Re: [PATCH v7 3/4] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[Lance Yang]

On Thu, Jun 13, 2024 at 4:34 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 10.06.24 14:06, Lance Yang wrote:
> > In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
> > folios, start the pagewalk first, then call split_huge_pmd_address() to
> > split the folio.
> >
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Suggested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >   include/linux/huge_mm.h |  6 ++++++
> >   mm/huge_memory.c        | 42 +++++++++++++++++++++--------------------
> >   mm/rmap.c               | 21 +++++++++++++++------
> >   3 files changed, 43 insertions(+), 26 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 088d66a54643..4670c6ee118b 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -415,6 +415,9 @@ static inline bool thp_migration_supported(void)
> >       return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
> >   }
> >
> > +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> > +                        pmd_t *pmd, bool freeze, struct folio *folio);
> > +
> >   #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> >   static inline bool folio_test_pmd_mappable(struct folio *folio)
> > @@ -477,6 +480,9 @@ static inline void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >               unsigned long address, bool freeze, struct folio *folio) {}
> >   static inline void split_huge_pmd_address(struct vm_area_struct *vma,
> >               unsigned long address, bool freeze, struct folio *folio) {}
> > +static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> > +                                      unsigned long address, pmd_t *pmd,
> > +                                      bool freeze, struct folio *folio) {}
> >
> >   #define split_huge_pud(__vma, __pmd, __address)     \
> >       do { } while (0)
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e6d26c2eb670..d2697cc8f9d4 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2581,6 +2581,27 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >       pmd_populate(mm, pmd, pgtable);
> >   }
> >
> > +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> > +                        pmd_t *pmd, bool freeze, struct folio *folio)
> > +{
> > +     VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
> > +     VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
> > +     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> > +     VM_BUG_ON(freeze && !folio);
>
> Curious: could we actually end up here without a folio right now? That
> would mean, that try_to_unmap_one() would be called with folio==NULL.

try_to_unmap_one() would not be called with folio==NULL, I guess.

I just moved 'VM_BUG_ON(freeze && !folio)' from __split_huge_pmd() to here,
and now __split_huge_pmd() will call split_huge_pmd_locked().

>
> > +
> > +     /*
> > +      * When the caller requests to set up a migration entry, we
> > +      * require a folio to check the PMD against. Otherwise, there
> > +      * is a risk of replacing the wrong folio.
> > +      */
> > +     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> > +         is_pmd_migration_entry(*pmd)) {
> > +             if (folio && folio != pmd_folio(*pmd))
> > +                     return;
> > +             __split_huge_pmd_locked(vma, pmd, address, freeze);
> > +     }
> > +}
> > +
> >   void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >               unsigned long address, bool freeze, struct folio *folio)
> >   {
> > @@ -2592,26 +2613,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >                               (address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE);
> >       mmu_notifier_invalidate_range_start(&range);
> >       ptl = pmd_lock(vma->vm_mm, pmd);
> > -
> > -     /*
> > -      * If caller asks to setup a migration entry, we need a folio to check
> > -      * pmd against. Otherwise we can end up replacing wrong folio.
> > -      */
> > -     VM_BUG_ON(freeze && !folio);
> > -     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> > -
> > -     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> > -         is_pmd_migration_entry(*pmd)) {
> > -             /*
> > -              * It's safe to call pmd_page when folio is set because it's
> > -              * guaranteed that pmd is present.
> > -              */
> > -             if (folio && folio != pmd_folio(*pmd))
> > -                     goto out;
> > -             __split_huge_pmd_locked(vma, pmd, range.start, freeze);
> > -     }
> > -
> > -out:
> > +     split_huge_pmd_locked(vma, range.start, pmd, freeze, folio);
> >       spin_unlock(ptl);
> >       mmu_notifier_invalidate_range_end(&range);
> >   }
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index ddffa30c79fb..b77f88695588 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       if (flags & TTU_SYNC)
> >               pvmw.flags = PVMW_SYNC;
> >
> > -     if (flags & TTU_SPLIT_HUGE_PMD)
> > -             split_huge_pmd_address(vma, address, false, folio);
> > -
> >       /*
> >        * For THP, we have to assume the worse case ie pmd for invalidation.
> >        * For hugetlb, it could be much worse if we need to do pud
> > @@ -1668,9 +1665,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       mmu_notifier_invalidate_range_start(&range);
> >
> >       while (page_vma_mapped_walk(&pvmw)) {
> > -             /* Unexpected PMD-mapped THP? */
> > -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> > -
> >               /*
> >                * If the folio is in an mlock()d vma, we must not swap it out.
> >                */
> > @@ -1682,6 +1676,21 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                       goto walk_done_err;
> >               }
> >
> > +             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);
>
> If, for some reason, split_huge_pmd_locked() would fail, we would keep
> looping and never hit the VM_BUG_ON_FOLIO() below. Maybe we'd want to
> let split_huge_pmd_locked() return whether splitting succeeded, and
> handle that case differently?

Hmm... after calling split_huge_pmd_locked(), we also do
"flags &= ~TTU_SPLIT_HUGE_PMD", preventing re-entry into this block,
then triggering the VM_BUG_ON_FOLIO() below if split_huge_pmd_locked()
fails, IIUC.

What do you think?

Thanks,
Lance

>
> > +                     continue;
> > +             }
> > +
> > +             /* Unexpected PMD-mapped THP? */
> > +             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> > +
> >               pfn = pte_pfn(ptep_get(pvmw.pte));
> >               subpage = folio_page(folio, pfn - folio_pfn(folio));
> >               address = pvmw.address;
>
> --
> Cheers,
>
> David / dhildenb
>