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);
--
2.50.1On Tue, Feb 17, 2026 at 04:45:07PM -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 (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
No need to ifdef this, a small additional header is not a big deal and it's
anyway included in seq_file.h.
> +
> #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[] = {
Flag names only needed when ARCH_KEEP_MEMBLOCK is set ...
> [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
... so this ifdef should go before the flagname declaration.
> +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);
phys_addr_t should be casted automatically to u64 IMO.
And please, no magic numbers.
> + seq_printf(m, "%s\t\t%pa\t(%s)\n",
> + map->name, &map->start, txtsz);
Let's not expose the physical address, name and size should be enough to
see which reserve_mem allocations succeeded.
> + }
You can define "reserve_mem" attribute separately, and leave the existing
memblock_debug_show() as it was.
> + } 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);
No need to create memblock directory in debugfs if there's nothing to show.
Could be something like
if (i!(IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK) || reserved_mem_count))
return;
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
>
>
--
Sincerely yours,
Mike.
Hi Mike, thanks a bunch for your review! Good ideas, I'll change the
implementation to follow the suggestions and improve the changelog, as
you suggested. I have some questions though, that I will comment inline,
below.
First of all, what should we do regarding patch 1? Should I resubmit as
part of V2, even with no changes - or pick it now and I only submit
patch 2 as V2, with changes?
On 21/02/2026 05:52, Mike Rapoport wrote:
> [...]
>> + string_get_size((u64)(map->size), 1, STRING_UNITS_2, txtsz, 16);
>
> phys_addr_t should be casted automatically to u64 IMO.
> And please, no magic numbers.
Specifically here, by magic number you mean my choice of 16, right? What
do you suggest me to pick? It's the length of the string carrying the
size of reserved_mem, some number must be selected for this
length...lemme know WDYT.
> [...]
> You can define "reserve_mem" attribute separately, and leave the existing
> memblock_debug_show() as it was.
>
>> + } 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);
>
> No need to create memblock directory in debugfs if there's nothing to show.
> Could be something like
>
> if (i!(IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK) || reserved_mem_count))
> return;
>
> root = debugfs_create_dir("memblock", NULL);
Very good suggestions here, but just let me clarify: so I could continue
showing the "reserved_mem_param" inside the "<debugfs>/memblock" folder,
just using a different function for that attribute?
I understood that, based on your (good) suggestion to hide the memblock
folder if ARCH_KEEP_MEMBLOCK is not defined and there is no reserved_mem
set ... just want to confirm to follow-up the implementation.
Cheers,
Guilherme
On Sun, Feb 22, 2026 at 10:58:27AM -0300, Guilherme G. Piccoli wrote:
> Hi Mike, thanks a bunch for your review! Good ideas, I'll change the
> implementation to follow the suggestions and improve the changelog, as
> you suggested. I have some questions though, that I will comment inline,
> below.
>
> First of all, what should we do regarding patch 1? Should I resubmit as
> part of V2, even with no changes - or pick it now and I only submit
> patch 2 as V2, with changes?
Please resend both patches together.
> On 21/02/2026 05:52, Mike Rapoport wrote:
> > [...]
> >> + string_get_size((u64)(map->size), 1, STRING_UNITS_2, txtsz, 16);
> >
> > phys_addr_t should be casted automatically to u64 IMO.
> > And please, no magic numbers.
>
> Specifically here, by magic number you mean my choice of 16, right? What
> do you suggest me to pick? It's the length of the string carrying the
> size of reserved_mem, some number must be selected for this
> length...lemme know WDYT.
sizeof(txtsz) should work :)
> > [...]
> > You can define "reserve_mem" attribute separately, and leave the existing
> > memblock_debug_show() as it was.
> >
> >> + } 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);
> >
> > No need to create memblock directory in debugfs if there's nothing to show.
> > Could be something like
> >
> > if (i!(IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK) || reserved_mem_count))
> > return;
> >
> > root = debugfs_create_dir("memblock", NULL);
>
> Very good suggestions here, but just let me clarify: so I could continue
> showing the "reserved_mem_param" inside the "<debugfs>/memblock" folder,
> just using a different function for that attribute?
