Re: Bisected stability regression in 6.6

[Helge Deller]

On 11/11/23 07:31, matoro wrote:
> Hi Helge, I have bisected a regression in 6.6 which is causing
> userspace segfaults at a significantly increased rate in kernel 6.6.
> There seems to be a pathological case triggered by the ninja build
> tool.  The test case I have been using is cmake with ninja backend to
> attempt to build the nghttp2 package.  In 6.6, this segfaults, not at
> the same location every time, but with enough reliability that I was
> able to use it as a bisection regression case, including immediately
> after a reboot.  In the kernel log, these show up as "trap #15: Data
> TLB miss fault" messages.  Now these messages can and do show up in
> 6.5 causing segfaults, but never immediately after a reboot and
> infrequently enough that the system is stable.  With kernel 6.6 I am
> completely unable to build nghttp2 under any circumstances.
>
> I have bisected this down to the following commit:
>
> $ git bisect good
> 3033cd4307681c60db6d08f398a64484b36e0b0f is the first bad commit
> commit 3033cd4307681c60db6d08f398a64484b36e0b0f
> Author: Helge Deller <deller@xxxxxx>
> Date:   Sat Aug 19 00:53:28 2023 +0200
>
>      parisc: Use generic mmap top-down layout and brk randomization
>
>      parisc uses a top-down layout by default that exactly fits the generic
>      functions, so get rid of arch specific code and use the generic version
>      by selecting ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.
>
>      Note that on parisc the stack always grows up and a "unlimited stack"
>      simply means that the value as defined in CONFIG_STACK_MAX_DEFAULT_SIZE_MB
>      should be used. So RLIM_INFINITY is not an indicator to use the legacy
>      memory layout.
>
>      Signed-off-by: Helge Deller <deller@xxxxxx>
>
>   arch/parisc/Kconfig             | 17 +++++++++++++
>   arch/parisc/kernel/process.c    | 14 -----------
>   arch/parisc/kernel/sys_parisc.c | 54 +----------------------------------------
>   mm/util.c                       |  5 +++-
>   4 files changed, 22 insertions(+), 68 deletions(-)

Thanks for your report!
I think it's quite unlikely that this patch introduces such a bad regression.
I'd suspect some other bad commmit, but I'll try to reproduce.

In any case, do you have CONFIG_BPF_JIT enabled? If so, could you try
to reproduce with CONFIG_BPF_JIT disabled?
The JIT is quite new in v6.6 and I did face some crashes and disabling
it helped me so far.

> I have tried applying ad4aa06e1d92b06ed56c7240252927bd60632efe
> ("parisc: Add nop instructions after TLB inserts") on top of 6.6, but
> it does NOT fix the issue.

Ok.

Helge