Re: [PATCH mm-unstable v1 1/4] mm/swap: stop using page->private on tail pages for THP_SWAP

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Aug 23, 2023 at 8:17 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 23.08.23 17:12, Yosry Ahmed wrote:
> > On Mon, Aug 21, 2023 at 9:09 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> Let's stop using page->private on tail pages, making it possible to
> >> just unconditionally reuse that field in the tail pages of large folios.
> >>
> >> The remaining usage of the private field for THP_SWAP is in the THP
> >> splitting code (mm/huge_memory.c), that we'll handle separately later.
> >>
> >> Update the THP_SWAP documentation and sanity checks in mm_types.h and
> >> __split_huge_page_tail().
> >>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> > The mm part looks good to me (with the added fixup):
> >
> > Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
>
> Thanks!
>
> >>   /**
> >> diff --git a/include/linux/swap.h b/include/linux/swap.h
> >> index bb5adc604144..84fe0e94f5cd 100644
> >> --- a/include/linux/swap.h
> >> +++ b/include/linux/swap.h
> >> @@ -339,6 +339,15 @@ static inline swp_entry_t folio_swap_entry(struct folio *folio)
> >>          return entry;
> >>   }
> >>
> >> +static inline swp_entry_t page_swap_entry(struct page *page)
> >> +{
> >> +       struct folio *folio = page_folio(page);
> >> +       swp_entry_t entry = folio_swap_entry(folio);
> >> +
> >> +       entry.val += page - &folio->page;
> >> +       return entry;
> >> +}
> >> +
> >>   static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
> >>   {
> >>          folio->private = (void *)entry.val;
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index cc2f65f8cc62..c04702ae71d2 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -2446,18 +2446,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
> >>          page_tail->index = head->index + tail;
> >>
> >>          /*
> >> -        * page->private should not be set in tail pages with the exception
> >> -        * of swap cache pages that store the swp_entry_t in tail pages.
> >> -        * Fix up and warn once if private is unexpectedly set.
> >> -        *
> >> -        * What of 32-bit systems, on which folio->_pincount overlays
> >> -        * head[1].private?  No problem: THP_SWAP is not enabled on 32-bit, and
> >> -        * pincount must be 0 for folio_ref_freeze() to have succeeded.
> >> +        * page->private should not be set in tail pages. Fix up and warn once
> >> +        * if private is unexpectedly set.
> >>           */
> >> -       if (!folio_test_swapcache(page_folio(head))) {
> >> -               VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
> >> +       if (unlikely(page_tail->private)) {
> >> +               VM_WARN_ON_ONCE_PAGE(true, page_tail);
> >>                  page_tail->private = 0;
> >>          }
> >
> > Could probably save a couple of lines here:
> >
> > if (VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail))
> >
> >         page_tail->private = 0;
> >
>
> That would mean that we eventually compile out the runtime check
>
> #define VM_WARN_ON_ONCE_PAGE(cond, page)  BUILD_BUG_ON_INVALID(cond)

I thought the warning would be compiled out but not the check, my bad.

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