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    SubjectRe: [PATCH 34/46] hugetlb: userfaultfd: when using MADV_SPLIT, round addresses to PAGE_SIZE
    On Fri, Jan 6, 2023 at 10:13 AM Peter Xu <peterx@redhat.com> 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@google.com>
    > > ---
    > > 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.

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