[PATCH 2/2] kho: fix restoring of contiguous ranges of order-0 pages

Mike Rapoport
2025-11-25T11:09:35Z

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

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.

Reported-by: Pratyush Yadav <pratyush@kernel.org>
Fixes: a667300bd53f ("kho: add support for preserving vmalloc allocations")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
kernel/liveupdate/kexec_handover.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index e64ee87fa62a..61d17ed1f423 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/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;
}

--
2.50.1