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

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2024/9/24 16:52, Muchun Song wrote:
> 
> 
>> On Sep 24, 2024, at 15:29, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>
>>
>>
>> On 2024/9/24 15:14, Muchun Song wrote:
>>>> 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.
>>
>> My bad, should set nr_ptes to 0 and call flush_tlb_mm() here, right?
> 
> Or add a new label "out" just below the "abort". Then go to out.

For this way, we also need to call flush_tlb_mm() first, like the
following:

if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
	spin_unlock(pml);
	flush_tlb_mm(mm);
	goto out;
}

> 
>>
>>>> +            }
>>>> +        }
>>>>     }
>>>>     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);
>>
>> Both are fine, will do.
>>
>> Thanks,
>> Qi
>>
>>>>
>>>>     mmu_notifier_invalidate_range_end(&range);
>>>> --
>>>> 2.20.1
> 
>