Re: [PATCH v1 01/10] mm: Expose clear_huge_page() unconditionally

[Yu Zhao <yuzhao@google.com>]

On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 27/06/2023 02:55, Yu Zhao wrote:
> > On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> In preparation for extending vma_alloc_zeroed_movable_folio() to
> >> allocate a arbitrary order folio, expose clear_huge_page()
> >> unconditionally, so that it can be used to zero the allocated folio in
> >> the generic implementation of vma_alloc_zeroed_movable_folio().
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >> ---
> >>  include/linux/mm.h | 3 ++-
> >>  mm/memory.c        | 2 +-
> >>  2 files changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index 7f1741bd870a..7e3bf45e6491 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
> >>   */
> >>  extern const struct attribute_group memory_failure_attr_group;
> >>
> >> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >>  extern void clear_huge_page(struct page *page,
> >>                             unsigned long addr_hint,
> >>                             unsigned int pages_per_huge_page);
> >> +
> >> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >
> > We might not want to depend on THP eventually. Right now, we still
> > have to, unless splitting is optional, which seems to contradict
> > 06/10. (deferred_split_folio()  is a nop without THP.)
>
> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
> think that helps us here.
>
> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
> parameter. So the generic/default version of the function now needs a way to
> clear a compound page.
>
> I guess I could do something like:
>
>  static inline
>  struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>                                    unsigned long vaddr, gfp_t gfp, int order)
>  {
>         struct folio *folio;
>
>         folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
>                                         order, vma, vaddr, false);
>         if (folio) {
> #ifdef CONFIG_LARGE_FOLIO
>                 clear_huge_page(&folio->page, vaddr, 1U << order);
> #else
>                 BUG_ON(order != 0);
>                 clear_user_highpage(&folio->page, vaddr);
> #endif
>         }
>
>         return folio;
>  }
>
> But that's pretty messy and there's no reason why other users might come along
> that pass order != 0 and will be surprised by the BUG_ON.

#ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
long vaddr, int order)
{
  // how do_huge_pmd_anonymous_page() allocs and clears
  vma_alloc_folio(..., *true*);
}
#else
#define alloc_anon_folio(vma, addr, order)
vma_alloc_zeroed_movable_folio(vma, addr)
#endif