Re: [PATCH 34/46] hugetlb: userfaultfd: when using MADV_SPLIT, round addresses to PAGE_SIZE

[James Houghton]

On Fri, Jan 6, 2023 at 10:13 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Thu, Jan 05, 2023 at 10:18:32AM +0000, James Houghton wrote:
> > MADV_SPLIT enables HugeTLB HGM which allows for UFFDIO_CONTINUE in
> > PAGE_SIZE chunks. If a huge-page-aligned address were to be provided,
> > userspace would be completely unable to take advantage of HGM. That
> > would then require userspace to know to provide
> > UFFD_FEATURE_EXACT_ADDRESS.
> >
> > This patch would make it harder to make a mistake. Instead of requiring
> > userspace to provide UFFD_FEATURE_EXACT_ADDRESS, always provide a usable
> > address.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 31 +++++++++++++++----------------
> >  1 file changed, 15 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 5af6db52f34e..5b6215e03fe1 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5936,28 +5936,27 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
> >                                                 unsigned long addr,
> >                                                 unsigned long reason)
> >  {
> > +     u32 hash;
> > +     struct vm_fault vmf;
> > +
> >       /*
> >        * Don't use the hpage-aligned address if the user has explicitly
> >        * enabled HGM.
> >        */
> >       if (hugetlb_hgm_advised(vma) && reason == VM_UFFD_MINOR)
> > -             haddr = address & PAGE_MASK;
> > -
> > -     u32 hash;
> > -     struct vm_fault vmf = {
> > -             .vma = vma,
> > -             .address = haddr,
> > -             .real_address = addr,
> > -             .flags = flags,
> > +             haddr = addr & PAGE_MASK;
> >
> > -             /*
> > -              * Hard to debug if it ends up being
> > -              * used by a callee that assumes
> > -              * something about the other
> > -              * uninitialized fields... same as in
> > -              * memory.c
> > -              */
> > -     };
> > +     vmf.vma = vma;
> > +     vmf.address = haddr;
> > +     vmf.real_address = addr;
> > +     vmf.flags = flags;
>
> Const fields here:
>
> mm/hugetlb.c: In function ‘hugetlb_handle_userfault’:
> mm/hugetlb.c:5961:17: error: assignment of member ‘vma’ in read-only object
>  5961 |         vmf.vma = vma;
>       |                 ^
> mm/hugetlb.c:5962:21: error: assignment of member ‘address’ in read-only object
>  5962 |         vmf.address = haddr;
>       |                     ^
> mm/hugetlb.c:5963:26: error: assignment of member ‘real_address’ in read-only object
>  5963 |         vmf.real_address = addr;

Thanks Peter for this and your other findings. Not sure why my
compiler (clang) let me do this. :/ Will send a v2 soon with this +
the other problems fixed.