don't bother you're looking
> at 1 pte only".

That's why I think this change is a lot more valuable when squashing
patch #4.

-- 
Cheers,

David