Re: [PATCH v7 2/8] mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support

[Stanislav Kinsburskii <skinsburskii@gmail.com>]

On Fri, Jul 10, 2026 at 03:03:12PM -0300, Jason Gunthorpe wrote:
> On Fri, Jul 10, 2026 at 10:02:55AM -0700, Stanislav Kinsburskii wrote:
> > The main customer for this new feature I have in mind is the MSHV driver
> > which backs VMs memory with HMM, requires userfaultfd support for
> > post-copy live migration and fast restore and it doesn't timeout.
> > 
> > I agree, that this current timeout value used by the other callers might
> > not be enough to repopulate the mappings with userfaultfd, but there
> > drivers would get -EFAULT for uderfaultfd-backed mappings without this
> > change anyway, so getting -EBUSY with the change instead doesn't look
> > like a significant change to the behaviour from my POV.
> 
> It sounds like it won't be reliable either then.
> 
> > > So, maybe the deadline should be resetting after every handled fault?
> > > ie the timeout really is only about the mmu notifier and we don't
> > > count the time spent handling faults or walking?
> >
> > The timeout was inherited from existing HMM users rather than introduced
> > as a new HMM policy. Some GPU drivers use HMM_RANGE_DEFAULT_TIMEOUT as a
> > budget for the whole range population operation, including HMM retries
> > and subsequent driver mapping work.
> 
> Yes, because we always had a timeout around the notifier because that
> scheme can sort of live lock. The timeout was to protect that only, ie
> limit the number of notifier retries.
> 
> Expanding the timeout to be outside what is bounded by the notifier
> retry is not right, and heavy stuff like mapping should be done after
> the hmm side succeeds and the notifiers concluded so they can rely on
> normal locking instead.
> 
> This is why I'm suggesting to reset the deadline as hmm makes forward
> progress, we really only want to bound the notifier retry loop not
> anything else.
> 

Sure, I'll modify accordingly.

Thanks,
Stanislav

> Jason