Re: Kernel crash due to memory corruption with v5.4.26-rt17 and PowerPC e500

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-05-29 17:38:39 [+0200], Mark Marshall wrote:
> Hi Sebastian & list,
Hi,

> I had assumed that my e-mail had got lost or overlooked, I was meaning to
> post a follow up message this week...
> 
> All I could find from the debugging and tracing that we added was that
> something was going wrong with the mm data structures somewhere in the
> exec code.  In the end I just spent a week or two pouring over the diffs
> of this code between the versions that I new worked and didn't work.
> 
> I eventually found the culprit.  On the working kernel versions there is
> a patch called "mm: Protect activate_mm() by preempt_[disable&enable]_rt()".
> This is commit f0b4a9cb253a on the V4.19.82-rt30 branch, for instance.
> Although the commit message talks about ARM, it seems that we need this for
> PowerPC too (I guess, any PowerPC with the "nohash" MMU?).

Could you drop me your config, please? I need to dig here a little and I
should have seen this on qemu, right?

> Could you please add this commit back to the RT branch?  I'm not sure how
> to find out the history of this commit.  For instance, why has it been
> removed from the RT patchset?  How are these things tracked, generally?

I dropped that patch in v5.4.3-rt1. I couldn't reproduce the issue that
was documented in the patch and the code that triggered the warning was
removed / reworked in commit
    b5466f8728527 ("ARM: mm: remove IPI broadcasting on ASID rollover")

So it looked like no longer needed and then got dropped during the
rebase.
In order to get it back into the RT queue I need to understand why it is
required. What exactly is it fixing. Let me stare at for a little…

> Best regards,
> Mark

Sebastian