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

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

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
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: