Re: [PATCH v3 08/14] mm: copy_pte_range() use pte_offset_map_rw_nolock()

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

On 2024/9/5 16:57, Muchun Song wrote:
> 
> 
> On 2024/9/4 16:40, Qi Zheng wrote:
>> In copy_pte_range(), we may modify the src_pte entry after holding the
>> src_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 get pmdval and do pmd_same() check 
>> after
>> the ptl is held.
> 
> See commit 3db82b9374ca92, copy_pte_range and retract_page_tables
> are using vma->anon_vma to be exclusive.
> 
> retract_page_tables()                    copy_page_range()
>      vma_interval_tree_foreach()              if (!vma_needs_copy())
>          if (READ_ONCE(vma->anon_vma))            return 0;
>              continue;                        copy_pte_range()
> 
> So I think mmap write lock here is also used for keeping ->anon_vma stable.
> And we do not need pmd_same().

Indeed, will change it in v4. Thanks!

> 
> Muchun,
> Thanks.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>> Hi Muchun, since the code has changed, I dropped your Reviewed-by tag 
>> here.
>>
>>   mm/memory.c | 18 +++++++++++++++++-
>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 06674f94b7a4e..47974cc4bd7f2 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1082,6 +1082,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, 
>> struct vm_area_struct *src_vma,
>>       struct mm_struct *src_mm = src_vma->vm_mm;
>>       pte_t *orig_src_pte, *orig_dst_pte;
>>       pte_t *src_pte, *dst_pte;
>> +    pmd_t pmdval;
>>       pte_t ptent;
>>       spinlock_t *src_ptl, *dst_ptl;
>>       int progress, max_nr, ret = 0;
>> @@ -1107,13 +1108,28 @@ copy_pte_range(struct vm_area_struct *dst_vma, 
>> struct vm_area_struct *src_vma,
>>           ret = -ENOMEM;
>>           goto out;
>>       }
>> -    src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_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.
>> +     */
>> +    src_pte = pte_offset_map_rw_nolock(src_mm, src_pmd, addr, &pmdval,
>> +                       &src_ptl);
>>       if (!src_pte) {
>>           pte_unmap_unlock(dst_pte, dst_ptl);
>>           /* ret == 0 */
>>           goto out;
>>       }
>>       spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>> +
>> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(src_pmd)))) {
>> +        pte_unmap_unlock(src_pte, src_ptl);
>> +        pte_unmap_unlock(dst_pte, dst_ptl);
>> +        /* ret == 0 */
>> +        goto out;
>> +    }
>> +
>>       orig_src_pte = src_pte;
>>       orig_dst_pte = dst_pte;
>>       arch_enter_lazy_mmu_mode();
>