[PATCH bpf-next] bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

[PATCH bpf-next] bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

[Matt Bobrowski]

This reverts commit e463b6de9da1 ("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: Matt Bobrowski <mattbobrowski@google.com>
---
 mm/bpf_memcontrol.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
index f95cd5d16f4c..716df49d7647 100644
--- a/mm/bpf_memcontrol.c
+++ b/mm/bpf_memcontrol.c
@@ -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)
 
-- 
2.52.0.457.g6b5491de43-goog

Re: [PATCH bpf-next] bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Wed, 21 Jan 2026 09:00:01 +0000 you wrote:
> This reverts commit e463b6de9da1 ("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.
> 
> [...]

Here is the summary with links:
  - [bpf-next] bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"
    https://git.kernel.org/bpf/bpf-next/c/d0f5d4f8f328

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH bpf-next] bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

[Roman Gushchin]

Matt Bobrowski <mattbobrowski@google.com> writes:

> This reverts commit e463b6de9da1 ("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: Matt Bobrowski <mattbobrowski@google.com>

Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks, Matt!