Re: [PATCH] mm: Compute mTHP order efficiently

[Dev Jain]

On 9/16/24 10:42, Barry Song wrote:
> On Fri, Sep 13, 2024 at 5:19 PM Dev Jain <dev.jain@xxxxxxx> wrote:
> > We use pte_range_none() to determine whether contiguous PTEs are empty
> > for an mTHP allocation. Instead of iterating the while loop for every
> > order, use some information, which is the first set PTE found, from the
> > previous iteration, to eliminate some cases. The key to understanding
> > the correctness of the patch is that the ranges we want to examine
> > form a strictly decreasing sequence of nested intervals.
> >
> > Suggested-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> I like this patch, but could we come up with a better subject for
> pte_range_none()?
> The subject is really incorrect.

Are you asking me to change "Compute mTHP order efficiently" to
something else?

> Also, I'd prefer the change for alloc_anon_folio() to be separated
> into its own patch.
> So, one patchset with two patches, please.

Fine by me.

> > ---
> >   mm/memory.c | 30 +++++++++++++++++++++++-------
> >   1 file changed, 23 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 3c01d68065be..ffc24a48ef15 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4409,26 +4409,27 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >          return ret;
> >   }
> >
> > -static bool pte_range_none(pte_t *pte, int nr_pages)
> > +static int pte_range_none(pte_t *pte, int nr_pages)
> >   {
> >          int i;
> >
> >          for (i = 0; i < nr_pages; i++) {
> >                  if (!pte_none(ptep_get_lockless(pte + i)))
> > -                       return false;
> > +                       return i;
> >          }
> >
> > -       return true;
> > +       return nr_pages;
> >   }
> >
> >   static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> >   {
> >          struct vm_area_struct *vma = vmf->vma;
> >   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +       pte_t *first_set_pte = NULL, *align_pte, *pte;
> >          unsigned long orders;
> >          struct folio *folio;
> >          unsigned long addr;
> > -       pte_t *pte;
> > +       int max_empty;
> >          gfp_t gfp;
> >          int order;
> >
> > @@ -4463,8 +4464,23 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> >          order = highest_order(orders);
> >          while (orders) {
> >                  addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > -               if (pte_range_none(pte + pte_index(addr), 1 << order))
> > +               align_pte = pte + pte_index(addr);
> > +
> > +               /* Range to be scanned known to be empty */
> > +               if (align_pte + (1 << order) <= first_set_pte)
> >                          break;
> > +
> > +               /* Range to be scanned contains first_set_pte */
> > +               if (align_pte <= first_set_pte)
> > +                       goto repeat;
> > +
> > +               /* align_pte > first_set_pte, so need to check properly */
> > +               max_empty = pte_range_none(align_pte, 1 << order);
> > +               if (max_empty == 1 << order)
> > +                       break;
> > +
> > +               first_set_pte = align_pte + max_empty;
> > +repeat:
> >                  order = next_order(&orders, order);
> >          }
> >
> > @@ -4579,7 +4595,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >          if (nr_pages == 1 && vmf_pte_changed(vmf)) {
> >                  update_mmu_tlb(vma, addr, vmf->pte);
> >                  goto release;
> > -       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> > +       } else if (nr_pages > 1 && pte_range_none(vmf->pte, nr_pages) != nr_pages) {
> >                  update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> >                  goto release;
> >          }
> > @@ -4915,7 +4931,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >                  update_mmu_tlb(vma, addr, vmf->pte);
> >                  ret = VM_FAULT_NOPAGE;
> >                  goto unlock;
> > -       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> > +       } else if (nr_pages > 1 && pte_range_none(vmf->pte, nr_pages) != nr_pages) {
> >                  update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> >                  ret = VM_FAULT_NOPAGE;
> >                  goto unlock;
> > --
> > 2.30.2
> Thanks
> Barry