Re: [PATCH v2 2/2] mm/mprotect: special-case small folios when applying write permissions

[Pedro Falcato <pfalcato@suse.de>]

On Tue, Mar 24, 2026 at 09:18:42PM +0100, David Hildenbrand (Arm) wrote:
> On 3/24/26 16:43, Pedro Falcato wrote:
> > The common order-0 case is important enough to want its own branch, and
> > avoids the hairy, large loop logic that the CPU does not seem to handle
> > particularly well.
> > 
> > While at it, encourage the compiler to inline batch PTE logic and resolve
> > constant branches by adding __always_inline strategically.
> > 
> > Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> > ---
> >  mm/mprotect.c | 17 ++++++++++++-----
> >  1 file changed, 12 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index 2eaf862e5734..2fda26107066 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)
> >  {
> > @@ -177,6 +177,13 @@ static void commit_anon_folio_batch(struct vm_area_struct *vma,
> >  	int sub_batch_idx = 0;
> >  	int len;
> >  
> > +	/* Optimize for the common order-0 case. */
> > +	if (likely(nr_ptes == 1)) {
> > +		prot_commit_flush_ptes(vma, addr, ptep, oldpte, ptent, 1,
> > +				       0, PageAnonExclusive(first_page), tlb);
> 
> To optimize that one, inlining prot_commit_flush_ptes() would be
> sufficient. Does inlining the other two really help? I don't think we
> can optimize out loops etc. for them?

Well, I'm getting meaningful (smaller) wins from adding those
__always_inline's. (and I also get a small win for __always_inline on
set_write_prot_commit_flush_ptes, but I didn't realize that until now).

> 
> I would have thought that specializing on nr_ptes==0 on an even higher
> level--where we call
> set_write_prot_commit_flush_ptes/prot_commit_flush_ptes() would allow
> for optimizing the loops entirely for nr_ptes==0?

That could also work, but then set_write_prot_commit_flush_ptes (holy cow
what a long name) would definitely need inlining. And might be a little uglier
overall.

This is the part where having data points other than my giga-fast-giga-powerful
zen5 could prove handy :/

-- 
Pedro