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

[Pratyush Yadav]

From: "Pratyush Yadav (Google)" <pratyush@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>
---

Notes:
    Ref: https://lore.kernel.org/linux-mm/CA+CK2bDvaGmfkCPCMWM6gPcd4FfUyD6e5yWE+kNcma1vT3Jw3g@xxxxxxxxxxxxxx/

 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