Re: [PATCH v2 2/7] mm: Optimize mprotect() by batch-skipping PTEs

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Apr 30, 2025 at 02:18:20PM +0100, Ryan Roberts wrote:
> On 30/04/2025 07:37, Dev Jain wrote:
> >
> >
> > On 29/04/25 6:49 pm, Lorenzo Stoakes wrote:
> >> Very very very nitty on subject (sorry I realise this is annoying :P) -
> >> generally don't need to capitalise 'Optimize' here :>)
> >>
> >> Generally I like the idea here. But some issues on impl.
> >>
> >> On Tue, Apr 29, 2025 at 10:53:31AM +0530, Dev Jain wrote:
> >>> In case of prot_numa, there are various cases in which we can skip to the
> >>> next iteration. Since the skip condition is based on the folio and not
> >>> the PTEs, we can skip a PTE batch.
> >>>
> >>> Signed-off-by: Dev Jain <dev.jain@arm.com>
> >>> ---
> >>>   mm/mprotect.c | 27 ++++++++++++++++++++-------
> >>>   1 file changed, 20 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/mm/mprotect.c b/mm/mprotect.c
> >>> index 70f59aa8c2a8..ec5d17af7650 100644
> >>> --- a/mm/mprotect.c
> >>> +++ b/mm/mprotect.c
> >>> @@ -91,6 +91,9 @@ static bool prot_numa_skip(struct vm_area_struct *vma,
> >>> struct folio *folio,
> >>>       bool toptier;
> >>>       int nid;
> >>>
> >>> +    if (folio_is_zone_device(folio) || folio_test_ksm(folio))
> >>> +        return true;
> >>> +
> >>
> >> Hm why not just put this here from the start? I think you should put this back
> >> in the prior commit.
> >>
> >>>       /* Also skip shared copy-on-write pages */
> >>>       if (is_cow_mapping(vma->vm_flags) &&
> >>>           (folio_maybe_dma_pinned(folio) ||
> >>> @@ -126,8 +129,10 @@ static bool prot_numa_skip(struct vm_area_struct *vma,
> >>> struct folio *folio,
> >>>   }
> >>>
> >>>   static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
> >>> -        unsigned long addr, pte_t oldpte, int target_node)
> >>> +        unsigned long addr, pte_t *pte, pte_t oldpte, int target_node,
> >>> +        int max_nr, int *nr)
> >>
> >> Hate this ptr to nr.
> >>
> >> Why not just return nr, if it's 0 then skip? Simple!
> >>
> >>>   {
> >>> +    const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> >>>       struct folio *folio;
> >>>       int ret;
> >>>
> >>> @@ -136,12 +141,16 @@ static bool prot_numa_avoid_fault(struct vm_area_struct
> >>> *vma,
> >>>           return true;
> >>>
> >>>       folio = vm_normal_folio(vma, addr, oldpte);
> >>> -    if (!folio || folio_is_zone_device(folio) ||
> >>> -        folio_test_ksm(folio))
> >>> +    if (!folio)
> >>>           return true;
> >>> +
> >>
> >> Very nitty, but stray extra line unless intended...
> >>
> >> Not sure why we can't just put this !folio check in prot_numa_skip()?
> >
> > Because we won't be able to batch if the folio is NULL.
> >
> > I think I really messed up by having separate patch 1 and 2. The real intent of
> > patch 1 was to do batching in patch 2 *and* not have insane indentation. Perhaps
> > I should merge them, or completely separate them logically, I'll figure this out.
>
> I'd be inclined to just merge into single patch...

Agreed!

