Re: [PATCH] kho: add support for deferred struct page init

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Tue, Dec 16, 2025 at 3:49 AM Evangelos Petrongonas <epetron@amazon.de> wrote:
>
> When `CONFIG_DEFERRED_STRUCT_PAGE_INIT` is enabled, struct page
> initialization is deferred to parallel kthreads that run later
> in the boot process.
>
> During KHO restoration, `deserialize_bitmap()` writes metadata for
> each preserved memory region. However, if the struct page has not been
> initialized, this write targets uninitialized memory, potentially
> leading to errors like:
> ```
> BUG: unable to handle page fault for address: ...
> ```
>
> Fix this by introducing `kho_get_preserved_page()`,  which ensures
> all struct pages in a preserved region are initialized by calling
> `init_deferred_page()` which is a no-op when deferred init is disabled
> or when the struct page is already initialized.
>
> Fixes: 8b66ed2c3f42 ("kho: mm: don't allow deferred struct page with KHO")

You are adding a new feature. Backporting this to stable is not needed.

> Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
> ---
> ### Notes
> @Jason, this patch should act as a temporary fix to make KHO play nice
> with deferred struct page init until you post your ideas about splitting
> "Physical Reservation" from "Metadata Restoration".
>
> ### Testing
> In order to test the fix, I modified the KHO selftest, to allocate more
> memory and do so from higher memory to trigger the incompatibility. The
> branch with those changes can be found in:
> https://git.infradead.org/?p=users/vpetrog/linux.git;a=shortlog;h=refs/heads/kho-deferred-struct-page-init
>
> In future patches, we might want to enhance the selftest to cover
> this case as well. However, properly adopting the test for this
> is much more work than the actual fix, therefore it can be deferred to a
> follow-up series.
>
> In addition attempting to run the selftest for arm (without my changes)
> fails with:
> ```
> ERROR:target/arm/internals.h:767:regime_is_user: code should not be reached
> Bail out! ERROR:target/arm/internals.h:767:regime_is_user: code should not be reached
> ./tools/testing/selftests/kho/vmtest.sh: line 113: 61609 Aborted
> ```
> I have not looked it up further, but can also do so as part of a
> selftest follow-up.
>
>  kernel/liveupdate/Kconfig          |  2 --
>  kernel/liveupdate/kexec_handover.c | 19 ++++++++++++++++++-
>  2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
> index d2aeaf13c3ac..9394a608f939 100644
> --- a/kernel/liveupdate/Kconfig
> +++ b/kernel/liveupdate/Kconfig
> @@ -1,12 +1,10 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>
>  menu "Live Update and Kexec HandOver"
> -       depends on !DEFERRED_STRUCT_PAGE_INIT
>
>  config KEXEC_HANDOVER
>         bool "kexec handover"
>         depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE
> -       depends on !DEFERRED_STRUCT_PAGE_INIT
>         select MEMBLOCK_KHO_SCRATCH
>         select KEXEC_FILE
>         select LIBFDT
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 9dc51fab604f..78cfe71e6107 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -439,6 +439,23 @@ static int kho_mem_serialize(struct kho_out *kho_out)
>         return err;
>  }
>
> +/*
> + * With CONFIG_DEFERRED_STRUCT_PAGE_INIT, struct pages in higher memory
> + * regions may not be initialized yet at the time KHO deserializes preserved
> + * memory. This function ensures all struct pages in the region are initialized.
> + */
> +static struct page *__init kho_get_preserved_page(phys_addr_t phys,
> +                                                 unsigned int order)
> +{
> +       unsigned long pfn = PHYS_PFN(phys);
> +       int nid = early_pfn_to_nid(pfn);
> +
> +       for (int i = 0; i < (1 << order); i++)
> +               init_deferred_page(pfn + i, nid);
> +
> +       return pfn_to_page(pfn);
> +}
> +
>  static void __init deserialize_bitmap(unsigned int order,
>                                       struct khoser_mem_bitmap_ptr *elm)
>  {
> @@ -449,7 +466,7 @@ static void __init deserialize_bitmap(unsigned int order,
>                 int sz = 1 << (order + PAGE_SHIFT);
>                 phys_addr_t phys =
>                         elm->phys_start + (bit << (order + PAGE_SHIFT));
> -               struct page *page = phys_to_page(phys);
> +               struct page *page = kho_get_preserved_page(phys, order);
>                 union kho_page_info info;
>
>                 memblock_reserve(phys, sz);

In deferred_init_memmap_chunk() we initialize deferred struct pages in
this iterator:

for_each_free_mem_range(i, nid, 0, &start, &end, NULL) {
 init_deferred_page()
}

However, since, memblock_reserve() is called, the memory is not going
to be part of the for_each_free_mem_range() iterator. So, I think the
proposed patch should work.

Pratyush, what happens if we deserialize a HugeTLB with HVO? Since HVO
optimizes out the unique backing struct pages for tail pages, blindly
iterating and initializing them via init_deferred_page() might corrupt
the shared struct page mapping.

Pasha