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

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Message-ID: <YtmkX73/wzptkZbu@xz-m1.local>
Date:   Thu, 21 Jul 2022 15:09:19 -0400
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 20/26] hugetlb: add support for high-granularity
 UFFDIO_CONTINUE

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?

-- 
Peter Xu