Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging

[Peter Xu]

On Sat, Mar 01, 2025 at 10:44:22AM -0500, Mathieu Desnoyers wrote:
> > > Also, I notice that do_wp_page() only calls handle_userfault
> > > VM_UFFD_WP when vm_fault flags does not have FAULT_FLAG_UNSHARE
> > > set.
> > 
> > AFAICT that's expected, unshare should only be set on reads, never writes.
> > So uffd-wp shouldn't trap any of those.
> 
> I'm confused by your comment. I thought unshare only applies to
> *write* faults. What am I missing ?

The major path so far to set unshare is here in GUP (ignoring two corner
cases used in either s390 and ksm):

	if (unshare) {
		fault_flags |= FAULT_FLAG_UNSHARE;
		/* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */
		VM_BUG_ON(fault_flags & FAULT_FLAG_WRITE);
	}

See the VM_BUG_ON() - if it's write it'll crash already.

"unshare", in its earliest form of patch, used to be called COR
(Copy-On-Read), which might be more straightforward in this case.. so it's
the counterpart of COW but for read cases where a copy is required. The
patchset that introduced it has more information (e.g. a7f2266041).

Thanks,

-- 
Peter Xu