Commit d0f5d4f8 authored by Matt Bobrowski's avatar Matt Bobrowski Committed by Alexei Starovoitov
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bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()" 

This reverts commit e463b6de ("bpf: drop KF_ACQUIRE flag on BPF
kfunc bpf_get_root_mem_cgroup()").

The original commit removed the KF_ACQUIRE flag from
bpf_get_root_mem_cgroup() under the assumption that it resulted in
simplified usage. This stemmed from the fact that
bpf_get_root_mem_cgroup() inherently returns a reference to an object
which technically isn't reference counted, therefore there is no
strong requirement to call a matching bpf_put_mem_cgroup() on the
returned reference.

Although technically correct, as per the arguments in the thread [0],
dropping the KF_ACQUIRE flag and losing reference tracking semantics
negatively impacted the usability of bpf_get_root_mem_cgroup() in
practice.

[0] https://lore.kernel.org/bpf/878qdx6yut.fsf@linux.dev/

Link: https://lore.kernel.org/bpf/CAADnVQ+6d1Lj4dteAv8u62d7kj3Ze5io6bqM0xeQd-UPk9ZgJQ@mail.gmail.com/


Signed-off-by: default avatarMatt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20260121090001.240166-1-mattbobrowski@google.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent ca674a03

mm/bpf_memcontrol.c

+7 −7
Original line number Diff line number Diff line
@@ -13,12 +13,12 @@ __bpf_kfunc_start_defs(); 
/**

 * bpf_get_root_mem_cgroup - Returns a pointer to the root memory cgroup

 *

 * Return: A pointer to the root memory cgroup. Notably, the pointer to the

 * returned struct mem_cgroup is trusted by default, so it's perfectably

 * acceptable to also pass this pointer into other BPF kfuncs (e.g.,

 * bpf_mem_cgroup_usage()). Additionally, this BPF kfunc does not make use of

 * KF_ACQUIRE semantics, so there's no requirement for the BPF program to call

 * bpf_put_mem_cgroup() on the pointer returned by this BPF kfunc.

 * The function has KF_ACQUIRE semantics, even though the root memory

 * cgroup is never destroyed after being created and doesn't require

 * reference counting. And it's perfectly safe to pass it to

 * bpf_put_mem_cgroup()

 *

 * Return: A pointer to the root memory cgroup.

 */

__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)

{
@@ -162,7 +162,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg) 
__bpf_kfunc_end_defs();



BTF_KFUNCS_START(bpf_memcontrol_kfuncs)

BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_RET_NULL)

BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_ACQUIRE | KF_RET_NULL)

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)