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 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?

> 
>> +            }
>> +        }
>>     }
>>     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