On Tue, Dec 23 2025, Pasha Tatashin wrote: > On Sat, Dec 6, 2025 at 6:03 PM Pratyush Yadav <[email protected]> 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 <[email protected]> >> --- >> 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(). > >> + 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
