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

[Wei Yang]

On Fri, Nov 29, 2019 at 03:18:01AM -0800, Matthew Wilcox wrote:
>On Fri, Nov 29, 2019 at 04:30:02PM +0800, Wei Yang wrote:
>> On Thu, Nov 28, 2019 at 02:39:04PM -0800, Matthew Wilcox wrote:
>> >On Thu, Nov 28, 2019 at 09:09:45PM +0000, Wei Yang wrote:
>> >> 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.
>> >
>> >mremap() can also move a mapping, see MREMAP_FIXED.
>> 
>> Hi, Matthew
>> 
>> Thanks for your comment.
>> 
>> I took a look into the MREMAP_FIXED case, but still not clear in which case it
>> fall into the situation Kirill mentioned.
>> 
>> Per my understanding, move mapping is achieved in two steps:
>> 
>>     * unmap some range in old vma if old_len >= new_len
>>     * move vma
>> 
>> If the length doesn't change, we are expecting to have the "copy" of old
>> vma. This doesn't change the THP PMD mapping.
>> 
>> So the change still happens in the unmap step, if I am correct.
>> 
>> Would you mind giving me more hint on the case when we would have the
>> situation as Kirill mentioned?
>
>Set up a THP mapping.
>Move it to an address which is no longer 2MB aligned.
>Unmap it.

Thanks Matthew

I got the point, thanks a lot :-)

-- 
Wei Yang
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