Re: [PATCH v3 0/3] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

[Stanislav Kinsburskii <skinsburskii@gmail.com>]

On Thu, May 21, 2026 at 04:33:09PM -0700, Andrew Morton wrote:
> On Wed, 20 May 2026 07:09:19 -0700 Stanislav Kinsburskii <skinsburskii@gmail.com> wrote:
> 
> > This series extends the HMM framework to support userfaultfd-backed memory
> > by allowing the mmap read lock to be dropped during hmm_range_fault().
> > 
> > Some page fault handlers — most notably userfaultfd — require the mmap lock
> > to be released so that userspace can resolve the fault. The current HMM
> > interface never sets FAULT_FLAG_ALLOW_RETRY, making it impossible to fault
> > in pages from userfaultfd-registered regions.
> > 
> > This series follows the established int *locked pattern from
> > get_user_pages_remote() in mm/gup.c. A new entry point,
> > hmm_range_fault_unlockable(), accepts an int *locked parameter. When the
> > mmap lock is dropped during fault resolution (VM_FAULT_RETRY or
> > VM_FAULT_COMPLETED), the function returns 0 with *locked = 0, signalling
> > the caller to restart its walk. The existing hmm_range_fault() is
> > refactored into a thin wrapper that passes NULL, preserving current
> > behavior for all existing callers.
> > 
> > Faulting hugetlb pages on the unlockable path is not supported because
> > walk_hugetlb_range() unconditionally holds and releases
> > hugetlb_vma_lock_read across the callback; if the mmap lock is dropped
> > inside the callback, the VMA may be freed before the walk framework's
> > unlock. Hugetlb pages already present in page tables are handled normally.
> > Possible approaches to lift this limitation are documented in
> > Documentation/mm/hmm.rst.
> 
> Thanks.  AI review identified one possible issue, possibly a duplicate
> from the v2 series?
> 
> 	https://sashiko.dev/#/patchset/177928604779.589431.14703161356676674288.stgit@skinsburskii
> 
> I'll take no action at this stage, shall await reviewer input.  Please
> poke me in a week or so if nothing has happened.
> 

Hi Andrew,

A gentle reminder as requested: do you think this change could be taken into
the mm tree?
It's beneficial not only for the MSHV driver, but can be used for
post-copy live migration of GPU states in future.

Thanks,
Stanislav

> Which is quite possible - things seem rather hectic at this time and
> we're almost at -rc5!