Re: [PATCH v3 09/14] mm: mremap: move_ptes() use pte_offset_map_rw_nolock()

[Qi Zheng]

On 2024/9/5 17:25, Muchun Song wrote:
> On 2024/9/4 16:40, Qi Zheng wrote:
> > In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so
> > convert it to using pte_offset_map_rw_nolock(). Since we may free the PTE
> > page in retract_page_tables() without holding the read lock of mmap_lock,
> > so we still need to do a pmd_same() check after holding the PTL.
>
> retract_page_tables() and move_ptes() are synchronized with
> i_mmap_lock, right?

Right, will remove the pmd_same() check in v4. Thanks!

> Muchun,
> Thanks.
>
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   mm/mremap.c | 20 ++++++++++++++++++--
> >   1 file changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 24712f8dbb6b5..16e54151395ad 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, 
> > pmd_t *old_pmd,
> >       spinlock_t *old_ptl, *new_ptl;
> >       bool force_flush = false;
> >       unsigned long len = old_end - old_addr;
> > +    pmd_t pmdval;
> >       int err = 0;
> >       /*
> > @@ -175,14 +176,29 @@ static int move_ptes(struct vm_area_struct *vma, 
> > pmd_t *old_pmd,
> >           err = -EAGAIN;
> >           goto out;
> >       }
> > -    new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl);
> > +    /*
> > +     * Since we may free the PTE page in retract_page_tables() without
> > +     * holding the read lock of mmap_lock, so we still need to do a
> > +     * pmd_same() check after holding the PTL.
> > +     */
> > +    new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &pmdval,
> > +                       &new_ptl);
> >       if (!new_pte) {
> >           pte_unmap_unlock(old_pte, old_ptl);
> >           err = -EAGAIN;
> >           goto out;
> >       }
> > -    if (new_ptl != old_ptl)
> > +    if (new_ptl != old_ptl) {
> >           spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> > +
> > +        if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(new_pmd)))) {
> > +            pte_unmap_unlock(new_pte, new_ptl);
> > +            pte_unmap_unlock(old_pte, old_ptl);
> > +            err = -EAGAIN;
> > +            goto out;
> > +        }
> > +    }
> > +
> >       flush_tlb_batched_pending(vma->vm_mm);
> >       arch_enter_lazy_mmu_mode();