Re: [PATCH v5 05/10] userfaultfd: add minor fault registration mode

[Peter Xu]

On Fri, Feb 12, 2021 at 10:21:45PM +0000, Matthew Wilcox wrote:
> On Thu, Feb 11, 2021 at 11:28:09AM -0800, Axel Rasmussen wrote:
> > Ah, I had added this just after VM_UFFD_WP, without noticing that this
> > would be sharing a bit with VM_LOCKED. That seems like not such a
> > great idea.
> > 
> > I don't see another unused bit, and I don't see some other obvious
> > candidate to share with. So, the solution that comes to mind is
> 
> it'd be even better if you didn't use the last unused bit for UFFD_WP.
> not sure how feasible that is, but you can see we're really short on
> bits here.

UFFD_WP is used now for anonymouse already.. And the support for hugetlbfs and
shmem is in rfc stage on the list.

Is it possible to use CONFIG_ARCH_USES_HIGH_VMA_FLAGS here?  So far uffd-wp is
only working for 64 bit x86 too due to enlarged pte space.  Maybe we can also
let minor mode to only support 64 bit hosts.

Thanks,

-- 
Peter Xu