Re: [RFC PATCH 06/10] liveupdate: hugetlb subsystem FLB state preservation

Pasha Tatashin
2025-12-30T16:37:58Z

On Mon, Dec 29, 2025 at 4:21 PM Pratyush Yadav <pratyush@kernel.org> wrote:
>
> On Tue, Dec 23 2025, Pasha Tatashin wrote:
>
> > On Sat, Dec 6, 2025 at 6:03 PM Pratyush Yadav <pratyush@kernel.org> wrote:
> >>
> >> HugeTLB manages its own pages. It allocates them on boot and uses those
> >> to fulfill hugepage requests.
> >>
> >> To support live update for a hugetlb-backed memfd, it is necessary to
> >> track how many pages of each hstate are coming from live update. This is
> >> needed to ensure the boot time allocations don't over-allocate huge
> >> pages, causing the rest of the system unexpected memory pressure.
> >>
> >> For example, say the system has 100G memory and it uses 90 1G huge
> >> pages, with 10G put aside for other processes. Now say 5 of those pages
> >> are preserved via KHO for live updating a huge memfd.
> >>
> >> But during boot, the system will still see that it needs 90 huge pages,
> >> so it will attempt to allocate those. When the file is later retrieved,
> >> those 5 pages also get added to the huge page pool, resulting in 95
> >> total huge pages. This exceeds the original expectation of 90 pages, and
> >> ends up wasting memory.
> >>
> >> LUO has file-lifecycle-bound (FLB) data to keep track of global state of
> >> a subsystem. Use it to track how many huge pages are used up for each
> >> hstate. When a file is preserved, it will increment to the counter, and
> >> when it is unpreserved, it will decrement it. During boot time
> >> allocations, this data can be used to calculate how many hugepages
> >> actually need to be allocated.
> >>
> >> Design note: another way of doing this would be to preserve the entire
> >> set of hugepages using the FLB, skip boot time allocation, and restore
> >> them all on FLB retrieve. The pain problem with that approach is that it
> >> would need to freeze all hstates after serializing them. This will need
> >> a lot more invasive changes in hugetlb since there are many ways folios
> >> can be added to or removed from a hstate. Doing it this way is simpler
> >> and less invasive.
> >>
> >> Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
> >> ---
> >> Documentation/mm/memfd_preservation.rst | 9 ++
> >> MAINTAINERS | 1 +
> >> include/linux/kho/abi/hugetlb.h | 66 +++++++++
> >> kernel/liveupdate/Kconfig | 12 ++
> >> mm/Makefile | 1 +
> >> mm/hugetlb.c | 1 +
> >> mm/hugetlb_internal.h | 15 ++
> >> mm/hugetlb_luo.c | 179 ++++++++++++++++++++++++
> >> 8 files changed, 284 insertions(+)
> >> create mode 100644 include/linux/kho/abi/hugetlb.h
> >> create mode 100644 mm/hugetlb_luo.c
> >>
> [...]
> >> +static int hugetlb_flb_retrieve(struct liveupdate_flb_op_args *args)
> >> +{
> >> + /*
> >> + * The FLB is only needed for boot-time calculation of how many
> >> + * hugepages are needed. This is done by early boot handlers already.
> >> + * Free the serialized state now.
> >> + */
> >
> > It should be done in this function.
>
> The calculations can't be done in retrieve. Retrieve happens only once
> and for the whole FLB. They will need to come from
> hugetlb_hstate_alloc_pages().
>
> Maybe you mean getting rid of liveupdate_flb_incoming_early()? Yeah,
> that I can do. It will make this function a no-op once we move the
> kho_restore_free() to finish().

Yeah, this is what I meant.

Thanks,
Pasha

>
> >
> >> + kho_restore_free(phys_to_virt(args->data));
> >
> > This should be moved to finish() after blackout.
>
> Sure.
>
> >
> >> +
> >> + /*
> >> + * HACK: But since LUO FLB still needs an obj, use ZERO_SIZE_PTR to
> >> + * satisfy it.
> >> + */
> >> + args->obj = ZERO_SIZE_PTR;
> >
> > Hopefully this is not needed any more with the updated FLB, please check :-)
>
> Yep. IIRC when I sent this series the older version of FLB was in
> mm-nonmm-unstable.
>
> >
> >> + return 0;
> >> +}
> >> +
> [...]
>
> --
> Regards,
> Pratyush Yadav