>
> >
> >>
> >>>       ret = prot_numa_skip(vma, folio, target_node);
> >>> -    if (ret)
> >>> +    if (ret) {
> >>> +        if (folio_test_large(folio) && max_nr != 1)
> >>> +            *nr = folio_pte_batch(folio, addr, pte, oldpte,
> >>> +                          max_nr, flags, NULL, NULL, NULL);
> >>
> >> So max_nr can <= 0 too? Shouldn't this be max_nr > 1?
> >>
> >>>           return ret;
> >>
> >> Again x = fn_return_bool(); if (x) { return x; } is a bit silly, just do if
> >> (fn_return_bool()) { return true; }.
> >>
> >> If we return the number of pages, then this can become really simple, like:
> >>
> >> I feel like maybe we should abstract the folio large handling here, though it'd
> >> be a tiny function so hm.
> >>
> >> Anyway assuming we leave it in place, and return number of pages processed, this
> >> can become:
> >>
> >> if (prot_numa_skip(vma, folio, target_node)) {
> >>     if (folio_test_large(folio) && max_nr > 1)
> >>         return folio_pte_batch(folio, addr, pte, oldpte, max_nr, flags,
> >>                 NULL, NULL, NULL);
> >>     return 1;
> >> }
> >>
> >> Which is neater I think!
> >>
> >>
> >>> +    }
> >>>       if (folio_use_access_time(folio))
> >>>           folio_xchg_access_time(folio,
> >>>               jiffies_to_msecs(jiffies));
> >>> @@ -159,6 +168,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> >>>       bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> >>>       bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> >>>       bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> >>> +    int nr;
> >>>
> >>>       tlb_change_page_size(tlb, PAGE_SIZE);
> >>>       pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> >>> @@ -173,8 +183,10 @@ static long change_pte_range(struct mmu_gather *tlb,
> >>>       flush_tlb_batched_pending(vma->vm_mm);
> >>>       arch_enter_lazy_mmu_mode();
> >>>       do {
> >>> +        nr = 1;
> >>>           oldpte = ptep_get(pte);
> >>>           if (pte_present(oldpte)) {
> >>> +            int max_nr = (end - addr) >> PAGE_SHIFT;
> >>
> >> Not a fan of open-coding this. Since we already provide addr, why not just
> >> provide end as well and have prot_numa_avoid_fault() calculate it?
> >>
> >>>               pte_t ptent;
> >>>
> >>>               /*
> >>> @@ -182,8 +194,9 @@ static long change_pte_range(struct mmu_gather *tlb,
> >>>                * pages. See similar comment in change_huge_pmd.
> >>>                */
> >>>               if (prot_numa &&
> >>> -                prot_numa_avoid_fault(vma, addr,
> >>> -                          oldpte, target_node))
> >>> +                prot_numa_avoid_fault(vma, addr, pte,
> >>> +                          oldpte, target_node,
> >>> +                              max_nr, &nr))
> >>>                       continue;
> >>>
> >>>               oldpte = ptep_modify_prot_start(vma, addr, pte);
> >>> @@ -300,7 +313,7 @@ static long change_pte_range(struct mmu_gather *tlb,
> >>>                   pages++;
> >>>               }
> >>>           }
> >>> -    } while (pte++, addr += PAGE_SIZE, addr != end);
> >>> +    } while (pte += nr, addr += nr * PAGE_SIZE, addr != end);
> >>
> >> This is icky, having 'nr' here like this.
>
> For better or worse, this is the pattern we have already established in other
> loops that are batching-aware. See zap_pte_range(), copy_pte_range(), etc. So
> I'd prefer to follow that pattern here, as Dev has done.

Yeah I'm fine with keeping this 'nr' stuff, I don't think there's a great
alternative.

>
> Thanks.
> Ryan

Cheers, Lorenzo

>
> >>
> >> But alternatives might be _even more_ icky (that is advancing both on
> >> prot_numa_avoid_fault() so probably we need to keep it like this.
> >>
> >> Maybe more a moan at the C programming language tbh haha!
> >>
> >>
> >>>       arch_leave_lazy_mmu_mode();
> >>>       pte_unmap_unlock(pte - 1, ptl);
> >>>
> >>> --
> >>> 2.30.2
> >>>
> >
>