Re: [PATCH v4 2/4] kho: replace kho_preserve_phys() with kho_preserve_pages()

[Pratyush Yadav <pratyush@kernel.org>]

Hi Mike,

On Wed, Sep 17 2025, Mike Rapoport wrote:

> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> to make it clear that KHO operates on pages rather than on a random
> physical address.
>
> The kho_preserve_pages() will be also used in upcoming support for
> vmalloc preservation.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  include/linux/kexec_handover.h |  5 +++--
>  kernel/kexec_handover.c        | 25 +++++++++++--------------
>  mm/memblock.c                  |  4 +++-
>  3 files changed, 17 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
> index 348844cffb13..cc5c49b0612b 100644
> --- a/include/linux/kexec_handover.h
> +++ b/include/linux/kexec_handover.h
> @@ -18,6 +18,7 @@ enum kho_event {
>  
>  struct folio;
>  struct notifier_block;
> +struct page;
>  
>  #define DECLARE_KHOSER_PTR(name, type) \
>  	union {                        \
> @@ -42,7 +43,7 @@ struct kho_serialization;
>  bool kho_is_enabled(void);
>  
>  int kho_preserve_folio(struct folio *folio);
> -int kho_preserve_phys(phys_addr_t phys, size_t size);
> +int kho_preserve_pages(struct page *page, unsigned int nr_pages);
>  struct folio *kho_restore_folio(phys_addr_t phys);
>  int kho_add_subtree(struct kho_serialization *ser, const char *name, void *fdt);
>  int kho_retrieve_subtree(const char *name, phys_addr_t *phys);
> @@ -65,7 +66,7 @@ static inline int kho_preserve_folio(struct folio *folio)
>  	return -EOPNOTSUPP;
>  }
>  
> -static inline int kho_preserve_phys(phys_addr_t phys, size_t size)
> +static inline int kho_preserve_pages(struct page *page, unsigned int nr_pages)
>  {
>  	return -EOPNOTSUPP;
>  }
> diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
> index f421acc58c1f..3ad59c5f9eaa 100644
> --- a/kernel/kexec_handover.c
> +++ b/kernel/kexec_handover.c
> @@ -698,26 +698,23 @@ int kho_preserve_folio(struct folio *folio)
>  EXPORT_SYMBOL_GPL(kho_preserve_folio);
>  
>  /**
> - * kho_preserve_phys - preserve a physically contiguous range across kexec.
> - * @phys: physical address of the range.
> - * @size: size of the range.
> + * kho_preserve_pages - preserve contiguous pages across kexec
> + * @page: first page in the list.
> + * @nr_pages: number of pages.
>   *
> - * Instructs KHO to preserve the memory range from @phys to @phys + @size
> - * across kexec.
> + * Preserve a contiguous list of order 0 pages. Must be restored using
> + * kho_restore_page() on each order 0 page.

This is not true. The pages are preserved with the maximum order
possible.

	while (pfn < end_pfn) {
		const unsigned int order =
			min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));

		err = __kho_preserve_order(track, pfn, order);
		[...]

So four 0-order pages will be preserved as one 2-order page. Restoring
them as four 0-order pages is wrong. And my proposed patch for checking
the magic [0] will uncover this exact bug.

I think you should either change the logic to always preserve at order
0, or maybe add a kho_restore_pages() that replicates the same order
calculation.

[0] https://lore.kernel.org/lkml/20250917125725.665-2-pratyush@kernel.org/

>   *
>   * Return: 0 on success, error code on failure
>   */
> -int kho_preserve_phys(phys_addr_t phys, size_t size)
> +int kho_preserve_pages(struct page *page, unsigned int nr_pages)
>  {
> -	unsigned long pfn = PHYS_PFN(phys);
> +	struct kho_mem_track *track = &kho_out.ser.track;
> +	const unsigned long start_pfn = page_to_pfn(page);
> +	const unsigned long end_pfn = start_pfn + nr_pages;
> +	unsigned long pfn = start_pfn;
>  	unsigned long failed_pfn = 0;
> -	const unsigned long start_pfn = pfn;
> -	const unsigned long end_pfn = PHYS_PFN(phys + size);
>  	int err = 0;
> -	struct kho_mem_track *track = &kho_out.ser.track;
> -
> -	if (!PAGE_ALIGNED(phys) || !PAGE_ALIGNED(size))
> -		return -EINVAL;
>  
>  	while (pfn < end_pfn) {
>  		const unsigned int order =
> @@ -737,7 +734,7 @@ int kho_preserve_phys(phys_addr_t phys, size_t size)
>  
>  	return err;
>  }
> -EXPORT_SYMBOL_GPL(kho_preserve_phys);
> +EXPORT_SYMBOL_GPL(kho_preserve_pages);
>  
>  /* Handling for debug/kho/out */
>  
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 117d963e677c..6ec3eaa4e8d1 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2516,8 +2516,10 @@ static int reserve_mem_kho_finalize(struct kho_serialization *ser)
>  
>  	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_phys(map->start, map->size);
> +		err |= kho_preserve_pages(page, nr_pages);

Unrelated to this patch, but since there is no
kho_restore_{phys,pages}(), won't the reserve_mem memory end up with
uninitialized struct pages, since preserved pages are
memblock_reserved_mark_noinit()?

That would also be a case for kho_restore_pages() I suppose?

-- 
Regards,
Pratyush Yadav