Re: Bisected stability regression in 6.6

[matoro <matoro_mailinglist_kernel@matoro.tk>]

On 2023-11-11 16:27, Sam James wrote:
> Helge Deller <deller@gmx.de> 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@gmx.de>
>>> 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@gmx.de>
>>> 
>>>   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?

Yes, I just tested this and it cleanly reverts on linux-6.6.y and the revert 
does fix the issue.

>> 
>> 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