Yes, something like
static int memblock_reserve_mem_show(struct seq_file *m, void *private)
{
...
}
DEFINE_SHOW_ATTRIBUTE(memblock_reserve_mem);
> I understood that, based on your (good) suggestion to hide the memblock
> folder if ARCH_KEEP_MEMBLOCK is not defined and there is no reserved_mem
> set ... just want to confirm to follow-up the implementation.
Yes, that's what I meant.
> Cheers,
> Guilherme
--
Sincerely yours,
Mike.
On 22/02/2026 16:19, Mike Rapoport wrote:
> [...]
>> First of all, what should we do regarding patch 1? Should I resubmit as
>> part of V2, even with no changes - or pick it now and I only submit
>> patch 2 as V2, with changes?
>
> Please resend both patches together.
>
Hi Mike! OK, thanks.
>> [...]
>> Specifically here, by magic number you mean my choice of 16, right? What
>> do you suggest me to pick? It's the length of the string carrying the
>> size of reserved_mem, some number must be selected for this
>> length...lemme know WDYT.
>
> sizeof(txtsz) should work :)
>
Ahh OK heh
I thought you were not happy with 16 as the size (this is an arbitrary
number itself), but it's just on the func argument - cool, will fix it!
>>> [...]
>> Very good suggestions here, but just let me clarify: so I could continue
>> showing the "reserved_mem_param" inside the "<debugfs>/memblock" folder,
>> just using a different function for that attribute?
>
> Yes, something like
>
> static int memblock_reserve_mem_show(struct seq_file *m, void *private)
> {
> ...
> }
> DEFINE_SHOW_ATTRIBUTE(memblock_reserve_mem);
>
>> I understood that, based on your (good) suggestion to hide the memblock
>> folder if ARCH_KEEP_MEMBLOCK is not defined and there is no reserved_mem
>> set ... just want to confirm to follow-up the implementation.
>
> Yes, that's what I meant.
>
Great, thanks a lot again!
Cheers,
Guilherme
Hi Guilherme,
kernel test robot noticed the following build warnings:
[auto build test WARNING on rppt-memblock/for-next]
[also build test WARNING on rppt-memblock/fixes akpm-mm/mm-everything linus/master v6.19 next-20260217]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Guilherme-G-Piccoli/mm-memblock-Print-out-errors-on-reserve_mem-parser/20260218-035935
base: https://git.kernel.org/pub/scm/linux/kernel/git/rppt/memblock.git for-next
patch link: https://lore.kernel.org/r/20260217195816.861684-3-gpiccoli%40igalia.com
patch subject: [PATCH 2/2] mm/memblock: Add reserve_mem debugfs info
config: xtensa-randconfig-r071-20260218 (https://download.01.org/0day-ci/archive/20260218/202602180841.dpP5OelN-lkp@intel.com/config)
compiler: xtensa-linux-gcc (GCC) 11.5.0
smatch version: v0.5.0-8994-gd50c5a4c
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260218/202602180841.dpP5OelN-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602180841.dpP5OelN-lkp@intel.com/
All warnings (new ones prefixed by >>):
>> mm/memblock.c:2721:27: warning: 'flagname' defined but not used [-Wunused-const-variable=]
2721 | static const char * const flagname[] = {
| ^~~~~~~~
vim +/flagname +2721 mm/memblock.c
1e4c64b71c9bf2 Steven Rostedt (Google 2024-06-13 2719)
54091ffecefa11 Guilherme G. Piccoli 2026-02-17 2720 #ifdef CONFIG_DEBUG_FS
