Re: [PATCH v12 mm-new 06/10] mm: bpf-thp: add support for global mode

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Sun, Oct 26, 2025 at 3:03 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> The per-process BPF-THP mode is unsuitable for managing shared resources
> such as shmem THP and file-backed THP. This aligns with known cgroup
> limitations for similar scenarios [0].
>
> Introduce a global BPF-THP mode to address this gap. When registered:
> - All existing per-process instances are disabled
> - New per-process registrations are blocked
> - Existing per-process instances remain registered (no forced unregistration)
>
> The global mode takes precedence over per-process instances. Updates are
> type-isolated: global instances can only be updated by new global
> instances, and per-process instances by new per-process instances.

...

>         spin_lock(&thp_ops_lock);
> -       /* Each process is exclusively managed by a single BPF-THP. */
> -       if (rcu_access_pointer(mm->bpf_mm.bpf_thp)) {
> +       /* Each process is exclusively managed by a single BPF-THP.
> +        * Global mode disables per-process instances.
> +        */
> +       if (rcu_access_pointer(mm->bpf_mm.bpf_thp) || rcu_access_pointer(bpf_thp_global)) {
>                 err = -EBUSY;
>                 goto out;
>         }

You didn't address the issue and instead doubled down
on this broken global approach.

This bait-and-switch patchset is frankly disingenuous.
'lets code up some per-mm hack, since people will hate it anyway,
and I'm not going to use it either, and add this global mode
as a fake "fallback"...'

The way the previous thread evolved and this followup hack
I don't see a genuine desire to find a solution.
Just relentless push for global mode.

Nacked-by: Alexei Starovoitov <ast@kernel.org>

Please carry it in all future patches.