[PATCH v1 13/13] kho: Introduce high-level memory allocation API

[Pasha Tatashin <pasha.tatashin@soleen.com>]

Currently, clients of KHO must manually allocate memory (e.g., via
alloc_pages), calculate the page order, and explicitly call
kho_preserve_folio(). Similarly, cleanup requires separate calls to
unpreserve and free the memory.

Introduce a high-level API to streamline this common pattern:

- kho_alloc_preserve(size): Allocates physically contiguous, zeroed
  memory and immediately marks it for preservation.
- kho_free_unpreserve(ptr, size): Unpreserves and frees the memory
  in the current kernel.
- kho_free_restore(ptr, size): Restores the struct page state of
  preserved memory in the new kernel and immediately frees it to the
  page allocator.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/kexec_handover.h     |  22 +++++--
 kernel/liveupdate/kexec_handover.c | 101 +++++++++++++++++++++++++++++
 2 files changed, 116 insertions(+), 7 deletions(-)

diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
index 80ece4232617..76c496e01877 100644
--- a/include/linux/kexec_handover.h
+++ b/include/linux/kexec_handover.h
@@ -2,8 +2,9 @@
 #ifndef LINUX_KEXEC_HANDOVER_H
 #define LINUX_KEXEC_HANDOVER_H
 
-#include <linux/types.h>
+#include <linux/err.h>
 #include <linux/errno.h>
+#include <linux/types.h>
 
 struct kho_scratch {
 	phys_addr_t addr;
@@ -48,6 +49,9 @@ int kho_preserve_pages(struct page *page, unsigned int nr_pages);
 int kho_unpreserve_pages(struct page *page, unsigned int nr_pages);
 int kho_preserve_vmalloc(void *ptr, struct kho_vmalloc *preservation);
 int kho_unpreserve_vmalloc(struct kho_vmalloc *preservation);
+void *kho_alloc_preserve(size_t size);
+void kho_free_unpreserve(void *mem, size_t size);
+void kho_free_restore(void *mem, size_t size);
 struct folio *kho_restore_folio(phys_addr_t phys);
 struct page *kho_restore_pages(phys_addr_t phys, unsigned int nr_pages);
 void *kho_restore_vmalloc(const struct kho_vmalloc *preservation);
@@ -101,6 +105,14 @@ static inline int kho_unpreserve_vmalloc(struct kho_vmalloc *preservation)
 	return -EOPNOTSUPP;
 }
 
+void *kho_alloc_preserve(size_t size)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+
+void kho_free_unpreserve(void *mem, size_t size) { }
+void kho_free_restore(void *mem, size_t size) { }
+
 static inline struct folio *kho_restore_folio(phys_addr_t phys)
 {
 	return NULL;
@@ -122,18 +134,14 @@ static inline int kho_add_subtree(const char *name, void *fdt)
 	return -EOPNOTSUPP;
 }
 
-static inline void kho_remove_subtree(void *fdt)
-{
-}
+static inline void kho_remove_subtree(void *fdt) { }
 
 static inline int kho_retrieve_subtree(const char *name, phys_addr_t *phys)
 {
 	return -EOPNOTSUPP;
 }
 
-static inline void kho_memory_init(void)
-{
-}
+static inline void kho_memory_init(void) { }
 
 static inline void kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
 				phys_addr_t scratch_phys, u64 scratch_len)
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index a905bccf5f65..9f05849fd68e 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -4,6 +4,7 @@
  * Copyright (C) 2023 Alexander Graf <graf@amazon.com>
  * Copyright (C) 2025 Microsoft Corporation, Mike Rapoport <rppt@kernel.org>
  * Copyright (C) 2025 Google LLC, Changyuan Lyu <changyuanl@google.com>
+ * Copyright (C) 2025 Pasha Tatashin <pasha.tatashin@soleen.com>
  */
 
 #define pr_fmt(fmt) "KHO: " fmt
@@ -1151,6 +1152,106 @@ void *kho_restore_vmalloc(const struct kho_vmalloc *preservation)
 }
 EXPORT_SYMBOL_GPL(kho_restore_vmalloc);
 
+/**
+ * kho_alloc_preserve - Allocate, zero, and preserve memory.
+ * @size: The number of bytes to allocate.
+ *
+ * Allocates a physically contiguous block of zeroed pages that is large
+ * enough to hold @size bytes. The allocated memory is then registered with
+ * KHO for preservation across a kexec.
+ *
+ * Note: The actual allocated size will be rounded up to the nearest
+ * power-of-two page boundary.
+ *
+ * @return A virtual pointer to the allocated and preserved memory on success,
+ * or an ERR_PTR() encoded error on failure.
+ */
+void *kho_alloc_preserve(size_t size)
+{
+	struct folio *folio;
+	int order, ret;
+
+	if (!size)
+		return ERR_PTR(-EINVAL);
+
+	order = get_order(size);
+	if (order > MAX_PAGE_ORDER)
+		return ERR_PTR(-E2BIG);
+
+	folio = folio_alloc(GFP_KERNEL | __GFP_ZERO, order);
+	if (!folio)
+		return ERR_PTR(-ENOMEM);
+
+	ret = kho_preserve_folio(folio);
+	if (ret) {
+		folio_put(folio);
+		return ERR_PTR(ret);
+	}
+
+	return folio_address(folio);
+}
+EXPORT_SYMBOL_GPL(kho_alloc_preserve);
+
+/**
+ * kho_free_unpreserve - Unpreserve and free memory.
+ * @mem:  Pointer to the memory allocated by kho_alloc_preserve().
+ * @size: The original size requested during allocation. This is used to
+ *        recalculate the correct order for freeing the pages.
+ *
+ * Unregisters the memory from KHO preservation and frees the underlying
+ * pages back to the system. This function should be called to clean up
+ * memory allocated with kho_alloc_preserve().
+ */
+void kho_free_unpreserve(void *mem, size_t size)
+{
+	struct folio *folio;
+	unsigned int order;
+
+	if (!mem || !size)
+		return;
+
+	order = get_order(size);
+	if (WARN_ON_ONCE(order > MAX_PAGE_ORDER))
+		return;
+
+	folio = virt_to_folio(mem);
+	WARN_ON_ONCE(kho_unpreserve_folio(folio));
+	folio_put(folio);
+}
+EXPORT_SYMBOL_GPL(kho_free_unpreserve);
+
+/**
+ * kho_free_restore - Restore and free memory after kexec.
+ * @mem:  Pointer to the memory (in the new kernel's address space)
+ * that was allocated by the old kernel.
+ * @size: The original size requested during allocation. This is used to
+ * recalculate the correct order for freeing the pages.
+ *
+ * This function is intended to be called in the new kernel (post-kexec)
+ * to take ownership of and free a memory region that was preserved by the
+ * old kernel using kho_alloc_preserve().
+ *
+ * It first restores the pages from KHO (using their physical address)
+ * and then frees the pages back to the new kernel's page allocator.
+ */
+void kho_free_restore(void *mem, size_t size)
+{
+	struct folio *folio;
+	unsigned int order;
+
+	if (!mem || !size)
+		return;
+
+	order = get_order(size);
+	if (WARN_ON_ONCE(order > MAX_PAGE_ORDER))
+		return;
+
+	folio = kho_restore_folio(__pa(mem));
+	if (!WARN_ON(!folio))
+		free_pages((unsigned long)mem, order);
+}
+EXPORT_SYMBOL_GPL(kho_free_restore);
+
 int kho_finalize(void)
 {
 	int ret;
-- 
2.52.0.rc1.455.g30608eb744-goog