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

[Hao Ge <hao.ge@linux.dev>]

Hi Hongfu


On 2026/4/17 10:07, 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>


Good catch!


> ---
>   tools/cgroup/memcg_slabinfo.py | 20 ++++++++++++++------
>   1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/tools/cgroup/memcg_slabinfo.py b/tools/cgroup/memcg_slabinfo.py
> index 6bf4bde77903..e67dcfae1263 100644
> --- a/tools/cgroup/memcg_slabinfo.py
> +++ b/tools/cgroup/memcg_slabinfo.py
> @@ -33,6 +33,13 @@ def err(s):
>       sys.exit(1)
>   
>   
> +def objexts_flags_mask():
> +    try:
> +        return int(prog.constant('__NR_OBJEXTS_FLAGS')) - 1
> +    except:
> +        return 0x7
> +
> +
>   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 +199,24 @@ 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_()

Here, we should also check whether it equals OBJEXTS_ALLOC_FAIL,

which indicates that the allocation of obj_exts has failed, Or have I 
missed something here?

Thanks

BR

Hao

> +            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
> +            mask = objexts_flags_mask()
> +            objext_vec = Object(prog, 'struct slabobj_ext *',
> +                                value=objext_vec_raw & ~mask)
>               for i in range(oo_objects(cache)):
> -                if objcg_vec[i].value_() in obj_cgroups:
> +                if objext_vec[i].objcg.value_() in obj_cgroups:
>                       stats[addr] += 1
>   
>           for addr in caches: