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

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Oct 29, 2025 at 9:33 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> 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.

To move forward, I'm happy to set the global mode aside for now and
potentially drop it in the next version. I'd really like to hear your
perspective on the per-process mode. Does this implementation meet
your needs?


--
Regards
Yafang