Re: [RFC PATCH 20/26] hugetlb: add support for high-granularity UFFDIO_CONTINUE

[James Houghton]

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
>