Re: [PATCH v2 2/3] mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==false

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

On Thu, Jun 20, 2024 at 7:46 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 18.06.24 01:11, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.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 improves the code’s readability. Currently, all new anonymous
> > folios calling folio_add_anon_rmap_ptes() are order-0. This ensures
> > that new folios cannot be partially exclusive; they are either entirely
> > exclusive or entirely shared.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > Tested-by: Shuai Yuan <yuanshuai@oppo.com>
> > ---
> >   mm/memory.c   |  8 ++++++++
> >   mm/swapfile.c | 13 +++++++++++--
> >   2 files changed, 19 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 1f24ecdafe05..620654c13b2f 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4339,6 +4339,14 @@ 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)) {
> > +             /*
> > +              * We currently only expect small !anon folios, for which we now
> > +              * that they are either fully exclusive or fully shared. If we
> > +              * ever get large folios here, we have to be careful.
> > +              */
> > +             VM_WARN_ON_ONCE(folio_test_large(folio));
> > +             folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
> >       } else {
> >               folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
> >                                       rmap_flags);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index ae1d2700f6a3..69efa1a57087 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1908,8 +1908,17 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
> >               VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
> >               if (pte_swp_exclusive(old_pte))
> >                       rmap_flags |= RMAP_EXCLUSIVE;
> > -
> > -             folio_add_anon_rmap_pte(folio, page, vma, addr, rmap_flags);
> > +             /*
> > +              * We currently only expect small !anon folios, for which we now that
> > +              * they are either fully exclusive or fully shared. If we ever get
> > +              * large folios here, we have to be careful.
> > +              */
> > +             if (!folio_test_anon(folio)) {
> > +                     VM_WARN_ON_ONCE(folio_test_large(folio));
>
> (comment applies to both cases)
>
> Thinking about Hugh's comment, we should likely add here:
>
> VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>
> [the check we are removing from __folio_add_anon_rmap()]
>
> and document for folio_add_new_anon_rmap() in patch #1, that when
> dealing with folios that might be mapped concurrently by others, the
> folio lock must be held.

I assume you mean something like the following for patch#1?

diff --git a/mm/rmap.c b/mm/rmap.c
index df1a43295c85..20986b25f1b2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1394,7 +1394,8 @@ void folio_add_anon_rmap_pmd(struct folio
*folio, struct page *page,
  *
  * Like folio_add_anon_rmap_*() but must only be called on *new* folios.
  * This means the inc-and-test can be bypassed.
- * The folio does not have to be locked.
+ * The folio doesn't necessarily need to be locked while it's
exclusive unless two threads
+ * map it concurrently. However, the folio must be locked if it's shared.
  *
  * If the folio is pmd-mappable, it is accounted as a THP.
  */
@@ -1406,6 +1407,7 @@ void folio_add_new_anon_rmap(struct folio
*folio, struct vm_area_struct *vma,
        int nr_pmdmapped = 0;

        VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
+       VM_WARN_ON_FOLIO(!exclusive && !folio_test_locked(folio), folio);
        VM_BUG_ON_VMA(address < vma->vm_start ||
                        address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
        __folio_set_swapbacked(folio);


>
> > +                     folio_add_new_anon_rmap(folio, vma, addr, rmap_flags);
> > +             } else {
> > +                     folio_add_anon_rmap_pte(folio, page, vma, addr, rmap_flags);
> > +             }
> >       } else { /* ksm created a completely new copy */
> >               folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
> >               folio_add_lru_vma(folio, vma);
>
> --
> Cheers,
>
> David / dhildenb
>