linux-kernel - Re: [PATCH v2 2/2] kho: simplify page initialization in kho_restore

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Message-ID: <aW99f7WAAAGtCfx9@kernel.org>
Date: Tue, 20 Jan 2026 15:05:03 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Graf <graf@...zon.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	kexec@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v2 2/2] kho: simplify page initialization in
 kho_restore_page()

On Fri, Jan 16, 2026 at 11:22:15AM +0000, Pratyush Yadav wrote:
> When restoring a page (from kho_restore_pages()) or folio (from
> kho_restore_folio()), KHO must initialize the struct page. The
> initialization differs slightly depending on if a folio is requested or
> a set of 0-order pages is requested.
> 
> Conceptually, it is quite simple to understand. When restoring 0-order
> pages, each page gets a refcount of 1 and that's it. When restoring a
> folio, head page gets a refcount of 1 and tail pages get 0.
> 
> kho_restore_page() tries to combine the two separate initialization flow
> into one piece of code. While it works fine, it is more complicated to
> read than it needs to be. Make the code simpler by splitting the two
> initalization paths into two separate functions. This improves
> readability by clearly showing how each type must be initialized.
> 
> Signed-off-by: Pratyush Yadav <pratyush@...nel.org>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>

> ---
> 
> Changes in v2:
> - Use unsigned long for nr_pages.
> 
>  kernel/liveupdate/kexec_handover.c | 40 +++++++++++++++++++-----------
>  1 file changed, 26 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 709484fbf9fd..92da76977684 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -219,11 +219,32 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
>  	return 0;
>  }
>  
> +/* For physically contiguous 0-order pages. */
> +static void kho_init_pages(struct page *page, unsigned long nr_pages)
> +{
> +	for (unsigned long i = 0; i < nr_pages; i++)
> +		set_page_count(page + i, 1);
> +}
> +
> +static void kho_init_folio(struct page *page, unsigned int order)
> +{
> +	unsigned long nr_pages = (1 << order);
> +
> +	/* Head page gets refcount of 1. */
> +	set_page_count(page, 1);
> +
> +	/* For higher order folios, tail pages get a page count of zero. */
> +	for (unsigned long i = 1; i < nr_pages; i++)
> +		set_page_count(page + i, 0);
> +
> +	if (order > 0)
> +		prep_compound_page(page, order);
> +}
> +
>  static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
>  {
>  	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
>  	unsigned long nr_pages;
> -	unsigned int ref_cnt;
>  	union kho_page_info info;
>  
>  	if (!page)
> @@ -241,20 +262,11 @@ static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
>  
>  	/* Clear private to make sure later restores on this page error out. */
>  	page->private = 0;
> -	/* Head page gets refcount of 1. */
> -	set_page_count(page, 1);
> -
> -	/*
> -	 * For higher order folios, tail pages get a page count of zero.
> -	 * For physically contiguous order-0 pages every pages gets a page
> -	 * count of 1
> -	 */
> -	ref_cnt = is_folio ? 0 : 1;
> -	for (unsigned long i = 1; i < nr_pages; i++)
> -		set_page_count(page + i, ref_cnt);
>  
> -	if (is_folio && info.order)
> -		prep_compound_page(page, info.order);
> +	if (is_folio)
> +		kho_init_folio(page, info.order);
> +	else
> +		kho_init_pages(page, nr_pages);
>  
>  	adjust_managed_page_count(page, nr_pages);
>  	return page;
> -- 
> 2.52.0.457.g6b5491de43-goog
> 

-- 
Sincerely yours,
Mike.