[PATCH v2] tools/cgroup/slabinfo: Fix use of slab.memcg_data

[PATCH v2] tools/cgroup/slabinfo: Fix use of slab.memcg_data

[Hongfu Li]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset=y, Size: 3609 bytes --]

After the introduce slabobj_ext to support slab object extensions, the
memcg_slabinfo tool broke. An attempt to run it produces a trace like
this:
Traceback (most recent call last):
  File "/usr/local/bin/drgn", line 8, in <module>
    sys.exit(_main())
             ^^^^^^^
  File "/usr/local/lib64/python3.11/site-packages/drgn/cli.py", line 688, in _main
    runpy.run_path(
  File "<frozen runpy>", line 291, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "/root/memcg_slabinfo.py", line 225, in <module>
    main()
  File "/root/memcg_slabinfo.py", line 195, in main
    objcg_vec_raw = slab.memcg_data.value_()
AttributeError: 'struct slab' has no member 'memcg_data'

Fixes: 21c690a349ba ("mm: introduce slabobj_ext to support slab object extensions")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>

Changes in v2:
- Skip slabs after masking when the base is zero (OBJEXTS_ALLOC_FAIL).
- Walk slab->obj_exts using base + stride * i (same indexing as slab_obj_ext()).
- Link to v1: https://lore.kernel.org/all/20260417020729.952897-1-lihongfu@kylinos.cn/
---
 tools/cgroup/memcg_slabinfo.py | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/tools/cgroup/memcg_slabinfo.py b/tools/cgroup/memcg_slabinfo.py
index 6bf4bde77903..b5d4e51671cd 100644
--- a/tools/cgroup/memcg_slabinfo.py
+++ b/tools/cgroup/memcg_slabinfo.py
@@ -33,6 +33,21 @@ def err(s):
     sys.exit(1)
 
 
+def objexts_flags_mask():
+    try:
+        return int(prog.constant('__NR_OBJEXTS_FLAGS')) - 1
+    except:
+        return 0x7
+
+
+def slab_obj_ext_stride(slab):
+    # Match slab_obj_ext() for both layouts: contiguous ext[] or ext-in-object
+    try:
+        return slab.stride.value_()
+    except AttributeError:
+        return prog.type('struct slabobj_ext').size
+
+
 def find_memcg_ids(css=prog['root_mem_cgroup'].css, prefix=''):
     if not list_empty(css.children.address_of_()):
         for css in list_for_each_entry('struct cgroup_subsys_state',
@@ -192,23 +207,30 @@ def main():
         # look over all slab folios and look for objects belonging
         # to the given memory cgroup
         for slab in for_each_slab(prog):
-            objcg_vec_raw = slab.memcg_data.value_()
-            if objcg_vec_raw == 0:
+            objext_vec_raw = slab.obj_exts.value_()
+            if objext_vec_raw == 0:
                 continue
             cache = slab.slab_cache
             if not cache:
                 continue
             addr = cache.value_()
             caches[addr] = cache
-            # clear the lowest bit to get the true obj_cgroups
-            objcg_vec = Object(prog, 'struct obj_cgroup **',
-                               value=objcg_vec_raw & ~1)
 
             if addr not in stats:
                 stats[addr] = 0
 
+            # clear OBJEXTS_FLAGS_MASK bits to get the true slabobj_ext base.
+            # If OBJEXTS_ALLOC_FAIL, masked base is 0 — not tied to any cgroup.
+            mask = objexts_flags_mask()
+            objext_base = objext_vec_raw & ~mask
+            if objext_base == 0:
+                continue
+
+            stride = slab_obj_ext_stride(slab)
             for i in range(oo_objects(cache)):
-                if objcg_vec[i].value_() in obj_cgroups:
+                objext = Object(prog, 'struct slabobj_ext *',
+                                value=objext_base + stride * i)
+                if objext.objcg.value_() in obj_cgroups:
                     stats[addr] += 1
 
         for addr in caches:
-- 
2.25.1

Re: [PATCH v2] tools/cgroup/slabinfo: Fix use of slab.memcg_data

[Harry Yoo (Oracle)]

On Tue, Apr 21, 2026 at 01:58:29PM +0800, Hongfu Li wrote:
> After the introduce slabobj_ext to support slab object extensions, the
> memcg_slabinfo tool broke. An attempt to run it produces a trace like
> this:
> Traceback (most recent call last):
>   File "/usr/local/bin/drgn", line 8, in <module>
>     sys.exit(_main())
>              ^^^^^^^
>   File "/usr/local/lib64/python3.11/site-packages/drgn/cli.py", line 688, in _main
>     runpy.run_path(
>   File "<frozen runpy>", line 291, in run_path
>   File "<frozen runpy>", line 98, in _run_module_code
>   File "<frozen runpy>", line 88, in _run_code
>   File "/root/memcg_slabinfo.py", line 225, in <module>
>     main()
>   File "/root/memcg_slabinfo.py", line 195, in main
>     objcg_vec_raw = slab.memcg_data.value_()
> AttributeError: 'struct slab' has no member 'memcg_data'
> 
> Fixes: 21c690a349ba ("mm: introduce slabobj_ext to support slab object extensions")
> Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
> 
> Changes in v2:

Changes between versions should not be part of the commit message.
to the changelog.

> - Skip slabs after masking when the base is zero (OBJEXTS_ALLOC_FAIL).
> - Walk slab->obj_exts using base + stride * i (same indexing as slab_obj_ext()).

These two changes looks good to me.

> - Link to v1: https://lore.kernel.org/all/20260417020729.952897-1-lihongfu@kylinos.cn/
> ---

"Changes since ..." should be under this '---' line so that it won't be
included in the commit message.

Not sure what kernel version you're using, but due to recent changes
in slab (v7.1-rc1), we need this:

diff --git a/tools/cgroup/memcg_slabinfo.py b/tools/cgroup/memcg_slabinfo.py
old mode 100644
new mode 100755
index b5d4e51671cd..e54e2bb65fe2
--- a/tools/cgroup/memcg_slabinfo.py
+++ b/tools/cgroup/memcg_slabinfo.py
@@ -101,7 +101,7 @@ def slub_get_slabinfo(s, cfg):
     nr_free = 0

     for node in range(cfg['nr_nodes']):
-        n = s.node[node]
+        n = s.per_node[node].node
         nr_slabs += n.nr_slabs.counter.value_()
         nr_objs += n.total_objects.counter.value_()
         nr_free += count_partial(n, count_free)

Perhaps you could make it work for kernel w/ and w/o the recent slab
changes. (By checking if the field exists and falling back)

Please address this and adjust the commit message accordingly.

-- 
Cheers,
Harry / Hyeonggon