Re: [PATCH 16/23] hugetlb/userfaultfd: Take care of UFFDIO_COPY_MODE_WP

Peter Xu
2021-04-22T01:14:29Z

On Wed, Apr 21, 2021 at 04:06:39PM -0700, Mike Kravetz wrote:
> On 3/22/21 5:49 PM, Peter Xu wrote:
> > Firstly, pass the wp_copy variable into hugetlb_mcopy_atomic_pte() thoughout
> > the stack. Then, apply the UFFD_WP bit if UFFDIO_COPY_MODE_WP is with
> > UFFDIO_COPY. Introduce huge_pte_mkuffd_wp() for it.
> >
> > Note that similar to how we've handled shmem, we'd better keep setting the
> > dirty bit even if UFFDIO_COPY_MODE_WP is provided, so that the core mm will
> > know this page contains valid data and never drop it.
>
> There is nothing wrong with setting the dirty bit in this manner to be
> consistent. But, since hugetlb pages are only managed by hugetlbfs, the
> core mm will not drop them.

Right, for this paragraph, how about I rephrase it as below?

Hugetlb pages are only managed by hugetlbfs, so we're safe even without
setting dirty bit in the huge pte if the page is installed as read-only.
However we'd better still keep the dirty bit set for a read-only UFFDIO_COPY
pte (when UFFDIO_COPY_MODE_WP bit is set), not only to match what we do with
shmem, but also because the page does contain dirty data that the kernel just
copied from the userspace.

>
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > include/asm-generic/hugetlb.h | 5 +++++
> > include/linux/hugetlb.h | 6 ++++--
> > mm/hugetlb.c | 22 +++++++++++++++++-----
> > mm/userfaultfd.c | 12 ++++++++----
> > 4 files changed, 34 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
> > index 8e1e6244a89d..548212eccbd6 100644
> > --- a/include/asm-generic/hugetlb.h
> > +++ b/include/asm-generic/hugetlb.h
> > @@ -27,6 +27,11 @@ static inline pte_t huge_pte_mkdirty(pte_t pte)
> > return pte_mkdirty(pte);
> > }
> >
> > +static inline pte_t huge_pte_mkuffd_wp(pte_t pte)
> > +{
> > + return pte_mkuffd_wp(pte);
> > +}
> > +
>
> Just want to verify that userfaultfd wp support is only enabled for
> x86_64 now? I only ask because there are arch specific hugetlb pte
> manipulation routines for some architectures.

Yes it's x86_64 only, as we have:

select HAVE_ARCH_USERFAULTFD_WP if X86_64 && USERFAULTFD

Here the helper defines the huge pte uffd-wp bit to be the same as the pte
level bit, across all archs. However should be fine since for any arch that
does not support uffd-wp, it'll be no-op:

static __always_inline pte_t pte_mkuffd_wp(pte_t pte)
{
return pte;
}

Meanwhile it shouldn't be called anywhere as well.

Here I can also define __HAVE_ARCH_HUGE_PTE_MKUFFD_WP, however I didn't do so
as I thought above should be enough. Then we can define it when really
necessary.

>
> > static inline pte_t huge_pte_modify(pte_t pte, pgprot_t newprot)
> > {
> > return pte_modify(pte, newprot);
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index a7f7d5f328dc..ef8d2b8427b1 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -141,7 +141,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte,
> > unsigned long dst_addr,
> > unsigned long src_addr,
> > enum mcopy_atomic_mode mode,
> > - struct page **pagep);
> > + struct page **pagep,
> > + bool wp_copy);
> > #endif /* CONFIG_USERFAULTFD */
> > bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
> > struct vm_area_struct *vma,
> > @@ -321,7 +322,8 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > unsigned long dst_addr,
> > unsigned long src_addr,
> > enum mcopy_atomic_mode mode,
> > - struct page **pagep)
> > + struct page **pagep,
> > + bool wp_copy)
> > {
> > BUG();
> > return 0;
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index def2c7ddf3ae..f0e55b341ebd 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4725,7 +4725,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > unsigned long dst_addr,
> > unsigned long src_addr,
> > enum mcopy_atomic_mode mode,
> > - struct page **pagep)
> > + struct page **pagep,
> > + bool wp_copy)
> > {
> > bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
> > struct address_space *mapping;
> > @@ -4822,17 +4823,28 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > hugepage_add_new_anon_rmap(page, dst_vma, dst_addr);
> > }
> >
> > - /* For CONTINUE on a non-shared VMA, don't set VM_WRITE for CoW. */
> > - if (is_continue && !vm_shared)
> > + /*
> > + * For either: (1) CONTINUE on a non-shared VMA, or (2) UFFDIO_COPY
> > + * with wp flag set, don't set pte write bit.
> > + */
> > + if (wp_copy || (is_continue && !vm_shared))
> > writable = 0;
> > else
> > writable = dst_vma->vm_flags & VM_WRITE;
> >
> > _dst_pte = make_huge_pte(dst_vma, page, writable);
> > - if (writable)
> > - _dst_pte = huge_pte_mkdirty(_dst_pte);
> > + /*
> > + * Always mark UFFDIO_COPY page dirty; note that this may not be
> > + * extremely important for hugetlbfs for now since swapping is not
> > + * supported, but we should still be clear in that this page cannot be
> > + * thrown away at will, even if write bit not set.
>
> As mentioned earlier there should not be any issue with hugetlb pages
> being thrown away without dirty set. Perhaps, the comment should
> reflect that this is mostly for consistency.

Yes, functional-wise it seems not necessary, however I also think it's a bit
more than being consistency with what we do with shmem (as also rephrased in
above commit message): UFFDIO_COPY page should be marked as dirty by the
definition of "dirty bit", imho, since the data is indeed dirty (kernel copied
that data from userspace)?

>
> Note to self: this may help when I get back to hugetlb soft dirty
> support.
>
> Other than that, patch looks good.

Thanks!

--
Peter Xu