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Date: Thu, 13 Jun 2024 17:21:07 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, fengwei.yin@...el.com, libang.li@...group.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org, maskray@...gle.com,
mhocko@...e.com, minchan@...nel.org, peterx@...hat.com, ryan.roberts@....com,
shy828301@...il.com, sj@...nel.org, songmuchun@...edance.com,
wangkefeng.wang@...wei.com, willy@...radead.org, xiehuan09@...il.com,
ziy@...dia.com, zokeefe@...gle.com
Subject: Re: [PATCH v7 3/4] mm/rmap: integrate PMD-mapped folio splitting into
pagewalk loop
On Thu, Jun 13, 2024 at 4:34 PM David Hildenbrand <david@...hat.com> 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@...hat.com>
> > Suggested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
> > ---
> > 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
>
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