[PATCH 7/9] kho: Report how much handover memory is preserved and restored

[Chris Down <chris@chrisdown.name>]

Most memory the previous kernel preserved stays reserved until its user
restores it through KHO, either to use it again or to free it. If
nothing does, it stays reserved for as long as the kernel runs. On a
hypervisor host that can be gigabytes, but amusingly nothing really
shows it reliably. The host simply has less memory than it should, which
is not exactly obvious.

All preserved memory is reserved in kho_preserved_memory_reserve(), and
everything restored through KHO goes through kho_restore_page(), so KHO
can count both easily. Let's add preserved_size and restored_size, in
bytes, to /sys/kernel/kexec/kho_incoming/.

For example, after a live update preserving a 64 MiB memfd:

                     preserved_size  restored_size
    before retrieval       67403776           8192
    after retrieval        67403776       67395584

The 8 KiB left are the incoming KHO FDT and LUO's FLB header.

Signed-off-by: Chris Down <chris@chrisdown.name>
---
 .../ABI/testing/sysfs-kernel-kexec-kdump      | 10 ++++++++
 Documentation/admin-guide/mm/kho.rst          |  3 +++
 kernel/liveupdate/kexec_handover.c            | 23 +++++++++++++++++++
 3 files changed, 36 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
index 5636cd060cb2..2b9f9917d2d4 100644
--- a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
+++ b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
@@ -79,3 +79,13 @@ Description:	read only
 		           the walk of its preserved memory map.
 		"failed"   The kernel rejected the handover.
 User:		system monitoring
+
+What:		/sys/kernel/kexec/kho_incoming/preserved_size
+What:		/sys/kernel/kexec/kho_incoming/restored_size
+Date:		Nov 2026
+Contact:	kexec@lists.infradead.org
+Description:	read only
+		Bytes preserved by the previous kernel and bytes restored
+		through KHO, respectively. Both are 0 unless the incoming
+		state is "revived".
+User:		system monitoring
diff --git a/Documentation/admin-guide/mm/kho.rst b/Documentation/admin-guide/mm/kho.rst
index 9d74d4ce18b5..37a62daa7622 100644
--- a/Documentation/admin-guide/mm/kho.rst
+++ b/Documentation/admin-guide/mm/kho.rst
@@ -91,6 +91,9 @@ sysfs Interfaces
 ``/sys/kernel/kexec/kho_incoming/state``
     Status of the incoming KHO handover.
 
+``/sys/kernel/kexec/kho_incoming/preserved_size`` and ``restored_size``
+    Bytes preserved by the previous kernel and bytes restored through KHO.
+
 See Documentation/ABI/testing/sysfs-kernel-kexec-kdump for the sysfs ABI.
 
 debugfs Interfaces
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index dfd6fa7e4a71..fe6b66270124 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -110,6 +110,8 @@ struct kho_in {
 	char previous_release[__NEW_UTS_LEN + 1];
 	u32 kexec_count;
 	enum kho_incoming_state state;
+	unsigned long preserved_pages;
+	atomic_long_t restored_pages;
 	struct kho_debugfs dbg;
 	struct kho_radix_tree radix_tree;
 };
@@ -552,6 +554,7 @@ static struct page *kho_restore_page(phys_addr_t phys, bool is_folio,
 		kho_init_pages(page, nr_pages);
 
 	adjust_managed_page_count(page, nr_pages);
+	atomic_long_add(nr_pages, &kho_in.restored_pages);
 	return page;
 }
 
@@ -643,6 +646,7 @@ static int __init kho_preserved_memory_reserve(unsigned long key, void *data)
 	info.magic = KHO_PAGE_MAGIC;
 	info.order = order;
 	page->private = info.page_private;
+	kho_in.preserved_pages += 1UL << order;
 
 	return 0;
 }
@@ -978,8 +982,26 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 }
 static struct kobj_attribute state_attr = __ATTR_RO(state);
 
+static ssize_t preserved_size_show(struct kobject *kobj,
+				   struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%llu\n", (u64)kho_in.preserved_pages << PAGE_SHIFT);
+}
+static struct kobj_attribute preserved_size_attr = __ATTR_RO(preserved_size);
+
+static ssize_t restored_size_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	u64 pages = atomic_long_read(&kho_in.restored_pages);
+
+	return sysfs_emit(buf, "%llu\n", pages << PAGE_SHIFT);
+}
+static struct kobj_attribute restored_size_attr = __ATTR_RO(restored_size);
+
 static struct attribute *kho_incoming_attrs[] = {
 	&state_attr.attr,
+	&preserved_size_attr.attr,
+	&restored_size_attr.attr,
 	NULL
 };
 
@@ -1785,6 +1807,7 @@ static void __init kho_mem_retrieve(void)
 	kho_in.fdt_phys = 0;
 	kho_in.radix_tree.root = NULL;
 	kho_in.state = KHO_INCOMING_FAILED;
+	kho_in.preserved_pages = 0;
 }
 
 static __init int kho_out_fdt_setup(void)
-- 
2.54.0