[PATCH 2/2] mm/memblock: Add reserve_mem debugfs info

Guilherme G. Piccoli
2026-02-17T19:58:48Z

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 (like unused memory "set" using
"mem=") nor in the kernel log (like the "crashkernel" parameter does).

Add here a new file under memblock debugfs showing properly set memory
reservations, with name, address and size as passed to "reserve_mem".

Notice this addition makes the memblock folder *always available* under
debugfs, regardless of ARCH_KEEP_MEMBLOCK and even if there is no
"reserve_mem=" setting in the command-line.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---
mm/memblock.c | 37 +++++++++++++++++++++++++++++++++++--
1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index 2d2646f7a120..58439de0a59b 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -18,6 +18,10 @@
#include <linux/memblock.h>
#include <linux/mutex.h>

+#ifdef CONFIG_DEBUG_FS
+#include <linux/string_helpers.h>
+#endif
+
#ifdef CONFIG_KEXEC_HANDOVER
#include <linux/libfdt.h>
#include <linux/kexec_handover.h>
@@ -2711,7 +2715,7 @@ static int __init reserve_mem(char *p)
}
__setup("reserve_mem=", reserve_mem);

-#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_ARCH_KEEP_MEMBLOCK)
+#ifdef CONFIG_DEBUG_FS
static const char * const flagname[] = {
[ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",
[ilog2(MEMBLOCK_MIRROR)] = "MIRROR",
@@ -2722,7 +2726,8 @@ static const char * const flagname[] = {
[ilog2(MEMBLOCK_KHO_SCRATCH)] = "KHO_SCRATCH",
};

-static int memblock_debug_show(struct seq_file *m, void *private)
+#ifdef CONFIG_ARCH_KEEP_MEMBLOCK
+static void memblock_debugfs_files(struct seq_file *m)
{
struct memblock_type *type = m->private;
struct memblock_region *reg;
@@ -2754,6 +2759,30 @@ static int memblock_debug_show(struct seq_file *m, void *private)
seq_printf(m, "%s\n", "NONE");
}
}
+}
+#else
+static void memblock_debugfs_files(struct seq_file *m) {}
+#endif /* CONFIG_ARCH_KEEP_MEMBLOCK */
+
+static int memblock_debug_show(struct seq_file *m, void *private)
+{
+ if (m->private == &reserved_mem_table[0]) {
+ struct reserve_mem_table *map;
+ char txtsz[16];
+
+ for (int i = 0; i < reserved_mem_count; i++) {
+ map = &reserved_mem_table[i];
+ if (!map->size)
+ continue;
+
+ memset(txtsz, 0, 16);
+ string_get_size((u64)(map->size), 1, STRING_UNITS_2, txtsz, 16);
+ seq_printf(m, "%s\t\t%pa\t(%s)\n",
+ map->name, &map->start, txtsz);
+ }
+ } else
+ memblock_debugfs_files(m);
+
return 0;
}
DEFINE_SHOW_ATTRIBUTE(memblock_debug);
@@ -2762,6 +2791,9 @@ static int __init memblock_init_debugfs(void)
{
struct dentry *root = debugfs_create_dir("memblock", NULL);

+ debugfs_create_file("reserve_mem_param", 0444, root,
+ &reserved_mem_table[0], &memblock_debug_fops);
+#ifdef CONFIG_ARCH_KEEP_MEMBLOCK
debugfs_create_file("memory", 0444, root,
&memblock.memory, &memblock_debug_fops);
debugfs_create_file("reserved", 0444, root,
@@ -2771,6 +2803,7 @@ static int __init memblock_init_debugfs(void)
&memblock_debug_fops);
#endif

+#endif /* CONFIG_ARCH_KEEP_MEMBLOCK */
return 0;
}
__initcall(memblock_init_debugfs);
--
2.50.1