Re: Bisected stability regression in 6.6

[Sam James]

Helge Deller <deller@xxxxxx> writes:

> 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.

matoro, does a revert apply cleanly? Does it help?

>
> 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