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

[Peter Xu]

On Fri, Feb 28, 2025 at 12:53:02PM -0500, Mathieu Desnoyers wrote:
> On 2025-02-28 11:32, Peter Xu wrote:
> > On Fri, Feb 28, 2025 at 09:59:00AM -0500, Mathieu Desnoyers wrote:
> > > For the VM use-case, I wonder if we could just add a userfaultfd
> > > "COW" event that would notify userspace when a COW happens ?
> > 
> > I don't know what's the best for KSM and how well this will work, but we
> > have such event for years..  See UFFDIO_REGISTER_MODE_WP:
> > 
> > https://man7.org/linux/man-pages/man2/userfaultfd.2.html
> 
> userfaultfd UFFDIO_REGISTER only seems to work if I pass an address
> resulting from a mmap mapping, but returns EINVAL if I pass a
> page-aligned address which sits within a private file mapping
> (e.g. executable data).

Yes, so far sync traps only supports RAM-based file systems, or anonymous.
Generic private file mappings (that stores executables and libraries) are
not yet supported.

> 
> 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.

> 
> AFAIU, as it stands now userfaultfd would not help tracking COW faults
> caused by stores to private file mappings. Am I missing something ?

I think you're right.  So we have UFFD_FEATURE_WP_ASYNC that should work on
most mappings.  That one is async, though, so more like soft-dirty.  It
might be doable to try making it sync too without a lot of changes based on
how async tracking works.

Thanks,

-- 
Peter Xu