Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

[Qi Zheng]

On 2024/9/5 14:32, Muchun Song wrote:
> > On Aug 30, 2024, at 14:54, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
> >
> >
> >
> > On 2024/8/29 16:10, Muchun Song wrote:
> > > On 2024/8/22 15:13, Qi Zheng 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 write lock of mmap_lock is not held, and 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.
> > > >
> > > > For the case where the ptl is released first and then the pml is acquired,
> > > > the PTE page may have been freed, so we must do pmd_same() check before
> > > > reacquiring the ptl.
> > > >
> > > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > > > ---
> > > >   mm/khugepaged.c | 16 +++++++++++++++-
> > > >   1 file changed, 15 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > index 53bfa7f4b7f82..15d3f7f3c65f2 100644
> > > > --- a/mm/khugepaged.c
> > > > +++ b/mm/khugepaged.c
> > > > @@ -1604,7 +1604,7 @@ 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)
> > > > @@ -1612,6 +1612,9 @@ 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++) {
> > > > @@ -1657,6 +1660,16 @@ 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);
> > > > +        /*
> > > > +         * We called pte_unmap() and release the ptl before acquiring
> > > > +         * the pml, which means we left the RCU critical section, so the
> > > > +         * PTE page may have been freed, so we must do pmd_same() check
> > > > +         * before reacquiring the ptl.
> > > > +         */
> > > > +        if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
> > > > +            spin_unlock(pml);
> > > > +            goto pmd_change;
> > > Seems we forget to flush TLB since we've cleared some pte entry?
> >
> > See comment above the ptep_clear():
> >
> > /*
> > * Must clear entry, or a racing truncate may re-remove it.
> > * TLB flush can be left until pmdp_collapse_flush() does it.
> > * PTE dirty? Shmem page is already dirty; file is read-only.
> > */
> >
> > The TLB flush was handed over to pmdp_collapse_flush(). If a
>
> But you skipped pmdp_collapse_flush().

I skip it only in !pmd_same() case, at which time it must be cleared
by other thread, which will be responsible for flushing TLB:

CPU 0				CPU 1
				pmd_clear
				spin_unlock
				flushing tlb
spin_lock
if (!pmd_same)	
	goto pmd_change;
pmdp_collapse_flush

Did I miss something?

> > concurrent thread free the PTE page at this time, the TLB will
> > also be flushed after pmd_clear().
> >
> > > > +        }
> > > >           if (ptl != pml)
> > > >               spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> > > >       }
> > > > @@ -1688,6 +1701,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> > > >           pte_unmap_unlock(start_pte, ptl);
> > > >       if (pml && pml != ptl)
> > > >           spin_unlock(pml);
> > > > +pmd_change:
> > > >       if (notified)
> > > >           mmu_notifier_invalidate_range_end(&range);
> > > >   drop_folio: