Commit 5904db98 authored by Roman Gushchin's avatar Roman Gushchin Committed by Alexei Starovoitov
Browse files

mm: introduce BPF kfuncs to deal with memcg pointers 



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: default avatarRoman Gushchin <roman.gushchin@linux.dev>
Link: https://lore.kernel.org/r/20251223044156.208250-3-roman.gushchin@linux.dev


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 4221de8c

mm/Makefile

+3 −0
Original line number Diff line number Diff line
@@ -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

mm/bpf_memcontrol.c

0 → 100644
+88 −0
Original line number Diff line number Diff line 
// 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

 *

 * It's fine to pass a css which belongs to any cgroup controller,

 * e.g. unified hierarchy's main css.

 *

 * Implements KF_ACQUIRE semantics.

 *

 * Return: A pointer to a mem_cgroup structure after bumping

 * the corresponding css's reference counter.

 */

__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_ACQUIRE | KF_RET_NULL | KF_RCU)

BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)



BTF_KFUNCS_END(bpf_memcontrol_kfuncs)



static const struct btf_kfunc_id_set bpf_memcontrol_kfunc_set = {

	.owner          = THIS_MODULE,

	.set            = &bpf_memcontrol_kfuncs,

};



static int __init bpf_memcontrol_init(void)

{

	int err;



	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_UNSPEC,

					&bpf_memcontrol_kfunc_set);

	if (err)

		pr_warn("error while registering bpf memcontrol kfuncs: %d", err);



	return err;

}

late_initcall(bpf_memcontrol_init);