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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

TL;DR - NAK this series as-is.

On Fri, Nov 28, 2025 at 10:53:53AM +0800, Yafang Shao wrote:
> Thank you for sharing this.
> However, BPF-THP is already deployed across our server fleet and both
> our users and my boss are satisfied with it. As such, we are not
> considering a switch. The current solution also offers us a valuable
> opportunity to experiment with additional policies in production.

Sorry Yafang, this isn't how upstream works.

I've not been paying attention to this series as I have been waiting for
you and Alexei to reach some kind of resolution before diving back in.

But your response here is _very_ concerning to me.

Of course you're welcome to deploy unmerged arbitrary patches to your
kernel (as long as you abide by the GPL naturally).

But we've made it _very_ clear that this is an - experimental - feature,
that might go away at any time, while we iterate and determine how useful
it might be to users in general.

Now it seems that exactly the thing I feared has already happened - people
ignoring the fact we are hiding this behind an, in effect,
CONFIG_EXPERIMENTAL_PLEASE_DO_NOT_RELY_ON_THIS flag.

This means that I am no longer confident this approach is going to work,
which inclines me to reject this proposal outright.

The bar is now a lot higher in my view, and now we're going to need
extensive and overwhelming evidence that whatever BPF hook we provide is
both future proof as to how we intend THP to develop and of use to more
than one user.

Again as David mentioned, you seem to be able to achieve what you want to
achieve via the extensions we added to PR_SET_THP_DISABLE.

That then reduces the number of users of this feature to 0 and again
inclines me to reject this approach entirely.

So for now it's a NAK.

>
> In summary, I am fine with either the per-MM or per-MEMCG method.
> Furthermore, I don't believe this is an either-or decision; both can
> be implemented to work together.

No, it is - the global approach is broken and we won't be having that.

>
>
> --
> Regards
> Yafang

Thanks, Lorenzo