Re: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp()

[David Hildenbrand]

On 14.12.22 15:26, Peter Xu wrote:
> On Wed, Dec 14, 2022 at 11:59:35AM +0100, David Hildenbrand wrote:
> > On 08.12.22 20:46, Peter Xu wrote:
> > > This patch is a cleanup to always wr-protect pte/pmd in mkuffd_wp paths.
> > >
> > > The reasons I still think this patch is worthwhile, are:
> > >
> > >     (1) It is a cleanup already; diffstat tells.
> > >
> > >     (2) It just feels natural after I thought about this, if the pte is uffd
> > >         protected, let's remove the write bit no matter what it was.
> > >
> > >     (2) Since x86 is the only arch that supports uffd-wp, it also redefines
> > >         pte|pmd_mkuffd_wp() in that it should always contain removals of
> > >         write bits.  It means any future arch that want to implement uffd-wp
> > >         should naturally follow this rule too.  It's good to make it a
> > >         default, even if with vm_page_prot changes on VM_UFFD_WP.
> > >
> > >     (3) It covers more than vm_page_prot.  So no chance of any potential
> > >         future "accident" (like pte_mkdirty() sparc64 or loongarch, even
> > >         though it just got its pte_mkdirty fixed <1 month ago).  It'll be
> > >         fairly clear when reading the code too that we don't worry anything
> > >         before a pte_mkuffd_wp() on uncertainty of the write bit.
> >
> > Don't necessarily agree with (3). If you'd have a broken pte_mkdirty() and
> > do the pte_mkdirty() after pte_mkuffd_wp() it would still be broken. Because
> > sparc64 and loongarch are simply broken.
>
> That's why I mentioned on the order of operations matters.
>
> > > We may call pte_wrprotect() one more time in some paths (e.g. thp split),
> > > but that should be fully local bitop instruction so the overhead should be
> > > negligible.
> > >
> > > Although this patch should logically also fix all the known issues on
> > > uffd-wp too recently on either page migration or numa balancing, but this
> > > is not the plan for that fix.  So no fixes, and stable doesn't need this.
> >
> > I don't see how this would fix do_numa_page(), where we only do a
> > pte_modify().
>
> Yes, this patch won't, because it's a pure cleanup.  Otherwise we need
> another line of wr-protect in numa recover path.
>
> I can remove that sentence in v2 commit log.

Feel free to add my

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Nothing jumped at me.

--
Thanks,

David / dhildenb