Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop
From: Lance Yang
Date: Tue May 07 2024 - 00:37:34 EST
Hey Baolin,
Thanks for taking time to review!
On Tue, May 7, 2024 at 11:40 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2024/5/1 12:26, 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>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> > include/linux/huge_mm.h | 20 ++++++++++++++++++++
> > mm/huge_memory.c | 42 +++++++++++++++++++++--------------------
> > mm/rmap.c | 24 +++++++++++++++++------
> > 3 files changed, 60 insertions(+), 26 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index c8d3ec116e29..38c4b5537715 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -409,6 +409,20 @@ 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);
> > +
> > +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > + unsigned long *start,
> > + unsigned long *end)
> > +{
> > + *start = ALIGN(*start, HPAGE_PMD_SIZE);
> > + *end = ALIGN_DOWN(*end, HPAGE_PMD_SIZE);
> > +
> > + VM_WARN_ON_ONCE(vma->vm_start > *start);
> > + VM_WARN_ON_ONCE(vma->vm_end < *end);
> > +}
> > +
> > #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> > static inline bool folio_test_pmd_mappable(struct folio *folio)
> > @@ -471,6 +485,12 @@ 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) {}
> > +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > + unsigned long *start,
> > + unsigned long *end) {}
> >
> > #define split_huge_pud(__vma, __pmd, __address) \
> > do { } while (0)
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 8261b5669397..145505a1dd05 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2584,6 +2584,27 @@ 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);
> > +
> > + /*
> > + * 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)
> > {
> > @@ -2595,26 +2616,7 @@ 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 7e2575d669a9..432601154583 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1636,9 +1636,6 @@ 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
> > @@ -1650,6 +1647,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > range.end = vma_address_end(&pvmw);
> > mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
> > address, range.end);
> > + if (flags & TTU_SPLIT_HUGE_PMD)
> > + align_huge_pmd_range(vma, &range.start, &range.end);
>
> I am not sure why need this alignment?
> (1) For a partially mapped THP, 'range.start' and 'range.end' can beyond
> the VMA limits. For a PMD mapped THP, I think the address is already THP
> size alignment returned from vma_address(&folio->page, vma).
> (2) The range.end is not used.
Thanks for pointing that out!
I agree that this alignment is indeed redundant, and we should remove it.
Especially for a partially mapped THP, it could cause 'range.start' and
'range.end' beyond the VMA limits, which could lead us into trouble.
>
> > if (folio_test_hugetlb(folio)) {
> > /*
> > * If sharing is possible, start and end will be adjusted
> > @@ -1664,9 +1663,6 @@ 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.
> > */
> > @@ -1678,6 +1674,22 @@ 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, range.start, pvmw.pmd, false,
> > + folio);
> > + pvmw.pmd = NULL;
> > + spin_unlock(pvmw.ptl);
>
> IMO, you should also make the 'pvmw.ptl = NULL;' after unlocking as
> page_vma_mapped_walk() did, in case some corner case met.
Yep, I'll also set pvmw.ptl to NULL here if any corner cases arise.
Thanks again for the review!
Lance
>
> > + flags &= ~TTU_SPLIT_HUGE_PMD;
> > + 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;