Re: next: i386-boot: clang-nightly: failed - intermittently - BUG: unable to handle page fault for address: 000024c0

[Naresh Kamboju]

On Wed, 17 May 2023 at 23:42, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Wed, May 17, 2023 at 8:21 AM Naresh Kamboju
> <naresh.kamboju@xxxxxxxxxx> wrote:
> >
> > Linux next-20230517 build with clang nightly for i386 boot fails intermittently.
>
> Keyword: intermittently. That will make tracking this down fun.
>
> Our CI also hit a boot failure on tip/master with the same splat:
> https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/4998374271/jobs/8957285746
> Though the CI pulled down a SHA
> 0932447780e1f9a43bf68ef7fe3d9b41b46d58fc
> which looks weird on
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=0932447780e1f9a43bf68ef7fe3d9b41b46d58fc
> >> Notice: this object is not reachable from any branch.
>
> That this failed in -next and -tip in the same way makes me wonder if
> something affecting this is coming in via -tip? Maybe the splat looks
> familiar to x86 folks?
>
> I haven't been able to reproduce locally when my machine is relatively
> load-less.  If I do a kernel build in the background, I was able to
> get QEMU to hang, but without any splat. That was using tip/master @
> f81d8f759e7f.
>
> Naresh, when you say "intermittent" do you have any data on the
> relative frequency of this boot failure? (Also, please make sure to
> use llvm@xxxxxxxxxxxxxxx in the future; we moved mailing lists years
> ago).

Noted:
It is reproducible only 3 times out of 100 runs.

>
> Looks like our CI report linked above has an additional splat though
> via apply_alternatives and optimize_nops.
>
> >> [ 0.166742] Code: Unable to access opcode bytes at 0x36.
>
> Peter, that smells like perhaps either:
> commit b6c881b248ef ("x86/alternative: Complicate optimize_nops() some more")
> commit 6c480f222128 ("x86/alternative: Rewrite optimize_nops() some")
>
> Looks like BP committed them May 11; maybe just recently they were
> merged into tip/master?
>
>
> >   - i386: boot/clang-nightly-lkftconfig - failed
> >

- Naresh