Re: [LSF/MM/BPF TOPIC] Using BPF in MM

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/28/26 01:57, Roman Gushchin wrote:
> [LSF/MM/BPF TOPIC] Using BPF in MM
> ----------------------------------
> 
> Over the last decade, BPF successfully penetrated into multiple kernel
> subsystems: started as a feature to filter (out) networking packets,
> it captured its place in networking, tracing, security, HID drivers,
> and scheduling. Memory management is a logical next step, and recently
> we saw a growing number of proposals in this area.
> 
> In (approximately) historical order:
>   - BPF OOM
>   - BPF-based memcg stats access (landed)
>   - BPF-based NUMA balancing
>   - eBPF-mm
>   - cache_ext (BPF Page Cache)
>   - memcg_ext

There was also the BPF THP control.

> 
> There are some obvious target which haven't been covered yet:
>   - BPF-driven readahead control
>   - BPF-driven KSM
>   - BPF-driven guest memory control
> 
> Despite a large number of suggestions only a relatively small feature
> (query memcg statistics from BPF) made it to upstream.
> 
> It looks like using BPF in the MM subsystem comes with a set of somewhat
> unique challenges and questions to be answered.

[...]

I think you are missing one of the most important points: Unclear ABI stability
guarantees.

One the one hand, we are told that there are no ABI stability guarantees, and
that we can change hooks (add/remove/modify) any time we want.

On the other hand, as soon as there is some ebpf program out there that we
break, you can rest assured that there will be trouble.

In the area of THP, where we don't even know which hooks we will need long term
and how they would look like, that was one of the reasons why the BPF THP
control was rejected.

-- 
Cheers,

David