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

[Peter Xu]

On Thu, Jul 21, 2022 at 12:44:58PM -0700, James Houghton wrote:
> 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.

Ah OK. :)

> 
> 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.

I just want to make sure this patch won't break existing uffd-minor users,
or it'll be an kernel abi breakage.

We'd still want to have e.g. existing compiled apps run like before, which
iiuc means we should only use sub-page mapping when len!=hpagesize here.

I'm not sure it's only about perf - the app may not even be prepared to
receive yet another page faults within the same huge page range.

-- 
Peter Xu