[PATCH] kho: fix restore order for NUMA-split preservations

[PATCH] kho: fix restore order for NUMA-split preservations

[Mukesh Pilaniya]

kho_preserve_pages() uses __kho_preserve_pages_order() to reduce the
order of a preservation so that it does not span multiple NUMA nodes.
kho_restore_pages() does not use that helper; it recomputes the order
from the pfn alignment and the remaining length only.

When a preservation was split at a NUMA boundary, restore picks a larger
order than preserve did and jumps over the next key. Those pages never
get restored: they stay memblock-reserved and keep the KHOP magic in
page->private, while kho_restore_pages() returns success for the pages
that were restored. The consumer has no way to notice that part of its
buffer was not restored.

Fix this by reading the order from the key being restored. The order is
already stored in page->private by kho_preserved_memory_reserve() and is
validated by kho_restore_page(). Advance by that order instead of
recomputing it. This also keeps restore independent of the incoming
kernel's NUMA topology.

Example: QEMU is started with two NUMA nodes:

  node0: [0x40000000, 0xC0400000)   2052 MiB
  node1: [0xC0400000, 0x1C0000000)  4092 MiB

The module preserves [0xC0000000, 0xC0800000) (8 MiB, 2048 pages). The
range is 8 MiB aligned and the node boundary sits exactly 4 MiB into it, so
preserve emits two order-10 keys, the first at 0xC0000000 and the second at
0xC0400000. Restore computes order 11 for the first page, restores the
first key and jumps straight to the end of the range, skipping the second
key.

Fixes: 91e74fa8b1bc ("kho: make sure preservations do not span multiple NUMA nodes")
Signed-off-by: Mukesh Pilaniya <mpilaniy@redhat.com>
---
 kernel/liveupdate/kexec_handover.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 6f1f55fe3181..7d30ad9cc512 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -511,7 +511,8 @@ static void kho_init_folio(struct page *page, unsigned int order)
 		prep_compound_page(page, order);
 }
 
-static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
+static struct page *kho_restore_page(phys_addr_t phys, bool is_folio,
+				     unsigned int *order)
 {
 	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
 	unsigned long nr_pages;
@@ -530,6 +531,9 @@ static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
 		return NULL;
 	nr_pages = (1 << info.order);
 
+	if (order)
+		*order = info.order;
+
 	/* Clear private to make sure later restores on this page error out. */
 	page->private = 0;
 
@@ -550,7 +554,7 @@ static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
  */
 struct folio *kho_restore_folio(phys_addr_t phys)
 {
-	struct page *page = kho_restore_page(phys, true);
+	struct page *page = kho_restore_page(phys, true, NULL);
 
 	return page ? page_folio(page) : NULL;
 }
@@ -573,13 +577,13 @@ struct page *kho_restore_pages(phys_addr_t phys, unsigned long nr_pages)
 	unsigned long pfn = start_pfn;
 
 	while (pfn < end_pfn) {
-		const unsigned int order =
-			min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));
-		struct page *page = kho_restore_page(PFN_PHYS(pfn), false);
+		unsigned int order;
+		struct page *page;
 
+		page = kho_restore_page(PFN_PHYS(pfn), false, &order);
 		if (!page)
 			return NULL;
-		pfn += 1 << order;
+		pfn += 1UL << order;
 	}
 
 	return pfn_to_page(start_pfn);

---
base-commit: 2637bdec0535a2044cd830eba9a79363651d9043
change-id: 20261103-kho-numa-5ce38a3ed9c9

Best regards,
--  
Mukesh Pilaniya <mpilaniy@redhat.com>