Re: [PATCH v5 02/22] liveupdate: luo_core: integrate with KHO

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 07, 2025 at 04:03:00PM -0500, Pasha Tatashin wrote:
> Integrate the LUO with the KHO framework to enable passing LUO state
> across a kexec reboot.
> 
> When LUO is transitioned to a "prepared" state, it tells KHO to
> finalize, so all memory segments that were added to KHO preservation
> list are getting preserved. After "Prepared" state no new segments
> can be preserved. If LUO is canceled, it also tells KHO to cancel the
> serialization, and therefore, later LUO can go back into the prepared
> state.
> 
> This patch introduces the following changes:
> - During the KHO finalization phase allocate FDT blob.
> - Populate this FDT with a LUO compatibility string ("luo-v1").
> 
> LUO now depends on `CONFIG_KEXEC_HANDOVER`. The core state transition
> logic (`luo_do_*_calls`) remains unimplemented in this patch.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/liveupdate.h         |   6 +
>  include/linux/liveupdate/abi/luo.h |  54 +++++++
>  kernel/liveupdate/luo_core.c       | 243 ++++++++++++++++++++++++++++-
>  kernel/liveupdate/luo_internal.h   |  17 ++
>  mm/mm_init.c                       |   4 +
>  5 files changed, 323 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/liveupdate/abi/luo.h
>  create mode 100644 kernel/liveupdate/luo_internal.h
> 
> diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
> index 730b76625fec..0be8804fc42a 100644
> --- a/include/linux/liveupdate.h
> +++ b/include/linux/liveupdate.h
> @@ -13,6 +13,8 @@
>  
>  #ifdef CONFIG_LIVEUPDATE
>  
> +void __init liveupdate_init(void);
> +
>  /* Return true if live update orchestrator is enabled */
>  bool liveupdate_enabled(void);
>  
> @@ -21,6 +23,10 @@ int liveupdate_reboot(void);
>  
>  #else /* CONFIG_LIVEUPDATE */
>  
> +static inline void liveupdate_init(void)
> +{
> +}

The common practice is to place brackets at the same line with function
declaration.

...

> +static int __init luo_early_startup(void)
> +{
> +	phys_addr_t fdt_phys;
> +	int err, ln_size;
> +	const void *ptr;
> +
> +	if (!kho_is_enabled()) {
> +		if (liveupdate_enabled())
> +			pr_warn("Disabling liveupdate because KHO is disabled\n");
> +		luo_global.enabled = false;
> +		return 0;
> +	}
> +
> +	/* Retrieve LUO subtree, and verify its format. */
> +	err = kho_retrieve_subtree(LUO_FDT_KHO_ENTRY_NAME, &fdt_phys);
> +	if (err) {
> +		if (err != -ENOENT) {
> +			pr_err("failed to retrieve FDT '%s' from KHO: %pe\n",
> +			       LUO_FDT_KHO_ENTRY_NAME, ERR_PTR(err));
> +			return err;
> +		}
> +
> +		return 0;
> +	}
> +
> +	luo_global.fdt_in = __va(fdt_phys);

phys_to_virt is clearer, isn't it?

> +	err = fdt_node_check_compatible(luo_global.fdt_in, 0,
> +					LUO_FDT_COMPATIBLE);

...

> +void __init liveupdate_init(void)
> +{
> +	int err;
> +
> +	err = luo_early_startup();
> +	if (err) {
> +		pr_err("The incoming tree failed to initialize properly [%pe], disabling live update\n",
> +		       ERR_PTR(err));
> +		luo_global.enabled = false;
> +	}
> +}
> +
> +/* Called during boot to create LUO fdt tree */

			 ^ create outgoing

> +static int __init luo_late_startup(void)
> +{
> +	int err;
> +
> +	if (!liveupdate_enabled())
> +		return 0;
> +
> +	err = luo_fdt_setup();
> +	if (err)
> +		luo_global.enabled = false;
> +
> +	return err;
> +}
> +late_initcall(luo_late_startup);

It would be nice to have a comment explaining why late_initcall() is fine
and why there's no need to initialize the outgoing fdt earlier.

> +/**
> + * luo_alloc_preserve - Allocate, zero, and preserve memory.

I think this and the "free" counterparts would be useful for any KHO users,
even those that don't need LUO.

> + * @size: The number of bytes to allocate.
> + *
> + * Allocates a physically contiguous block of zeroed pages that is large
> + * enough to hold @size bytes. The allocated memory is then registered with
> + * KHO for preservation across a kexec.
> + *
> + * Note: The actual allocated size will be rounded up to the nearest
> + * power-of-two page boundary.
> + *
> + * @return A virtual pointer to the allocated and preserved memory on success,
> + * or an ERR_PTR() encoded error on failure.
> + */
> +void *luo_alloc_preserve(size_t size)
> +{
> +	struct folio *folio;
> +	int order, ret;
> +
> +	if (!size)
> +		return ERR_PTR(-EINVAL);
> +
> +	order = get_order(size);
> +	if (order > MAX_PAGE_ORDER)
> +		return ERR_PTR(-E2BIG);

High order allocations would likely fail or at least cause a heavy reclaim.
For now it seems that we won't be needing really large contiguous chunks so
maybe limiting this to PAGE_ALLOC_COSTLY_ORDER?

Later if we'd need higher order allocations we can try to allocate with
__GFP_NORETRY or __GFP_RETRY_MAYFAIL with a fallback to vmalloc.

> +
> +	folio = folio_alloc(GFP_KERNEL | __GFP_ZERO, order);
> +	if (!folio)
> +		return ERR_PTR(-ENOMEM);
> +
> +	ret = kho_preserve_folio(folio);
> +	if (ret) {
> +		folio_put(folio);
> +		return ERR_PTR(ret);
> +	}
> +
> +	return folio_address(folio);
> +}
> +

-- 
Sincerely yours,
Mike.