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

[Matthew Wilcox]

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.