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

[Fadouse <fadouse@proton.me>]

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Hi Evangelos, Mike, Pasha, Pratyush,

I independently hit a crash in the LUO/memfd restore path with
CONFIG_DEFERRED_STRUCT_PAGE_INIT=y, on a local build based on dd9b004b7ff3
(x86_64 QEMU, 6.19.0-rc1 timeframe).

In my reproducer, stage1 preserves a memfd via LUO and kexecs into stage2;
stage2 calls LIVEUPDATE_SESSION_FINISH without retrieving files. I observed
a reliable crash in adjust_managed_page_count() from kho_restore_page().

Minimal excerpt:

stage2: start
stage2: retrieved session fd=4
BUG: unable to handle page fault for address: 0000000000001410
RIP: adjust_managed_page_count+0x29/0x40
Call Trace:
   kho_restore_page+0x18a/0x1c0
   kho_restore_folio+0xe/0x60
   memfd_luo_finish+0xe6/0x160
   luo_file_finish+0x188/0x240
   luo_session_finish+0x2c/0x80
   luo_session_ioctl+0xf5/0x170
   __x64_sys_ioctl+0x91/0xe0

Applying the patch in <20251216084913.86342-1-epetron@amazon.de> makes the
issue no longer reproduce for me.

I can share full logs and the small two-stage initramfs reproducer if 
needed.

Thanks,
YanXin Li

Tested-by: YanXin Li <fadouse@proton.me>

On 12/16/2025 4:49 PM, Evangelos Petrongonas 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")
> 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);

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