kho: fix restoring of contiguous ranges of order-0 pages

When contiguous ranges of order-0 pages are restored, kho_restore_page()
calls prep_compound_page() with the first page in the range and order as
parameters and then kho_restore_pages() calls split_page() to make sure
all pages in the range are order-0.

However, since split_page() is not intended to split compound pages and
with VM_DEBUG enabled it will trigger a VM_BUG_ON_PAGE().

Update kho_restore_page() so that it will use prep_compound_page() when it
restores a folio and make sure it properly sets page count for both large
folios and ranges of order-0 pages.

Link: https://lkml.kernel.org/r/20251125110917.843744-3-rppt@kernel.org
Fixes: a667300 ("kho: add support for preserving vmalloc allocations")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reported-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: rppt

kernel/liveupdate/kexec_handover.c

@@ -219,11 +219,11 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
 	return 0;
 }
 
-static struct page *kho_restore_page(phys_addr_t phys)
+static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
 {
 	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
+	unsigned int nr_pages, ref_cnt;
 	union kho_page_info info;
-	unsigned int nr_pages;
 
 	if (!page)
 		return NULL;
@@ -243,11 +243,16 @@ static struct page *kho_restore_page(phys_addr_t phys)
 	/* Head page gets refcount of 1. */
 	set_page_count(page, 1);
 
-	/* For higher order folios, tail pages get a page count of zero. */
+	/*
+	 * 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 int i = 1; i < nr_pages; i++)
-		set_page_count(page + i, 0);
+		set_page_count(page + i, ref_cnt);
 
-	if (info.order > 0)
+	if (is_folio && info.order)
 		prep_compound_page(page, info.order);
 
 	adjust_managed_page_count(page, nr_pages);
@@ -262,7 +267,7 @@ static struct page *kho_restore_page(phys_addr_t phys)
  */
 struct folio *kho_restore_folio(phys_addr_t phys)
 {
-	struct page *page = kho_restore_page(phys);
+	struct page *page = kho_restore_page(phys, true);
 
 	return page ? page_folio(page) : NULL;
 }
@@ -287,11 +292,10 @@ struct page *kho_restore_pages(phys_addr_t phys, unsigned int nr_pages)
 	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));
+		struct page *page = kho_restore_page(PFN_PHYS(pfn), false);
 
 		if (!page)
 			return NULL;
-		split_page(page, order);
 		pfn += 1 << order;
 	}