Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in pti_clone_pagetable()

From: Song Liu
Date: Wed Aug 28 2019 - 16:33:37 EST

> On Aug 28, 2019, at 1:05 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Wed, 28 Aug 2019, Song Liu wrote:
>>> On Aug 28, 2019, at 8:51 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>> On Wed, 28 Aug 2019, Dave Hansen wrote:
>>>> On 8/28/19 7:24 AM, Thomas Gleixner wrote:
>>>>> From: Song Liu <songliubraving@xxxxxx>
>>>>> pti_clone_pmds() assumes that the supplied address is either:
>>>>> - properly PUD/PMD aligned
>>>>> or
>>>>> - the address is actually mapped which means that independent
>>>>> of the mapping level (PUD/PMD/PTE) the next higher mapping
>>>>> exist.
>>>>> If that's not the case the unaligned address can be incremented by PUD or
>>>>> PMD size wrongly. All callers supply mapped and/or aligned addresses, but
>>>>> for robustness sake, it's better to handle that case proper and to emit a
>>>>> warning.
>>>> Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>>> Song, did you ever root-cause the performance regression? I thought
>>>> there were still some mysteries there.
>>> See Peter's series to rework the ftrace code patching ...
>> Thanks Thomas.
>> Yes, in summary, enabling ftrace or kprobe-on-ftrace causes the kernel
>> to split PMDs in kernel text mapping.
>> Related question: while Peter's patches fix it for 5.3 kernel, they don't
>> apply cleanly over 5.2 kernel (which we are using). So I wonder what is
>> the best solution for 5.2 kernel. May patch also fixes the issue:
>> How about we apply this patch to upstream 5.2 kernel?
> That's not how it works. We fix stuff upstream and it gets backported to
> all affected kernels not just to the one you care about.

Agreed. I am trying to back port Peter's patch set to 5.2 kernel. There
are 9 dependencies and some manual changes.

> Aside of that I really disagree with that hack. You completely fail to
> explain why that commit in question broke it and instead of fixing the
> underlying issue you create a horrible workaround.
> It took me ~10 minutes to analyze the root cause and I'm just booting the
> test box with a proper fix which can be actually tagged for stable and can
> be removed from upstream again once ftrace got moved over to text poke.
> I'll post it once it's confirmed to work and I wrote a comprehensible
> changelog.

This sounds great. Thanks!