memblock: Add reserve_mem debugfs info

When using the "reserve_mem" parameter, users aim at having an
area that (hopefully) persists across boots, so pstore infrastructure
(like ramoops module) can make use of that to save oops/ftrace logs,
for example.

There is no easy way to determine if this kernel parameter is properly
set though; the kernel doesn't show information about this memory in
memblock debugfs, neither in /proc/iomem nor dmesg. This is a relevant
information for tools like kdumpst[0], to determine if it's reliable
to use the reserved area as ramoops persistent storage; checking only
/proc/cmdline is not sufficient as it doesn't tell if the reservation
effectively succeeded or not.

Add here a new file under memblock debugfs showing properly set memory
reservations, with name and size as passed to "reserve_mem". Notice that
if no "reserve_mem=" is passed on command-line or if the reservation
attempts fail, the file is not created.

[0] https://aur.archlinux.org/packages/kdumpst

Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Link: https://patch.msgid.link/20260324012839.1991765-2-gpiccoli@igalia.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/tools/testing/memblock/linux/string_helpers.h b/tools/testing/memblock/linux/string_helpers.h
new file mode 100644
index 000000000000..dbf015cfff31
--- /dev/null
+++ b/tools/testing/memblock/linux/string_helpers.h
@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_STRING_HELPERS_H_
+#define _LINUX_STRING_HELPERS_H_
+
+/*
+ * Header stub to avoid test build breakage; we don't need to
+ * actually implement string_get_size() as it's not used in the tests.
+ */
+
+#endif