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
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---
V3:
- Added header stub to avoid build failure (thanks Mike!).
- Refactored the debugfs creation routine to avoid too many
ifdefs, also changing (!(a || b)) to (!a && !b) in the function
(thanks SJ and Mike for the suggestions in this one!).
Notice that I've inverted the order of file creation, putting
physmem (and its ifdef) *before* - the reason for that is purely
because in my view, the code looks better this way, the ifdefs
are a bit more far from each other. I'm totally OK in changing
it, up yo you folks.
Cheers!
mm/memblock.c | 56 ++++++++++++++++---
tools/testing/memblock/linux/string_helpers.h | 10 ++++
2 files changed, 58 insertions(+), 8 deletions(-)
create mode 100644 tools/testing/memblock/linux/string_helpers.h
diff --git a/mm/memblock.c b/mm/memblock.c
index ac08d7f8c15e..f4219f434ce5 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -17,6 +17,7 @@
#include <linux/seq_file.h>
#include <linux/memblock.h>
#include <linux/mutex.h>
+#include <linux/string_helpers.h>
#ifdef CONFIG_KEXEC_HANDOVER
#include <linux/libfdt.h>
@@ -2710,7 +2711,8 @@ 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
+#ifdef CONFIG_ARCH_KEEP_MEMBLOCK
static const char * const flagname[] = {
[ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",
[ilog2(MEMBLOCK_MIRROR)] = "MIRROR",
@@ -2757,19 +2759,57 @@ static int memblock_debug_show(struct seq_file *m, void *private)
}
DEFINE_SHOW_ATTRIBUTE(memblock_debug);
-static int __init memblock_init_debugfs(void)
+static inline void memblock_debugfs_make_dirs(struct dentry *root)
{
- struct dentry *root = debugfs_create_dir("memblock", NULL);
-
- debugfs_create_file("memory", 0444, root,
- &memblock.memory, &memblock_debug_fops);
- debugfs_create_file("reserved", 0444, root,
- &memblock.reserved, &memblock_debug_fops);
#ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
debugfs_create_file("physmem", 0444, root, &physmem,
&memblock_debug_fops);
#endif
+ debugfs_create_file("memory", 0444, root,
+ &memblock.memory, &memblock_debug_fops);
+ debugfs_create_file("reserved", 0444, root,
+ &memblock.reserved, &memblock_debug_fops);
+}
+#else
+
+static inline void memblock_debugfs_make_dirs(struct dentry *root) { }
+
+#endif /* CONFIG_ARCH_KEEP_MEMBLOCK */
+
+static int memblock_reserve_mem_show(struct seq_file *m, void *private)
+{
+ 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, sizeof(txtsz));
+ string_get_size(map->size, 1, STRING_UNITS_2, txtsz, sizeof(txtsz));
+ seq_printf(m, "%s\t\t(%s)\n", map->name, txtsz);
+ }
+
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(memblock_reserve_mem);
+
+static int __init memblock_init_debugfs(void)
+{
+ struct dentry *root;
+
+ if (!IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK) && !reserved_mem_count)
+ return 0;
+
+ root = debugfs_create_dir("memblock", NULL);
+
+ if (reserved_mem_count)
+ debugfs_create_file("reserve_mem_param", 0444, root, NULL,
+ &memblock_reserve_mem_fops);
+
+ memblock_debugfs_make_dirs(root);
return 0;
}
__initcall(memblock_init_debugfs);
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
--
2.50.1On Wed, Mar 18, 2026 at 03:56:33PM -0300, Guilherme G. Piccoli wrote:
> 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.
...
> -static int __init memblock_init_debugfs(void)
> +static inline void memblock_debugfs_make_dirs(struct dentry *root)
This does not make dirs but rather exposes files representing memblock
arrays.
How about calling this function
memblock_debugfs_expose_arrays()?
> {
> - struct dentry *root = debugfs_create_dir("memblock", NULL);
> -
> - debugfs_create_file("memory", 0444, root,
> - &memblock.memory, &memblock_debug_fops);
> - debugfs_create_file("reserved", 0444, root,
> - &memblock.reserved, &memblock_debug_fops);
> #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
> debugfs_create_file("physmem", 0444, root, &physmem,
> &memblock_debug_fops);
> #endif
> + debugfs_create_file("memory", 0444, root,
> + &memblock.memory, &memblock_debug_fops);
> + debugfs_create_file("reserved", 0444, root,
> + &memblock.reserved, &memblock_debug_fops);
No need to move these after PHYS_MAP attribute
> +}
--
Sincerely yours,
Mike.
On 19/03/2026 13:34, Mike Rapoport wrote:
> [...]
>> -static int __init memblock_init_debugfs(void)
>> +static inline void memblock_debugfs_make_dirs(struct dentry *root)
>
> This does not make dirs but rather exposes files representing memblock
> arrays.
>
> How about calling this function
> memblock_debugfs_expose_arrays()?
>
I sincerely couldn't agree more - it should have have been a braino, why
on earth I called it this way?! Apologies, will respin soon...will wait
until weekend in case someone else has opinions.
>> {
>> - struct dentry *root = debugfs_create_dir("memblock", NULL);
>> -
>> - debugfs_create_file("memory", 0444, root,
>> - &memblock.memory, &memblock_debug_fops);
>> - debugfs_create_file("reserved", 0444, root,
>> - &memblock.reserved, &memblock_debug_fops);
>> #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
>> debugfs_create_file("physmem", 0444, root, &physmem,
>> &memblock_debug_fops);
>> #endif
>> + debugfs_create_file("memory", 0444, root,
>> + &memblock.memory, &memblock_debug_fops);
>> + debugfs_create_file("reserved", 0444, root,
>> + &memblock.reserved, &memblock_debug_fops);
>
> No need to move these after PHYS_MAP attribute
>
OK, your call, will change it in the respin as well.
Thanks,
Guilherme
On Thu, Mar 19, 2026 at 02:14:00PM -0300, Guilherme G. Piccoli wrote:
> On 19/03/2026 13:34, Mike Rapoport wrote:
> > [...]
> >> {
> >> - struct dentry *root = debugfs_create_dir("memblock", NULL);
> >> -
> >> - debugfs_create_file("memory", 0444, root,
> >> - &memblock.memory, &memblock_debug_fops);
> >> - debugfs_create_file("reserved", 0444, root,
> >> - &memblock.reserved, &memblock_debug_fops);
> >> #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
> >> debugfs_create_file("physmem", 0444, root, &physmem,
> >> &memblock_debug_fops);
> >> #endif
> >> + debugfs_create_file("memory", 0444, root,
> >> + &memblock.memory, &memblock_debug_fops);
> >> + debugfs_create_file("reserved", 0444, root,
> >> + &memblock.reserved, &memblock_debug_fops);
> >
> > No need to move these after PHYS_MAP attribute
> >
>
> OK, your call, will change it in the respin as well.
> Thanks,
That's actually more of a rule in the kernel: don't introduce unnecessary
changes :)
> Guilherme
>
--
Sincerely yours,
Mike.
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