Re: [PATCH v4 03/30] kho: drop notifiers

[Pratyush Yadav]

On Mon, Sep 29 2025, Pasha Tatashin wrote:

> From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
>
> The KHO framework uses a notifier chain as the mechanism for clients to
> participate in the finalization process. While this works for a single,
> central state machine, it is too restrictive for kernel-internal
> components like pstore/reserve_mem or IMA. These components need a
> simpler, direct way to register their state for preservation (e.g.,
> during their initcall) without being part of a complex,
> shutdown-time notifier sequence. The notifier model forces all
> participants into a single finalization flow and makes direct
> preservation from an arbitrary context difficult.
> This patch refactors the client participation model by removing the
> notifier chain and introducing a direct API for managing FDT subtrees.
>
> The core kho_finalize() and kho_abort() state machine remains, but
> clients now register their data with KHO beforehand.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
[...]
> diff --git a/mm/memblock.c b/mm/memblock.c
> index e23e16618e9b..c4b2d4e4c715 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2444,53 +2444,18 @@ int reserve_mem_release_by_name(const char *name)
>  #define MEMBLOCK_KHO_FDT "memblock"
>  #define MEMBLOCK_KHO_NODE_COMPATIBLE "memblock-v1"
>  #define RESERVE_MEM_KHO_NODE_COMPATIBLE "reserve-mem-v1"
> -static struct page *kho_fdt;
> -
> -static int reserve_mem_kho_finalize(struct kho_serialization *ser)
> -{
> -	int err = 0, i;
> -
> -	for (i = 0; i < reserved_mem_count; i++) {
> -		struct reserve_mem_table *map = &reserved_mem_table[i];
> -		struct page *page = phys_to_page(map->start);
> -		unsigned int nr_pages = map->size >> PAGE_SHIFT;
> -
> -		err |= kho_preserve_pages(page, nr_pages);
> -	}
> -
> -	err |= kho_preserve_folio(page_folio(kho_fdt));
> -	err |= kho_add_subtree(ser, MEMBLOCK_KHO_FDT, page_to_virt(kho_fdt));
> -
> -	return notifier_from_errno(err);
> -}
> -
> -static int reserve_mem_kho_notifier(struct notifier_block *self,
> -				    unsigned long cmd, void *v)
> -{
> -	switch (cmd) {
> -	case KEXEC_KHO_FINALIZE:
> -		return reserve_mem_kho_finalize((struct kho_serialization *)v);
> -	case KEXEC_KHO_ABORT:
> -		return NOTIFY_DONE;
> -	default:
> -		return NOTIFY_BAD;
> -	}
> -}
> -
> -static struct notifier_block reserve_mem_kho_nb = {
> -	.notifier_call = reserve_mem_kho_notifier,
> -};
>  
>  static int __init prepare_kho_fdt(void)
>  {
>  	int err = 0, i;
> +	struct page *fdt_page;
>  	void *fdt;
>  
> -	kho_fdt = alloc_page(GFP_KERNEL);
> -	if (!kho_fdt)
> +	fdt_page = alloc_page(GFP_KERNEL);
> +	if (!fdt_page)
>  		return -ENOMEM;
>  
> -	fdt = page_to_virt(kho_fdt);
> +	fdt = page_to_virt(fdt_page);
>  
>  	err |= fdt_create(fdt, PAGE_SIZE);
>  	err |= fdt_finish_reservemap(fdt);
> @@ -2499,7 +2464,10 @@ static int __init prepare_kho_fdt(void)
>  	err |= fdt_property_string(fdt, "compatible", MEMBLOCK_KHO_NODE_COMPATIBLE);
>  	for (i = 0; i < reserved_mem_count; i++) {
>  		struct reserve_mem_table *map = &reserved_mem_table[i];
> +		struct page *page = phys_to_page(map->start);
> +		unsigned int nr_pages = map->size >> PAGE_SHIFT;
>  
> +		err |= kho_preserve_pages(page, nr_pages);
>  		err |= fdt_begin_node(fdt, map->name);
>  		err |= fdt_property_string(fdt, "compatible", RESERVE_MEM_KHO_NODE_COMPATIBLE);
>  		err |= fdt_property(fdt, "start", &map->start, sizeof(map->start));
> @@ -2507,13 +2475,14 @@ static int __init prepare_kho_fdt(void)
>  		err |= fdt_end_node(fdt);
>  	}
>  	err |= fdt_end_node(fdt);
> -
>  	err |= fdt_finish(fdt);
>  
> +	err |= kho_preserve_folio(page_folio(fdt_page));
> +	err |= kho_add_subtree(MEMBLOCK_KHO_FDT, fdt);
> +
>  	if (err) {
>  		pr_err("failed to prepare memblock FDT for KHO: %d\n", err);
> -		put_page(kho_fdt);
> -		kho_fdt = NULL;
> +		put_page(fdt_page);

This adds subtree to KHO even if the FDT might be invalid. And then
leaves a dangling reference in KHO to the FDT in case of an error. I
think you should either do this check after
kho_preserve_folio(page_folio(fdt_page)) and do a clean error check for
kho_add_subtree(), or call kho_remove_subtree() in the error block.

I prefer the former since if kho_add_subtree() is the one that fails,
there is little sense in removing a subtree that was never added.

>  	}
>  
>  	return err;
> @@ -2529,13 +2498,6 @@ static int __init reserve_mem_init(void)
>  	err = prepare_kho_fdt();
>  	if (err)
>  		return err;
> -
> -	err = register_kho_notifier(&reserve_mem_kho_nb);
> -	if (err) {
> -		put_page(kho_fdt);
> -		kho_fdt = NULL;
> -	}
> -
>  	return err;
>  }
>  late_initcall(reserve_mem_init);

-- 
Regards,
Pratyush Yadav