Re: [PATCH 1/2] kho: make sure preservations do not span multiple NUMA nodes

[Mike Rapoport <rppt@kernel.org>]

On Mon, Mar 09, 2026 at 12:34:06PM +0000, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush@kernel.org>
> 
> The KHO restoration machinery is not capable of dealing with
> preservations that span multiple NUMA nodes. kho_preserve_folio()
> guarantees the preservation will only span one NUMA node since folios
> can't span multiple nodes.
> 
> This leaves kho_preserve_pages(). While semantically
> kho_preserve_pages() only deals with 0-order pages, so all preservations
> should be single page only, in practice it combines preservations to
> higher orders for efficiency. This can result in a preservation spanning
> multiple nodes. Break up the preservations into a smaller order if that
> happens.
> 
> Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>

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

> ---
> 
> Notes:
>     Ref: https://lore.kernel.org/linux-mm/CA+CK2bDvaGmfkCPCMWM6gPcd4FfUyD6e5yWE+kNcma1vT3Jw3g@mail.gmail.com/
> 
>  kernel/liveupdate/kexec_handover.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index cc68a3692905..bc9bd18294ee 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -869,9 +869,17 @@ int kho_preserve_pages(struct page *page, unsigned long nr_pages)
>  	}
>  
>  	while (pfn < end_pfn) {
> -		const unsigned int order =
> +		unsigned int order =
>  			min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));
>  
> +		/*
> +		 * Make sure all the pages in a single preservation are in the
> +		 * same NUMA node. The restore machinery can not cope with a
> +		 * preservation spanning multiple NUMA nodes.
> +		 */
> +		while (pfn_to_nid(pfn) != pfn_to_nid(pfn + (1UL << order) - 1))
> +			order--;
> +
>  		err = __kho_preserve_order(track, pfn, order);
>  		if (err) {
>  			failed_pfn = pfn;
> -- 
> 2.53.0.473.g4a7958ca14-goog
> 

-- 
Sincerely yours,
Mike.