Re: tools/cgroup/memcg_slabinfo.py is broken

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 16, 2022 at 12:52:36PM -0800, Roman Gushchin wrote:
> The problem can be fixed by explicitly casting struct page * to struct
> slab * for slab pages. The tools works as expected with this fix, e.g.:

This feels like a quick fix, and not really correct.

> @@ -77,7 +77,8 @@ def count_partial(n, fn):
>  
>  
>  def count_free(page):
> -    return page.objects - page.inuse
> +    slab = cast('struct slab *', page)
> +    return slab.objects - slab.inuse

count_free() should take a slab, not a page.

>  def slub_get_slabinfo(s, cfg):
> @@ -193,10 +194,11 @@ def main():
>          # look over all slab pages, belonging to non-root memcgs
>          # and look for objects belonging to the given memory cgroup
>          for page in for_each_slab_page(prog):

for_each_slab_page() should be renamed for_each_slab().
It should return a slab, not a page.  And it should definitely skip
over tail pages (it works today by coincidence because tail pages do
not have PG_slab set).

count_partial() should use struct slab, and slab_list, not lru.

... I think that's it.  But I'm no pythonist, much less dragoneer.