Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

[Johannes Weiner]

On Thu, Dec 18, 2025 at 10:53:57PM -0500, Johannes Weiner wrote:
> On Thu, Dec 18, 2025 at 06:09:50PM -0800, Shakeel Butt wrote:
> > On Wed, Dec 17, 2025 at 04:45:06PM -0500, Johannes Weiner wrote:
> > > So starting in patch 27, the tryget can fail if the memcg is offlined,
> > 
> > offlined or on its way to free? It is css_tryget() without online.
> 
> Sorry, I did mean freeing.
> 
> But in the new scheme, they will happen much closer together than
> before, since charges don't hold a reference to the css anymore.
> 
> So when css_killed_work_fn() does
> 
> 		offline_css(css);
> 		css_put(css);
> 
> on rmdir, that's now the css_put() we expect to drop the refcount to 0
> even with folios in circulation.
> 
> The race is then:
> 
> 	get_mem_cgroup_from_folio()	cgroup_rmdir()
> 	  memcg = folio_memcg(folio);
>             folio->objcg->memcg
> 					  offline_css()
>                                             reparent_objcgs()
> 					      objcg->memcg = objcg->memcg->parent
> 					  css_put() -> 0
> 	  !css_tryget(&memcg->css)
> 
> and the retry ensures we'll look up objcg->memcg again and find the
> live parent and new owner of the folio.

But yes, none of this happens until patch 27.