[PATCH V2 2/2] mm/memblock: Add reserve_mem debugfs info

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

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>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---


Thanks a lot for the suggestions Mike! I'm not sure if you would
prefer a Co-Developed-by or Suggested-by instead of CC, you helped
a lot improving the code. Lemme know and either I can re-submit
(with potential other changes) or even, you can change while merging.

V2: (all suggestions by Mike Rapoport)

- Commit message (showing use case);
- Drop ifdef on include "string_helpers.h";
- Don't show the address of reserve_mem, only name and size;
- Fixed flag names inside ARCH_KEEP_MEMBLOCK ifdef;
- Make use of its own show_attribute instead of refactoring
  the memblock one;
- Use sizeof() instead of magical numbers for the size;
- Don't show memblock directory if no reserve_mem succeeded
  and ARCH_KEEP_MEMBLOCK isn't defined (keeping current behavior).


 mm/memblock.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index 2d2646f7a120..d816796ab919 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>
@@ -2711,7 +2712,8 @@ 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",
@@ -2758,10 +2760,40 @@ static int memblock_debug_show(struct seq_file *m, void *private)
 }
 DEFINE_SHOW_ATTRIBUTE(memblock_debug);
 
+#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 = debugfs_create_dir("memblock", NULL);
+	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);
+#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