[LSF/MM/BPF TOPIC] HugeTLB file preservation across live update

[Pratyush Yadav <pratyush@kernel.org>]

Hi all,

I would like to propose the topic of HugeTLB file preservation across
live update for discussion at the upcoming LSF/MM/BPF summit.

Background
==========

Support for performing a live update was recently added to the kernel
via the Live Update Orchestrator (LUO) subsystem [0]. Live update allows
a full kernel update over kexec without terminating running workloads.
The main target is live updating the hypervisor without terminating VMs.

For more information on live update and LUO, see its kernel
documentation [1], the LUO patchset [0], or this LPC 2025 presentation
[2].

MEMFD preservation
==================

MEMFD is the first file type that added support for live update. This
support was merged with the LUO series. The memfd preservation code can
be found in mm/memfd_luo.c [3].

MEMFD can use pages backed by shmem or HugeTLB. The live update support
in mainline currently only works with shmem-backed MEMFDs.

HugeTLB preservation
====================

While shmem support for live update is useful for many things, using it
for VM memory (and other memory-intensive workloads) is inefficient. The
next step for MEMFD support for live update is to allow preserving
HugeTLB-backed files.

Note that we do not attempt to preserve the whole HugeTLB file system.
We only preserve the memory pages for the files specified by userspace.
The HugeTLB file is re-created fresh on the next kernel and the pages
are inserted back in. This lets us keep the maintenance footprint
smaller.

Current status
--------------

This support was sent out on the list as an RFC [4] in December 2025,
and was also presented at LPC 2025 [5] in the Live Update
microconference.

While the patchset has got some initial reviews from live update
maintainers, it has not yet had any review from HugeTLB developers or
maintainers.

Goals of the discussion
-----------------------

This session will aim to present the topic, explain why it is needed,
show how the support works, and most importantly, collect feedback on
how to proceed with this work.

PS: if any HugeTLB developers or maintainers are reading this, it would
be great to have your initial thoughts on the RFC [4] so I can present a
more polished patchset at the conference and make the most of the
discussion.

Key attendees
=============

Live update:
  - Alexander Graf
  - David Matlack
  - Jason Gunthorpe
  - Mike Rapoport
  - Pasha Tatashin

HugeTLB:
  - David Hildenbrand
  - Muchun Song
  - Oscar Salvador

References
==========

[0] https://lore.kernel.org/linux-mm/20251125165850.3389713-1-pasha.tatashin@soleen.com/T/#u
[1] https://docs.kernel.org/core-api/liveupdate.html
[2] https://lpc.events/event/19/contributions/2052/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memfd_luo.c
[4] https://lore.kernel.org/linux-mm/20251206230222.853493-1-pratyush@kernel.org/T/#u
[5] https://lpc.events/event/19/contributions/2044/

-- 
Regards,
Pratyush Yadav