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 16:52, Muchun Song wrote:
> > On Sep 24, 2024, at 15:29, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
> >
> >
> >
> > 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?
>
> Or add a new label "out" just below the "abort". Then go to out.

For this way, we also need to call flush_tlb_mm() first, like the
following:

if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
	spin_unlock(pml);
	flush_tlb_mm(mm);
	goto out;
}

> > > > +            }
> > > > +        }
> > > >     }
> > > >     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