ARM64: Adding write-protect bit for Userfaultfd

[Catalin Marinas]

On Wed, May 17, 2023 at 11:43:29AM -0400, Mohamed Husain Noor Mohamed wrote:
> On Tue, May 2, 2023 at 9:36 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > On Fri, Apr 28, 2023 at 01:35:44PM -0400, Mohamed Husain Noor Mohamed wrote:
> > > I am Mohamed Husain. I am a graduate student, working on a research
> > > project using a userfaultfd for distributed shared memory.
> > > We are trying to use Write-Protect mode in ARM64 but based on the
> > > kernel commits we see the support only exists for the x86 kernel.
> > > https://lwn.net/Articles/777258/
> > >
> > > I am trying to add the write-protect support, so I am looking for the
> > > unused bits in the PTE. Do you guys have any suggestions on the bits I
> > > could use, or does it require hardware support?
> >
> > Unfortunately, we are pretty short on bits. The architecture only gives
> > us bits 55 to 58 and they are all used. We could move PTE_PROT_NONE to
> > another position (e.g. 60) since this is only used when !PTE_VALID and
> > therefore doesn't affect the actual page attributes and free up a bit.
> >
> > Alternatively, we could hijack bits 59-62 but there may be out of tree
> > patches making use of the PBHA imp def feature (AFAICT disabled on the
> > mainline kernel). Well, I guess one could make the userfaultfd wp
> > feature conditional.
> 
> We were able to implement the feature with bits 60 and 61. As you
> mentioned we made it a conditional feature.
> 
> PTE_UFFD_WP -> 60

I forgot about permission overlays (FEAT_POE in the 2022 architecture
extensions), Joey mentioned this already. We have bit 59 left as
currently we don't use the Attribute Index Extension (part of the same
2022 spec).

> PTE_SWP_UFFD_WP -> 61

Presumably this is only checked on swap ptes. Can you not use the same
bit, 59?

> We want to submit the patch(pull request) since our research requires
> this feature in the vanilla kernel. Could you please share the steps
> for that?

Documentation/process/submitting-patches.rst is a good starting point ;).

The patches would need to be reviewed on the list first (e.g. on
linux-arm-kernel) together with justification since they are using
precious spare bits in the pte. In the past we turned down the use of
a spare bit for soft-dirty ptes (needed by CRIU), though the suggestion
was to rely on UFFD (and it looks like this one may need a bit as well).

-- 
Catalin