Re: [PATCH v1 5/7] mm: pgtable: try to reclaim empty PTE page in madvise(MADV_DONTNEED)

[Will Deacon <will@kernel.org>]

On Thu, Oct 17, 2024 at 08:43:43PM +0200, Jann Horn wrote:
> +arm64 maintainers in case they have opinions on the break-before-make aspects

Thanks, Jann.

> On Thu, Oct 17, 2024 at 11:48 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> > +void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
> > +                    struct mmu_gather *tlb)
> > +{
> > +       pmd_t pmdval;
> > +       spinlock_t *pml, *ptl;
> > +       pte_t *start_pte, *pte;
> > +       int i;
> > +
> > +       start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pmdval, &ptl);
> > +       if (!start_pte)
> > +               return;
> > +
> > +       pml = pmd_lock(mm, pmd);
> > +       if (ptl != pml)
> > +               spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> > +
> > +       if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd))))
> > +               goto out_ptl;
> > +
> > +       /* Check if it is empty PTE page */
> > +       for (i = 0, pte = start_pte; i < PTRS_PER_PTE; i++, pte++) {
> > +               if (!pte_none(ptep_get(pte)))
> > +                       goto out_ptl;
> > +       }
> > +       pte_unmap(start_pte);
> > +
> > +       pmd_clear(pmd);
> > +
> > +       if (ptl != pml)
> > +               spin_unlock(ptl);
> > +       spin_unlock(pml);
> 
> At this point, you have cleared the PMD and dropped the locks
> protecting against concurrency, but have not yet done a TLB flush. If
> another thread concurrently repopulates the PMD at this point, can we
> get incoherent TLB state in a way that violates the arm64
> break-before-make rule?

Sounds like it, yes, unless there's something that constrains the new
PMD value to be some function of what it was in the first place?

Will