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

[Pratyush Yadav]

On Tue, Oct 07 2025, Pasha Tatashin wrote:

> On Tue, Oct 7, 2025 at 8:10 AM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>>
>> On Mon, Oct 06 2025, Pasha Tatashin wrote:
>>
>> > On Mon, Oct 6, 2025 at 1:01 PM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>> >>
>> >> 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 agree, I do not like these err |= stuff, we should be checking
>> > errors cleanly, and do proper clean-ups.
>>
>> Yeah, this is mainly a byproduct of using FDTs. Getting and setting
>> simple properties also needs error checking and that can get tedious
>> real quick. Which is why this pattern has shown up I suppose.
>
> Exactly. This is also why it's important to replace FDT with something
> more sensible for general-purpose live update purposes.
>
> By the way, I forgot to address this comment in the v5 of the KHO
> series I sent out yesterday. Could you please take another look? If
> everything else is good, I will refresh that series so we can ask
> Andrew to take in the KHO patches. That would simplify the LUO series.

Good idea. Will take a look.

[...]

-- 
Regards,
Pratyush Yadav