Re: [PATCH 2/2] mm/page_vma_mapped: page table boundary is already guaranteed

[Wei Yang]

On Thu, Nov 28, 2019 at 11:31:43AM +0300, Kirill A. Shutemov wrote:
>On Thu, Nov 28, 2019 at 09:03:21AM +0800, Wei Yang wrote:
>> The check here is to guarantee pvmw->address iteration is limited in one
>> page table boundary. To be specific, here the address range should be in
>> one PMD_SIZE.
>> 
>> If my understanding is correct, this check is already done in the above
>> check:
>> 
>>     address >= __vma_address(page, vma) + PMD_SIZE
>> 
>> The boundary check here seems not necessary.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>
>NAK.
>
>THP can be mapped with PTE not aligned to PMD_SIZE. Consider mremap().
>

Hi, Kirill

Thanks for your comment during Thanks Giving Day. Happy holiday:-)

I didn't think about this case before, thanks for reminding. Then I tried to
understand your concern.

mremap() would expand/shrink a memory mapping. In this case, probably shrink
is in concern. Since pvmw->page and pvmw->vma are not changed in the loop, the
case you mentioned maybe pvmw->page is the head of a THP but part of it is
unmapped.

This means the following condition stands:

    vma->vm_start <= vma_address(page) 
    vma->vm_end <=   vma_address(page) + page_size(page)

Since we have checked address with vm_end, do you think this case is also
guarded?

Not sure my understanding is correct, look forward your comments.

>> Test:
>>    more than 48 hours kernel build test shows this code is not touched.
>
>Not an argument. I doubt mremap(2) is ever called in kernel build
>workload.
>
>-- 
> Kirill A. Shutemov

-- 
Wei Yang
Help you, Help me