Re: [PATCH v3 4/9] mm: introduce skip_none_ptes()

[David Hildenbrand]

On 14.11.24 10:20, Qi Zheng wrote:
> On 2024/11/14 16:04, David Hildenbrand wrote:
> > >    static unsigned long zap_pte_range(struct mmu_gather *tlb,
> > >                    struct vm_area_struct *vma, pmd_t *pmd,
> > >                    unsigned long addr, unsigned long end,
> > > @@ -1682,13 +1704,17 @@ static unsigned long zap_pte_range(struct
> > > mmu_gather *tlb,
> > >            pte_t ptent = ptep_get(pte);
> > >            int max_nr;
> > > -        nr = 1;
> > > -        if (pte_none(ptent))
> > > -            continue;
> > > -
> > >            if (need_resched())
> > >                break;
> > > +        nr = skip_none_ptes(pte, addr, end);
> > > +        if (nr) {
> > > +            addr += PAGE_SIZE * nr;
> > > +            if (addr == end)
> > > +                break;
> > > +            pte += nr;
> > > +        }
> > > +
> > >            max_nr = (end - addr) / PAGE_SIZE;
> >
> > I dislike calculating max_nr twice, once here and once in skip_non_ptes.
> >
> > Further, you're missing to update ptent here.
>
> Oh, my bad. However, with [PATCH v3 5/9], there will be no problem, but
> there are still two ptep_get() and max_nr calculation.
>
> If you inline it you can
> > avoid another ptep_get().
>
> Do you mean to inline the skip_none_ptes() into do_zap_pte_range()?

Effectively moving this patch after #5, and have it be something like:

diff --git a/mm/memory.c b/mm/memory.c
index 1949f5e0fece5..4f5d1e4c6688e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1667,8 +1667,21 @@ static inline int do_zap_pte_range(struct mmu_gather *tlb,
        pte_t ptent = ptep_get(pte);
        int max_nr = (end - addr) / PAGE_SIZE;
 
-       if (pte_none(ptent))
-               return 1;
+       /* Skip all consecutive pte_none(). */
+       if (pte_none(ptent)) {
+               int nr;
+
+               for (nr = 1; nr < max_nr; nr++) {
+                       ptent = ptep_get(pte + nr);
+                       if (!pte_none(ptent))
+                               break;
+               }
+               max_nr -= nr;
+               if (!max_nr)
+                       return nr;
+               pte += nr;
+               addr += nr * PAGE_SIZE;
+       }
 
        if (pte_present(ptent))
                return zap_present_ptes(tlb, vma, pte, ptent, max_nr,


In the context of this patch this makes most sense.

Regarding "count_pte_none" comment, I assume you talk about patch #7.

Can't you simply return the number of pte_none that you skipped here using another
input variable, if really required?

--
Cheers,

David / dhildenb