[PATCH] tools/mm: add script to display /proc/buddyinfo from vmcore

[Aaron Tomlin <atomlin@atomlin.com>]

When analysing crash dumps (vmcores), inspecting the buddy allocator's free
page distribution across memory zones can be useful for diagnosing
allocation failures, fragmentation, and watermark exhaustion.

Introduce tools/mm/show_buddyinfo.py, a drgn-based script that traverses
online NUMA nodes and populated memory zones to inspect free_area[].nr_free
and reconstruct the standard /proc/buddyinfo output.

For example:
❯ sudo drgn --no-default-symbols -c /proc/kcore -s vmlinux tools/mm/show_buddyinfo.py
Node 0, zone DMA           0      0      0      0      0      0      0      0      1      2      2
Node 0, zone DMA32       907    831    581    423    214     85      3      1      2      2      0
Node 0, zone Normal        1      1    143    731    552    182     32      0      0      0      0

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 tools/mm/show_buddyinfo.py | 48 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100755 tools/mm/show_buddyinfo.py

diff --git a/tools/mm/show_buddyinfo.py b/tools/mm/show_buddyinfo.py
new file mode 100755
index 000000000000..70372329e7c1
--- /dev/null
+++ b/tools/mm/show_buddyinfo.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env drgn
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2026 Aaron Tomlin <atomlin@atomlin.com>
+
+from collections.abc import Iterator
+
+from drgn import Object, Program, get_default_prog
+from drgn.helpers.linux.nodemask import for_each_online_node
+
+DESC = """
+This is a drgn-based Python script that displays /proc/buddyinfo from a
+vmcore. For further information on drgn, please refer to
+https://github.com/osandov/drgn.
+"""
+
+
+def buddyinfo():
+    """Display the buddyinfo."""
+
+    def walk_zones_in_node(prog: Program) -> Iterator[tuple[Object, Object]]:
+        for node_nr in for_each_online_node(prog):
+            try:
+                pgdat = prog["node_data"][node_nr]
+            except (KeyError, Exception):
+                pgdat = prog["contig_page_data"].address_of_()
+            for zone in pgdat.node_zones:
+                yield pgdat, zone
+
+    def populated_zone(zone: Object) -> bool:
+        """Determine if the specified zone is populated."""
+        return zone.present_pages.value_() > 0
+
+    prog = get_default_prog()
+    for pgdat, zone in walk_zones_in_node(prog):
+        if populated_zone(zone):
+            print(
+                f"Node {pgdat.node_id.value_()}, zone "
+                f"{zone.name.string_().decode('utf-8'):8s} ",
+                end="",
+            )
+            for order in range(len(zone.free_area)):
+                print(f"{zone.free_area[order].nr_free.value_():6} ", end="")
+            print()
+
+
+if __name__ == "__main__":
+    buddyinfo()
-- 
2.55.0