Re: [PATCH v4 07/13] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

[Muchun Song <muchun.song@linux.dev>]

> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@bytedance.com> 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@bytedance.com>
> ---
> 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.

> +            }
> +        }
>    }
>    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);

> 
>    mmu_notifier_invalidate_range_end(&range);
> --
> 2.20.1