linux-kernel - Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in pti_clone

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Date:   Wed, 28 Aug 2019 17:58:20 +0000
From:   Song Liu <songliubraving@...com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <jroedel@...e.de>,
        "Andy Lutomirski" <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rik van Riel" <riel@...riel.com>
Subject: Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in
 pti_clone_pagetable()



> On Aug 28, 2019, at 8:51 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Wed, 28 Aug 2019, Dave Hansen wrote:
>> On 8/28/19 7:24 AM, Thomas Gleixner wrote:
>>> From: Song Liu <songliubraving@...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@...ux.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