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SubjectRe: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in pti_clone_pagetable()
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> On Aug 28, 2019, at 8:51 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Wed, 28 Aug 2019, Dave Hansen wrote:
>> On 8/28/19 7:24 AM, Thomas Gleixner wrote:
>>> From: Song Liu <songliubraving@fb.com>
>>>
>>> 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@linux.intel.com>
>>
>> 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:

https://lore.kernel.org/lkml/20190823052335.572133-1-songliubraving@fb.com/

How about we apply this patch to upstream 5.2 kernel?

Thanks,
Song

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