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

[Roman Gushchin <roman.gushchin@linux.dev>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Mon, 18 Aug 2025 at 19:02, 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.
>> + *
>> + * Returns 1 if the forward progress was achieved and some memory was freed.
>> + * Returns a negative value if an error has been occurred.
>> + */
>> +__bpf_kfunc int bpf_out_of_memory(struct mem_cgroup *memcg__nullable,
>> +                                 int order, bool wait_on_oom_lock)
>
> I think this bool should be a u64 flags instead, just to make it
> easier to extend behavior in the future.

I like it, will change in the next version.

Thanks for the idea and also for reviewing the series!