Re: [PATCH v8 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Lance Yang <ioworker0@gmail.com>]

On Tue, Jun 18, 2024 at 2:04 AM David Hildenbrand <david@redhat.com> wrote:
>
> Sorry for taking so long to review ... getting there. Mostly nits.

No worries at all :)

Thanks for taking time to review!

>
> > @@ -497,6 +499,13 @@ static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> >                                        unsigned long address, pmd_t *pmd,
> >                                        bool freeze, struct folio *folio) {}
> >
> > +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
> > +                                      unsigned long addr, pmd_t *pmdp,
> > +                                      struct folio *folio)
> > +{
> > +     return false;
> > +}
> > +
> >   #define split_huge_pud(__vma, __pmd, __address)     \
> >       do { } while (0)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e766d3f3a302..425374ae06ed 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2688,6 +2688,82 @@ static void unmap_folio(struct folio *folio)
> >       try_to_unmap_flush();
> >   }
> >
> > +static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
> > +                                         unsigned long addr, pmd_t *pmdp,
> > +                                         struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
> > +     VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>
> I would drop these (that's exactly what the single caller checks). In

Agreed. I will drop these.

> any case don't place them above the variable declaration ;)

Yep, I see.

>
> > +
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     int ref_count, map_count;
> > +     pmd_t orig_pmd = *pmdp;
> > +     struct page *page;
> > +
> > +     if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
> > +             return false;
> > +
> > +     page = pmd_page(orig_pmd);
> > +     if (unlikely(page_folio(page) != folio))
> > +             return false;
>
> I'm curious, how could that happen? And how could it happen that we have
> !pmd_trans_huge() ? Didn't rmap walking code make sure that this PMD
> maps the folio already, and we are holding the PTL?

Makes sense to me. I was adding these just in case, but it's probably too much.

Let's drop them ;)

>
> > +
> > +     if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
> > +             return false;
> > +
> > +     orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
> > +
> > +     /*
> > +      * Syncing against concurrent GUP-fast:
> > +      * - clear PMD; barrier; read refcount
> > +      * - inc refcount; barrier; read PMD
> > +      */
> > +     smp_mb();
> > +
> > +     ref_count = folio_ref_count(folio);
> > +     map_count = folio_mapcount(folio);
> > +
> > +     /*
> > +      * Order reads for folio refcount and dirty flag
> > +      * (see comments in __remove_mapping()).
> > +      */
> > +     smp_rmb();
> > +
> > +     /*
> > +      * If the folio or its PMD is redirtied at this point, or if there
> > +      * are unexpected references, we will give up to discard this folio
> > +      * and remap it.
> > +      *
> > +      * The only folio refs must be one from isolation plus the rmap(s).
> > +      */
> > +     if (folio_test_dirty(folio) || pmd_dirty(orig_pmd) ||
> > +         ref_count != map_count + 1) {
> > +             set_pmd_at(mm, addr, pmdp, orig_pmd);
> > +             return false;
> > +     }
> > +
> > +     folio_remove_rmap_pmd(folio, page, vma);
> > +     zap_deposited_table(mm, pmdp);
> > +     add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > +     if (vma->vm_flags & VM_LOCKED)
> > +             mlock_drain_local();
> > +     folio_put(folio);
> > +
> > +     return true;
> > +}
> > +
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > +                        pmd_t *pmdp, struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > +     VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > +     VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> > +
> > +     if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > +             return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> > +
> > +     return false;
> > +}
> > +
> >   static void remap_page(struct folio *folio, unsigned long nr)
> >   {
> >       int i = 0;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index dacf24bc82f0..7d97806f74cd 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1678,16 +1678,23 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                       goto walk_abort;
> >               }
> >
> > -             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > -                     /*
> > -                      * We temporarily have to drop the PTL and start once
> > -                      * again from that now-PTE-mapped page table.
> > -                      */
> > -                     split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > -                                           false, folio);
> > -                     flags &= ~TTU_SPLIT_HUGE_PMD;
> > -                     page_vma_mapped_walk_restart(&pvmw);
> > -                     continue;
> > +             if (!pvmw.pte) {
> > +                     if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > +                                               folio))
> > +                             goto walk_done;
> > +
> > +                     if (flags & TTU_SPLIT_HUGE_PMD) {
> > +                             /*
> > +                              * We temporarily have to drop the PTL and start
> > +                              * once again from that now-PTE-mapped page
> > +                              * table.
>
> Nit: it's not a PTE-mapped page table.
>
> Maybe
>
> "... restart so we can process the PTE-mapped THP."

Nice. Will adjust as you suggested.

>
>
>
> >               }
> >
> >               /* Unexpected PMD-mapped THP? */
>
> Nothing else jumped at me :)

Thanks again for your time!
Lance

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