Re: [PATCH v4 1/8] mm: make PTE_MARKER_SWAPIN_ERROR more general

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Jul 10, 2023 at 10:19 AM Axel Rasmussen
<axelrasmussen@google.com> wrote:
>
> On Sat, Jul 8, 2023 at 6:08 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Fri,  7 Jul 2023 14:55:33 -0700 Axel Rasmussen <axelrasmussen@google.com> wrote:
> >
> > > Future patches will re-use PTE_MARKER_SWAPIN_ERROR to implement
> > > UFFDIO_POISON, so make some various preparations for that:
> > >
> > > First, rename it to just PTE_MARKER_POISONED. The "SWAPIN" can be
> > > confusing since we're going to re-use it for something not really
> > > related to swap. This can be particularly confusing for things like
> > > hugetlbfs, which doesn't support swap whatsoever. Also rename some
> > > various helper functions.
> > >
> > > Next, fix pte marker copying for hugetlbfs. Previously, it would WARN on
> > > seeing a PTE_MARKER_SWAPIN_ERROR, since hugetlbfs doesn't support swap.
> > > But, since we're going to re-use it, we want it to go ahead and copy it
> > > just like non-hugetlbfs memory does today. Since the code to do this is
> > > more complicated now, pull it out into a helper which can be re-used in
> > > both places. While we're at it, also make it slightly more explicit in
> > > its handling of e.g. uffd wp markers.
> > >
> > > For non-hugetlbfs page faults, instead of returning VM_FAULT_SIGBUS for
> > > an error entry, return VM_FAULT_HWPOISON. For most cases this change
> > > doesn't matter, e.g. a userspace program would receive a SIGBUS either
> > > way. But for UFFDIO_POISON, this change will let KVM guests get an MCE
> > > out of the box, instead of giving a SIGBUS to the hypervisor and
> > > requiring it to somehow inject an MCE.
> > >
> > > Finally, for hugetlbfs faults, handle PTE_MARKER_POISONED, and return
> > > VM_FAULT_HWPOISON_LARGE in such cases. Note that this can't happen today
> > > because the lack of swap support means we'll never end up with such a
> > > PTE anyway, but this behavior will be needed once such entries *can*
> > > show up via UFFDIO_POISON.
> > >
> > > --- a/include/linux/mm_inline.h
> > > +++ b/include/linux/mm_inline.h
> > > @@ -523,6 +523,25 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
> > >       return atomic_read(&mm->tlb_flush_pending) > 1;
> > >  }
> > >
> > > +/*
> > > + * Computes the pte marker to copy from the given source entry into dst_vma.
> > > + * If no marker should be copied, returns 0.
> > > + * The caller should insert a new pte created with make_pte_marker().
> > > + */
> > > +static inline pte_marker copy_pte_marker(
> > > +             swp_entry_t entry, struct vm_area_struct *dst_vma)
> > > +{
> > > +     pte_marker srcm = pte_marker_get(entry);
> > > +     /* Always copy error entries. */
> > > +     pte_marker dstm = srcm & PTE_MARKER_POISONED;
> > > +
> > > +     /* Only copy PTE markers if UFFD register matches. */
> > > +     if ((srcm & PTE_MARKER_UFFD_WP) && userfaultfd_wp(dst_vma))
> > > +             dstm |= PTE_MARKER_UFFD_WP;
> > > +
> > > +     return dstm;
> > > +}
> >
> > Breaks the build with CONFIG_MMU=n (arm allnoconfig).  pte_marker isn't
> > defined.
> >
> > I'll slap #ifdef CONFIG_MMU around this function, but probably somethng more
> > fine-grained could be used, like CONFIG_PTE_MARKER_UFFD_WP.  Please
> > consider.
>
> Whoops, sorry about this. This function "ought" to be in
> include/linux/swapops.h where it would be inside a #ifdef CONFIG_MMU
> anyway, but it can't be because it uses userfaultfd_wp() so there'd be
> a circular include. I think just wrapping it in CONFIG_MMU is the
> right way.
>
> But, this has also made me realize we need to not advertise
> UFFDIO_POISON as supported unless we have CONFIG_MMU. I don't want
> HAVE_ARCH_USERFAULTFD_WP for that, because it's only enabled on
> x86_64, whereas I want to support at least arm64 as well. I don't see
> a strong reason not to just use CONFIG_MMU for this too; this feature
> depends on the API in swapops.h, which uses that ifdef, so I don't see
> a lot of value out of creating a new but equivalent config option.

Actually, I'm being silly. CONFIG_USERFAULTFD depends on CONFIG_MMU,
so we don't need to worry about most of this.

Andrew's fix to just wrap the helper in CONFIG_MMU is enough.

>
> I'll make the needed changes (and also address Peter's comment above)
> and send out a v5.
>
> >
> > btw, both copy_pte_marker() and pte_install_uffd_wp_if_needed() look
> > far too large to justify inlining.  Please review the desirability of
> > this.

As far as inlining goes, I'm not opposed to un-inlining this, I was
mainly copying that pattern from existing helpers in swapops.h.

One question is, if it weren't inline, where should it go? There is no
mm/swapops.c which I would say is otherwise the proper place for it. I
don't see any other good place for the functions to go. The one I'm
introducing isn't userfaultfd-specific so userfaultfd.c seems wrong.

> >
> >