linux-kernel - Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg pointers

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Message-ID: <CAADnVQLkza5_95qc0vGYBLUu-4FN_cZEcVywTs5XemTE9O-ZtQ@mail.gmail.com>
Date: Tue, 28 Oct 2025 10:12:59 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: bot+bpf-ci@...nel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	LKML <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Andrii Nakryiko <andrii@...nel.org>, inwardvessel <inwardvessel@...il.com>, 
	linux-mm <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...nel.org>, Song Liu <song@...nel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, Tejun Heo <tj@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Eduard <eddyz87@...il.com>, Yonghong Song <yonghong.song@...ux.dev>, Chris Mason <clm@...a.com>, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>
Subject: Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg pointers

On Tue, Oct 28, 2025 at 9:11 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>
> bot+bpf-ci@...nel.org writes:
>
> > ```
> > commit ec8e2e1769704b4f69261f36e6aaf035d47c5f0f
> > Author: Roman Gushchin <roman.gushchin@...ux.dev>
> >
> > mm: introduce BPF kfuncs to deal with memcg pointers
> >
> > This commit introduces two new BPF kfuncs for converting css pointers
> > to memcg pointers with proper reference counting: bpf_get_mem_cgroup()
> > and bpf_put_mem_cgroup().
> >
> >> diff --git a/mm/Makefile b/mm/Makefile
> >> index 051e88c69..2d8f9beb3 100644
> >> --- a/mm/Makefile
> >> +++ b/mm/Makefile
> >> @@ -107,6 +107,7 @@ obj-$(CONFIG_MEMCG) += swap_cgroup.o
> >>  endif
> >>  ifdef CONFIG_BPF_SYSCALL
> >>  obj-y += bpf_oom.o
> >> +obj-$(CONFIG_MEMCG) += bpf_memcontrol.o
> >>  endif
> >>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
> >>  obj-$(CONFIG_GUP_TEST) += gup_test.o
> >> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> >> new file mode 100644
> >> index 000000000..1e4609774
> >> --- /dev/null
> >> +++ b/mm/bpf_memcontrol.c
> >> @@ -0,0 +1,88 @@
> >> +// SPDX-License-Identifier: GPL-2.0-or-later
> >> +/*
> >> + * Memory Controller-related BPF kfuncs and auxiliary code
> >> + *
> >> + * Author: Roman Gushchin <roman.gushchin@...ux.dev>
> >> + */
> >> +
> >> +#include <linux/memcontrol.h>
> >> +#include <linux/bpf.h>
> >> +
> >> +__bpf_kfunc_start_defs();
> >> +
> >> +/**
> >> + * bpf_get_mem_cgroup - Get a reference to a memory cgroup
> >> + * @css: pointer to the css structure
> >> + *
> >> + * Returns a pointer to a mem_cgroup structure after bumping
> >> + * the corresponding css's reference counter.
> >> + *
> >> + * It's fine to pass a css which belongs to any cgroup controller,
> >> + * e.g. unified hierarchy's main css.
> >> + *
> >> + * Implements KF_ACQUIRE semantics.
> >> + */
> >> +__bpf_kfunc struct mem_cgroup *
> >> +bpf_get_mem_cgroup(struct cgroup_subsys_state *css)
> >> +{
> >> +    struct mem_cgroup *memcg = NULL;
> >> +    bool rcu_unlock = false;
> >> +
> >> +    if (!root_mem_cgroup)
> >> +            return NULL;
> >> +
> >> +    if (root_mem_cgroup->css.ss != css->ss) {
> >                                             ^^^^^^^
> >
> > Can this dereference a NULL css pointer?  The function checks css for
> > NULL at line 42 with "if (css && css_tryget(css))", which suggests css
> > can be NULL.  If a BPF program calls this function with a NULL css,
> > wouldn't it crash here before reaching that check?
>
> Here css passed as an argument to bpf_get_mem_cgroup() can't be NULL,
> because the BPF verifier guarantees that it's a valid css pointer.
>
> However the result of rcu_dereference_raw(cgroup->subsys[ssid]) can be
> NULL, this is why the "if (css && css_tryget(css))" check is required
> down below.

Yeah. Not sure how feasible it is to teach AI about KF_RCU semantics.