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

[Samiullah Khawaja <skhawaja@google.com>]

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>
>---
>
>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
>
>

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>