Re: [PATCH v4 07/13] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

On 2024/9/24 15:14, Muchun Song wrote:
> > On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
> > In collapse_pte_mapped_thp(), we may modify the pte and pmd entry after
> > acquring the ptl, so convert it to using pte_offset_map_rw_nolock(). At
> > this time, the pte_same() check is not performed after the PTL held. So we
> > should get pgt_pmd and do pmd_same() check after the ptl held.
> >
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> > mm/khugepaged.c | 14 +++++++++++---
> > 1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 6498721d4783a..8ab79c13d077f 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1605,7 +1605,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> >     if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
> >         pml = pmd_lock(mm, pmd);
> >
> > -    start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
> > +    start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl);
> >     if (!start_pte)        /* mmap_lock + page lock should prevent this */
> >         goto abort;
> >     if (!pml)
> > @@ -1613,6 +1613,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> >     else if (ptl != pml)
> >         spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> >
> > +    if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
> > +        goto abort;
> > +
> >     /* step 2: clear page table and adjust rmap */
> >     for (i = 0, addr = haddr, pte = start_pte;
> >          i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
> > @@ -1645,7 +1648,6 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> >         nr_ptes++;
> >     }
> >
> > -    pte_unmap(start_pte);
> >     if (!pml)
> >         spin_unlock(ptl);
> >
> > @@ -1658,13 +1660,19 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> >     /* step 4: remove empty page table */
> >     if (!pml) {
> >         pml = pmd_lock(mm, pmd);
> > -        if (ptl != pml)
> > +        if (ptl != pml) {
> >             spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> > +            if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
> > +                spin_unlock(pml);
> > +                goto abort;
>
> Drop the reference of folio and the mm counter twice at the label of abort and the step 3.

My bad, should set nr_ptes to 0 and call flush_tlb_mm() here, right?

> > +            }
> > +        }
> >     }
> >     pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd);
> >     pmdp_get_lockless_sync();
> >     if (ptl != pml)
> >         spin_unlock(ptl);
> > +    pte_unmap(start_pte);
> >     spin_unlock(pml);
>
> Why not?
>
> pte_unmap_unlock(start_pte, ptl);
> if (pml != ptl)
>          spin_unlock(pml);

Both are fine, will do.

Thanks,
Qi

> >     mmu_notifier_invalidate_range_end(&range);
> > --
> > 2.20.1