Re: [PATCH v3] tools/cgroup/slabinfo: Fix use of slab.memcg_data

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

Hi Hongfu


On 2026/4/22 15:39, 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")
> Fixes: 5ba6bc27b1f9 ("slab: decouple pointer to barn from kmem_cache_node")
> Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
> ---
> v3:
> - Add a compatibility accessor for per-node cache state (per_node[nid].node vs node[nid])
> - Link to v2: https://lore.kernel.org/all/20260421055829.3930289-1-lihongfu@kylinos.cn/
> 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 | 45 ++++++++++++++++++++++++++++------
>   1 file changed, 38 insertions(+), 7 deletions(-)
>
> diff --git a/tools/cgroup/memcg_slabinfo.py b/tools/cgroup/memcg_slabinfo.py
> index 6bf4bde77903..f49512831d6a 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',
> @@ -68,6 +83,13 @@ def oo_objects(s):
>       return s.oo.x & OO_MASK
>   
>   
> +def kmem_cache_get_node(s, nid):
> +    try:
> +        return s.per_node[nid].node
> +    except AttributeError:
> +        return s.node[nid]
> +

This gives me another idea.

So actually, we don't need to forcibly convert memcg_data to obj_exts, 
right?

like this:

try:
       raw = slab.memcg_data.value_()
       new_layout = False
   except AttributeError:
       raw = slab.obj_exts.value_()
       new_layout = True

and then take the new path based on new_layout.

But I'm not sure if we need to do this, as it would indeed make the 
helper handle more and more cases.

Thanks

Best Regards

Hao

> +
>   def count_partial(n, fn):
>       nr_objs = 0
>       for slab in list_for_each_entry('struct slab', n.partial.address_of_(),
> @@ -86,7 +108,9 @@ def slub_get_slabinfo(s, cfg):
>       nr_free = 0
>   
>       for node in range(cfg['nr_nodes']):
> -        n = s.node[node]
> +        n = kmem_cache_get_node(s, node)
> +        if not n.value_():
> +            continue
>           nr_slabs += n.nr_slabs.counter.value_()
>           nr_objs += n.total_objects.counter.value_()
>           nr_free += count_partial(n, count_free)
> @@ -192,23 +216,30 @@ 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: