Re: [PATCH v1 06/14] mm: introduce bpf_out_of_memory() bpf kfunc

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Aug 18, 2025 at 10:02 AM Roman Gushchin
<roman.gushchin@linux.dev> wrote:
>
> Introduce bpf_out_of_memory() bpf kfunc, which allows to declare
> an out of memory events and trigger the corresponding kernel OOM
> handling mechanism.
>
> It takes a trusted memcg pointer (or NULL for system-wide OOMs)
> as an argument, as well as the page order.
>
> If the wait_on_oom_lock argument is not set, only one OOM can be
> declared and handled in the system at once, so if the function is
> called in parallel to another OOM handling, it bails out with -EBUSY.
> This mode is suited for global OOM's: any concurrent OOMs will likely
> do the job and release some memory. In a blocking mode (which is
> suited for memcg OOMs) the execution will wait on the oom_lock mutex.
>
> The function is declared as sleepable. It guarantees that it won't
> be called from an atomic context. It's required by the OOM handling
> code, which is not guaranteed to work in a non-blocking context.
>
> Handling of a memcg OOM almost always requires taking of the
> css_set_lock spinlock. The fact that bpf_out_of_memory() is sleepable
> also guarantees that it can't be called with acquired css_set_lock,
> so the kernel can't deadlock on it.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  mm/oom_kill.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 25fc5e744e27..df409f0fac45 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1324,10 +1324,55 @@ __bpf_kfunc int bpf_oom_kill_process(struct oom_control *oc,
>         return 0;
>  }
>
> +/**
> + * bpf_out_of_memory - declare Out Of Memory state and invoke OOM killer
> + * @memcg__nullable: memcg or NULL for system-wide OOMs
> + * @order: order of page which wasn't allocated
> + * @wait_on_oom_lock: if true, block on oom_lock
> + * @constraint_text__nullable: custom constraint description for the OOM report
> + *
> + * Declares the Out Of Memory state and invokes the OOM killer.
> + *
> + * OOM handlers are synchronized using the oom_lock mutex. If wait_on_oom_lock
> + * is true, the function will wait on it. Otherwise it bails out with -EBUSY
> + * if oom_lock is contended.
> + *
> + * Generally it's advised to pass wait_on_oom_lock=true for global OOMs
> + * and wait_on_oom_lock=false for memcg-scoped OOMs.

From the changelog description I was under impression that it's vice
versa, for global OOMs you would not block (wait_on_oom_lock=false),
for memcg ones you would (wait_on_oom_lock=true).

> + *
> + * Returns 1 if the forward progress was achieved and some memory was freed.
> + * Returns a negative value if an error has been occurred.

s/has been occurred/has occurred or occured


> + */
> +__bpf_kfunc int bpf_out_of_memory(struct mem_cgroup *memcg__nullable,
> +                                 int order, bool wait_on_oom_lock)
> +{
> +       struct oom_control oc = {
> +               .memcg = memcg__nullable,
> +               .order = order,
> +       };
> +       int ret;
> +
> +       if (oc.order < 0 || oc.order > MAX_PAGE_ORDER)
> +               return -EINVAL;
> +
> +       if (wait_on_oom_lock) {
> +               ret = mutex_lock_killable(&oom_lock);
> +               if (ret)
> +                       return ret;
> +       } else if (!mutex_trylock(&oom_lock))
> +               return -EBUSY;
> +
> +       ret = out_of_memory(&oc);
> +
> +       mutex_unlock(&oom_lock);
> +       return ret;
> +}
> +
>  __bpf_kfunc_end_defs();
>
>  BTF_KFUNCS_START(bpf_oom_kfuncs)
>  BTF_ID_FLAGS(func, bpf_oom_kill_process, KF_SLEEPABLE | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_out_of_memory, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>  BTF_KFUNCS_END(bpf_oom_kfuncs)
>
>  static const struct btf_kfunc_id_set bpf_oom_kfunc_set = {
> --
> 2.50.1
>