Re: [PATCH bpf-next v2 2/7] mm: introduce BPF kfuncs to deal with memcg pointers

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Dec 19, 2025 at 6:13 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> To effectively operate with memory cgroups in BPF there is a need
> to convert css pointers to memcg pointers. A simple container_of
> cast which is used in the kernel code can't be used in BPF because
> from the verifier's point of view that's a out-of-bounds memory access.
>
> Introduce helper get/put kfuncs which can be used to get
> a refcounted memcg pointer from the css pointer:
>   - bpf_get_mem_cgroup,
>   - bpf_put_mem_cgroup.
>
> bpf_get_mem_cgroup() can take both memcg's css and the corresponding
> cgroup's "self" css. It allows it to be used with the existing cgroup
> iterator which iterates over cgroup tree, not memcg tree.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  mm/Makefile         |  3 ++
>  mm/bpf_memcontrol.c | 88 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 91 insertions(+)
>  create mode 100644 mm/bpf_memcontrol.c
>
> diff --git a/mm/Makefile b/mm/Makefile
> index 9175f8cc6565..79c39a98ff83 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -106,6 +106,9 @@ obj-$(CONFIG_MEMCG) += memcontrol.o vmpressure.o
>  ifdef CONFIG_SWAP
>  obj-$(CONFIG_MEMCG) += swap_cgroup.o
>  endif
> +ifdef CONFIG_BPF_SYSCALL
> +obj-$(CONFIG_MEMCG) += bpf_memcontrol.o
> +endif
>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
>  obj-$(CONFIG_GUP_TEST) += gup_test.o
>  obj-$(CONFIG_DMAPOOL_TEST) += dmapool_test.o
> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> new file mode 100644
> index 000000000000..03d435fc4f10
> --- /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@linux.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 (mem_cgroup_disabled() || !root_mem_cgroup)
> +               return NULL;
> +
> +       if (root_mem_cgroup->css.ss != css->ss) {
> +               struct cgroup *cgroup = css->cgroup;
> +               int ssid = root_mem_cgroup->css.ss->id;
> +
> +               rcu_read_lock();
> +               rcu_unlock = true;
> +               css = rcu_dereference_raw(cgroup->subsys[ssid]);
> +       }
> +
> +       if (css && css_tryget(css))
> +               memcg = container_of(css, struct mem_cgroup, css);
> +
> +       if (rcu_unlock)
> +               rcu_read_unlock();
> +
> +       return memcg;
> +}
> +
> +/**
> + * bpf_put_mem_cgroup - Put a reference to a memory cgroup
> + * @memcg: memory cgroup to release
> + *
> + * Releases a previously acquired memcg reference.
> + * Implements KF_RELEASE semantics.
> + */
> +__bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
> +{
> +       css_put(&memcg->css);
> +}
> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
> +BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL | KF_RCU)
> +BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_TRUSTED_ARGS | KF_RELEASE)

This is an unusual combination of flags.
KF_RCU is a weaker KF_TRUSTED_ARGS, so just use KF_RCU.
We have an odd selftest kmod that specifies both,
but it's unnecessary there as well.
Just KF_ACQUIRE | KF_RET_NULL | KF_RCU will do.

Similarly KF_RELEASE implies KF_TRUSTED_ARGS.
That's even documented Documentation/bpf/kfuncs.rst,
so just use KF_RELEASE for bpf_put_mem_cgroup.

pw-bot: cr