Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[Barry Song <21cnbao@gmail.com>]

> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index fb63d9256f09..241d55a92a47 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1847,12 +1847,25 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
> >
> >   /* We support batch unmapping of PTEs for lazyfree large folios */
> >   static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
> > -                     struct folio *folio, pte_t *ptep)
> > +                                           struct folio *folio, pte_t *ptep,
> > +                                           struct vm_area_struct *vma)
> >   {
> >       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > +     unsigned long next_pmd, vma_end, end_addr;
> >       int max_nr = folio_nr_pages(folio);
> >       pte_t pte = ptep_get(ptep);
> >
> > +     /*
> > +      * Limit the batch scan within a single VMA and within a single
> > +      * page table.
> > +      */
> > +     vma_end = vma->vm_end;
> > +     next_pmd = ALIGN(addr + 1, PMD_SIZE);
> > +     end_addr = addr + (unsigned long)max_nr * PAGE_SIZE;
> > +
> > +     if (end_addr > min(next_pmd, vma_end))
> > +             return false;
>
> May I suggest that we clean all that up as we fix it?
>
> Maybe something like this:
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3b74bb19c11dd..11fbddc6ad8d6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1845,23 +1845,38 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
>   #endif
>   }
>
> -/* We support batch unmapping of PTEs for lazyfree large folios */
> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
> -                       struct folio *folio, pte_t *ptep)
> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> +               struct page_vma_mapped_walk *pvmw, enum ttu_flags flags,
> +               pte_t pte)
>   {
>          const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> -       int max_nr = folio_nr_pages(folio);
> -       pte_t pte = ptep_get(ptep);
> +       struct vm_area_struct *vma = pvmw->vma;
> +       unsigned long end_addr, addr = pvmw->address;
> +       unsigned int max_nr;
> +
> +       if (flags & TTU_HWPOISON)
> +               return 1;
> +       if (!folio_test_large(folio))
> +               return 1;
> +
> +       /* We may only batch within a single VMA and a single page table. */
> +       end_addr = min_t(unsigned long, ALIGN(addr + 1, PMD_SIZE), vma->vm_end);

Is this pmd_addr_end()?

> +       max_nr = (end_addr - addr) >> PAGE_SHIFT;
>
> +       /* We only support lazyfree batching for now ... */
>          if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
> -               return false;
> +               return 1;
>          if (pte_unused(pte))
> -               return false;
> -       if (pte_pfn(pte) != folio_pfn(folio))
> -               return false;
> +               return 1;
> +       /* ... where we must be able to batch the whole folio. */
> +       if (pte_pfn(pte) != folio_pfn(folio) || max_nr != folio_nr_pages(folio))
> +               return 1;
> +       max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
> +                                NULL, NULL, NULL);
>
> -       return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
> -                              NULL, NULL) == max_nr;
> +       if (max_nr != folio_nr_pages(folio))
> +               return 1;
> +       return max_nr;
>   }
>
>   /*
> @@ -2024,9 +2039,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>                          if (pte_dirty(pteval))
>                                  folio_mark_dirty(folio);
>                  } else if (likely(pte_present(pteval))) {
> -                       if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
> -                           can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
> -                               nr_pages = folio_nr_pages(folio);
> +                       nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
>                          end_addr = address + nr_pages * PAGE_SIZE;
>                          flush_cache_range(vma, address, end_addr);
>
>
> Note that I don't quite understand why we have to batch the whole thing or fallback to
> individual pages. Why can't we perform other batches that span only some PTEs? What's special
> about 1 PTE vs. 2 PTEs vs. all PTEs?
>
>
> Can someone enlighten me why that is required?

It's probably not a strict requirement — I thought cases where the
count is greater than 1 but less than nr_pages might not provide much
practical benefit, except perhaps in very rare edge cases, since
madv_free() already calls split_folio().

if (folio_test_large(folio)) {
                        bool any_young, any_dirty;
                        nr = madvise_folio_pte_batch(addr, end, folio, pte,
                                                     ptent,
&any_young, &any_dirty);


                        if (nr < folio_nr_pages(folio)) {
                                ...
                                err = split_folio(folio);
                                ...
                       }
}

Another reason is that when we extend this to non-lazyfree anonymous
folios [1], things get complicated: checking anon_exclusive and updating
folio_try_share_anon_rmap_pte with the number of PTEs becomes tricky if
a folio is partially exclusive and partially shared.

[1] https://lore.kernel.org/linux-mm/20250513084620.58231-1-21cnbao@gmail.com/

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry