On Thu, Jul 21, 2022 at 12:09 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Wed, Jul 20, 2022 at 01:58:06PM -0700, James Houghton wrote: > > > > > > @@ -335,12 +337,16 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm, > > > > > > copied = 0; > > > > > > page = NULL; > > > > > > vma_hpagesize = vma_kernel_pagesize(dst_vma); > > > > > > + if (use_hgm) > > > > > > + vma_altpagesize = PAGE_SIZE; > > > > > > > > > > Do we need to check the "len" to know whether we should use sub-page > > > > > mapping or original hpage size? E.g. any old UFFDIO_CONTINUE code will > > > > > still want the old behavior I think. > > > > > > > > I think that's a fair point; however, if we enable HGM and the address > > > > and len happen to be hstate-aligned > > > > > > The address can, but len (note! not "end" here) cannot? > > > > They both (dst_start and len) need to be hpage-aligned, otherwise we > > won't be able to install hstate-sized PTEs. Like if we're installing > > 4K at the beginning of a 1G hpage, we can't install a PUD, because we > > only want to install that 4K. > > I'm still confused... > > Shouldn't one of the major goals of sub-page mapping is to grant user the > capability to do UFFDIO_CONTINUE with len<hpagesize (so we install pages in > sub-page level)? If so, why len needs to be always hpagesize aligned? Sorry I misunderstood what you were asking. We allow both to be PAGE_SIZE-aligned. :) That is indeed the goal of HGM. If dst_start and len were both hpage-aligned, then we *could* set `use_hgm = false`, and everything would still work. That's what I thought you were asking about. I don't see any reason to do this though, as `use_hgm = true` will only grant additional functionality, and `use_hgm = false` would only -- at best -- be a minor performance optimization in this case. - James > > -- > Peter Xu >
