[RFC PATCH 4/4] kho: Introduce kho_split_preserved_pages() helper

[Pranjal Shrivastava <praan@google.com>]

A driver may need to split a high-order allocation that has already
been preserved. If the pages are split using split_page() manually,
the refcounts would change but KHO won't record the change in the
preserved page-type, resulting in a metadata mismatch during
restoration in the new kernel.

Introduce kho_split_preserved_pages() to handle splitting of preserved
pages. The helper follows an unpreserve -> split -> re-preserve sequence,
while ensuring that the KHO radix tree is updated with the correct
KHO_PAGE_SPLIT type bits.

The helper returns 0 on success, or a negative error code if the
re-preservation fails. Callers must ensure the provided order matches
the original allocation and that the operation is serialized against
other preservation API calls.

Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 include/linux/kexec_handover.h     |  7 +++++++
 kernel/liveupdate/kexec_handover.c | 23 +++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
index 8968c56d2d73..452e38bb2076 100644
--- a/include/linux/kexec_handover.h
+++ b/include/linux/kexec_handover.h
@@ -24,6 +24,7 @@ int kho_preserve_folio(struct folio *folio);
 void kho_unpreserve_folio(struct folio *folio);
 int kho_preserve_pages(struct page *page, unsigned long nr_pages);
 void kho_unpreserve_pages(struct page *page, unsigned long nr_pages);
+int kho_split_preserved_pages(struct page *page, unsigned int order);
 int kho_preserve_vmalloc(void *ptr, struct kho_vmalloc *preservation);
 void kho_unpreserve_vmalloc(struct kho_vmalloc *preservation);
 void *kho_alloc_preserve(size_t size);
@@ -65,6 +66,12 @@ static inline int kho_preserve_pages(struct page *page, unsigned int nr_pages)
 
 static inline void kho_unpreserve_pages(struct page *page, unsigned int nr_pages) { }
 
+static inline int kho_split_preserved_pages(struct page *page,
+					    unsigned int order)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int kho_preserve_vmalloc(void *ptr,
 				       struct kho_vmalloc *preservation)
 {
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index f6ca5e24c740..ea08248901b5 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -1018,6 +1018,29 @@ void kho_unpreserve_pages(struct page *page, unsigned long nr_pages)
 }
 EXPORT_SYMBOL_GPL(kho_unpreserve_pages);
 
+/**
+ * kho_split_preserved_pages - split contiguous pages that are preserved
+ * @page: first page in the list.
+ * @order: the order of the original allocation.
+ *
+ * This function allows to split a high-order allocation that has been
+ * preserved across kexec. It unpreserves the pages, splits them using
+ * split_page() and then re-preserves them as individual pages.
+ *
+ * This function MUST only be called on pages that are currently preserved.
+ * The @order provided MUST match the order used during the initial
+ * preservation.
+ *
+ * Return: 0 on success, or a negative error code on failure.
+ */
+int kho_split_preserved_pages(struct page *page, unsigned int order)
+{
+	kho_unpreserve_pages(page, 1UL << order);
+	split_page(page, order);
+	return kho_preserve_pages(page, 1UL << order);
+}
+EXPORT_SYMBOL_GPL(kho_split_preserved_pages);
+
 /* vmalloc flags KHO supports */
 #define KHO_VMALLOC_SUPPORTED_FLAGS	(VM_ALLOC | VM_ALLOW_HUGE_VMAP)
 
-- 
2.55.0.rc0.799.gd6f94ed593-goog