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.
