linux-kernel - Re: Large modules with 6.15 [was: [PATCH v4 6/6] percpu/x86: Enable strict percpu checks via named AS qualifiers]

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Message-ID: <34a2bfa5-30e1-4ba0-ac36-bf07a0d60d97@intel.com>
Date: Sat, 7 Jun 2025 07:12:02 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Jiri Slaby <jirislaby@...nel.org>, x86@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-bcachefs@...r.kernel.org,
 linux-arch@...r.kernel.org, netdev@...r.kernel.org,
 Nadav Amit <nadav.amit@...il.com>, Dennis Zhou <dennis@...nel.org>,
 Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Shung-Hsi Yu <shung-hsi.yu@...e.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: Large modules with 6.15 [was: [PATCH v4 6/6] percpu/x86: Enable
 strict percpu checks via named AS qualifiers]

On 6/7/25 01:52, Uros Bizjak wrote:
> Let me reiterate what the patch brings to the table. It prevents
> invalid references of per cpu variables to non-percpu locations.

Yes, it's a very useful mechanism. That's exactly why I want to preserve
it for developers or things like 0day that do build tests and don't care
about modules quadrupling in size.

> Hiding these checks behind the CONFIG_EXPERT option breaks the
> intention of the patch. IMO, it should be always enabled to avoid
> errors, mentioned in the previous paragraph, already during the
> development time.

I agree, it should always be enabled ... eventually. But now now. That's
why we're having this conversation: it's breaking too many things and
needs to be disabled.

> I'm much more inclined to James' proposal. Maybe we can disable these
> checks in v6.15 stable series, but leave them in v6.16? This would
> leave a couple of months for distributions to update libbpf.

I'd be worried that it will hit a whole bunch of folks doing 6.16 work.
I was expecting to revert it _everywhere_ for now, if we go the revert
route.

James, by partial revert, did you mean to revert in stable but not
mainline? I assumed you meant to just revert patch 6/6 of the series
(stable and mainline) but leave 1-5 in place so turning it back on later
was easier.