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

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

Jason