493f349e38d022 Yuwei Guan 2023-05-19 @2721 static const char * const flagname[] = {
493f349e38d022 Yuwei Guan 2023-05-19 2722 [ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",
493f349e38d022 Yuwei Guan 2023-05-19 2723 [ilog2(MEMBLOCK_MIRROR)] = "MIRROR",
493f349e38d022 Yuwei Guan 2023-05-19 2724 [ilog2(MEMBLOCK_NOMAP)] = "NOMAP",
493f349e38d022 Yuwei Guan 2023-05-19 2725 [ilog2(MEMBLOCK_DRIVER_MANAGED)] = "DRV_MNG",
4f155af0ae4464 Anshuman Khandual 2024-02-09 2726 [ilog2(MEMBLOCK_RSRV_NOINIT)] = "RSV_NIT",
4c78cc596bb8d3 Mike Rapoport (Microsoft 2025-05-09 2727) [ilog2(MEMBLOCK_RSRV_KERN)] = "RSV_KERN",
d59f43b5748092 Alexander Graf 2025-05-09 2728 [ilog2(MEMBLOCK_KHO_SCRATCH)] = "KHO_SCRATCH",
493f349e38d022 Yuwei Guan 2023-05-19 2729 };
6d03b885f0926a Benjamin Herrenschmidt 2010-07-06 2730
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
Hi Guilherme,
kernel test robot noticed the following build warnings:
[auto build test WARNING on rppt-memblock/for-next]
[also build test WARNING on rppt-memblock/fixes akpm-mm/mm-everything linus/master v6.19 next-20260217]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Guilherme-G-Piccoli/mm-memblock-Print-out-errors-on-reserve_mem-parser/20260218-035935
base: https://git.kernel.org/pub/scm/linux/kernel/git/rppt/memblock.git for-next
patch link: https://lore.kernel.org/r/20260217195816.861684-3-gpiccoli%40igalia.com
patch subject: [PATCH 2/2] mm/memblock: Add reserve_mem debugfs info
config: s390-randconfig-002-20260218 (https://download.01.org/0day-ci/archive/20260218/202602180820.Vi6LlvfF-lkp@intel.com/config)
compiler: clang version 23.0.0git (https://github.com/llvm/llvm-project e86750b29fa0ff207cd43213d66dabe565417638)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260218/202602180820.Vi6LlvfF-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602180820.Vi6LlvfF-lkp@intel.com/
All warnings (new ones prefixed by >>):
>> mm/memblock.c:2721:27: warning: unused variable 'flagname' [-Wunused-const-variable]
2721 | static const char * const flagname[] = {
| ^~~~~~~~
1 warning generated.
vim +/flagname +2721 mm/memblock.c
1e4c64b71c9bf2 Steven Rostedt (Google 2024-06-13 2719)
54091ffecefa11 Guilherme G. Piccoli 2026-02-17 2720 #ifdef CONFIG_DEBUG_FS
493f349e38d022 Yuwei Guan 2023-05-19 @2721 static const char * const flagname[] = {
493f349e38d022 Yuwei Guan 2023-05-19 2722 [ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",
493f349e38d022 Yuwei Guan 2023-05-19 2723 [ilog2(MEMBLOCK_MIRROR)] = "MIRROR",
493f349e38d022 Yuwei Guan 2023-05-19 2724 [ilog2(MEMBLOCK_NOMAP)] = "NOMAP",
493f349e38d022 Yuwei Guan 2023-05-19 2725 [ilog2(MEMBLOCK_DRIVER_MANAGED)] = "DRV_MNG",
4f155af0ae4464 Anshuman Khandual 2024-02-09 2726 [ilog2(MEMBLOCK_RSRV_NOINIT)] = "RSV_NIT",
4c78cc596bb8d3 Mike Rapoport (Microsoft 2025-05-09 2727) [ilog2(MEMBLOCK_RSRV_KERN)] = "RSV_KERN",
d59f43b5748092 Alexander Graf 2025-05-09 2728 [ilog2(MEMBLOCK_KHO_SCRATCH)] = "KHO_SCRATCH",
493f349e38d022 Yuwei Guan 2023-05-19 2729 };
6d03b885f0926a Benjamin Herrenschmidt 2010-07-06 2730
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
© 2016 - 2026 Red Hat, Inc.