linux-kernel - Re: [PATCH RFC 2/3] mm: do_swap_page: use folio_add_new_anon_rmap() if folio_test

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Date: Thu, 13 Jun 2024 21:58:35 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chrisl@...nel.org, 
	linux-kernel@...r.kernel.org, mhocko@...e.com, ryan.roberts@....com, 
	baolin.wang@...ux.alibaba.com, yosryahmed@...gle.com, shy828301@...il.com, 
	surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org, 
	ying.huang@...el.com, yuzhao@...gle.com
Subject: Re: [PATCH RFC 2/3] mm: do_swap_page: use folio_add_new_anon_rmap()
 if folio_test_anon(folio)==false

On Thu, Jun 13, 2024 at 9:23 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 13.06.24 02:07, Barry Song wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > For the !folio_test_anon(folio) case, we can now invoke folio_add_new_anon_rmap()
> > with the rmap flags set to either EXCLUSIVE or non-EXCLUSIVE. This action will
> > suppress the VM_WARN_ON_FOLIO check within __folio_add_anon_rmap() while initiating
> > the process of bringing up mTHP swapin.
> >
> >   static __always_inline void __folio_add_anon_rmap(struct folio *folio,
> >                   struct page *page, int nr_pages, struct vm_area_struct *vma,
> >                   unsigned long address, rmap_t flags, enum rmap_level level)
> >   {
> >           ...
> >           if (unlikely(!folio_test_anon(folio))) {
> >                   VM_WARN_ON_FOLIO(folio_test_large(folio) &&
> >                                    level != RMAP_LEVEL_PMD, folio);
> >           }
> >           ...
> >   }
> >
> > It also enhances the code’s readability.
> >
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > ---
> >   mm/memory.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 2f94921091fb..9c962f62f928 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4339,6 +4339,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (unlikely(folio != swapcache && swapcache)) {
> >               folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> >               folio_add_lru_vma(folio, vma);
> > +     } else if (!folio_test_anon(folio)) {
> > +             folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
>
> So, with large folio swapin, we would never end up here if any swp PTE
> is !exclusive, because we would make sure to allocate a large folio only
> for suitable regions, correct?
>
> It can certainly happen during swapout + refault with large folios. But
> there, we will always have folio_test_anon() still set and wouldn't run
> into this code path.
>
> (it will all be better with per-folio PAE bit, but that will take some
> more time until I actually get to implement it properly, handling all
> the nasty corner cases)
>
> Better add a comment regarding why we are sure that the complete thing
> is exclusive (e.g., currently only small folios).

No, patch 1/3 enables both cases to call folio_add_new_anon_rmap(). Currently,
small folios could be non-exclusive. I suppose your question is
whether we should
ensure that all pages within a mTHP have the same exclusivity status,
rather than
always being exclusive?

>
> >       } else {
> >               folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
> >                                       rmap_flags);
>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry