[PATCH v5 3/9] mm/mremap: Use pmd/pud_poplulate to update page table entries

[Aneesh Kumar K.V]

On 5/20/21 7:48 AM, Peter Xu wrote:
> On Wed, May 19, 2021 at 10:16:07AM +0530, Aneesh Kumar K.V wrote:
>>> On Thu, Apr 22, 2021 at 11:13:17AM +0530, Aneesh Kumar K.V wrote:
>>>> pmd/pud_populate is the right interface to be used to set the respective
>>>> page table entries. Some architectures like ppc64 do assume that set_pmd/pud_at
>>>> can only be used to set a hugepage PTE. Since we are not setting up a hugepage
>>>> PTE here, use the pmd/pud_populate interface.
> 
> [1]
> 
>> Can you try this change?
>>
>> modified   mm/mremap.c
>> @@ -279,7 +279,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>>   	pmd_clear(old_pmd);
>>   
>>   	VM_BUG_ON(!pmd_none(*new_pmd));
>> -	pmd_populate(mm, new_pmd, (pgtable_t)pmd_page_vaddr(pmd));
>> +	pmd_populate(mm, new_pmd, pmd_pgtable(pmd));
>>   
>>   	if (new_ptl != old_ptl)
>>   		spin_unlock(new_ptl);
> 
> I reported this issue today somewhere else:
> 
> https://lore.kernel.org/linux-mm/YKVemB5DuSqLFmmz@t490s/
> 
> And came to this same line after the bisection.
> 
> This seems to work at least for my userfaultfd test on shmem, however I don't
> fully understand the commit message [1] on: How do we guarantee we're not
> moving a thp pte?
> 

move_page_tables() checks for pmd_trans_huge() and ends up calling 
move_huge_pmd if it is a THP entry.

-aneesh