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From:   Song Liu <songliubraving@fb.com>
To:     Thomas Gleixner <tglx@linutronix.de>
CC:     Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>, Joerg Roedel <jroedel@suse.de>,
        "Andy Lutomirski" <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rik van Riel" <riel@surriel.com>
Subject: Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in
 pti_clone_pagetable()
Thread-Topic: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in
 pti_clone_pagetable()
Thread-Index: AQHVXa1SnM+Gh7oAvk+r6MnSURCjbqcQtC2AgAABSwCAACOGgA==
Date:   Wed, 28 Aug 2019 17:58:20 +0000
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> On Aug 28, 2019, at 8:51 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> On Wed, 28 Aug 2019, Dave Hansen wrote:
>> On 8/28/19 7:24 AM, Thomas Gleixner wrote:
>>> From: Song Liu <songliubraving@fb.com>
>>>=20
>>> pti_clone_pmds() assumes that the supplied address is either:
>>>=20
>>> - 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.
>>>=20
>>> 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, b=
ut
>>> for robustness sake, it's better to handle that case proper and to emit=
 a
>>> warning.
>>=20
>> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
>>=20
>> Song, did you ever root-cause the performance regression?  I thought
>> there were still some mysteries there.
>=20
> See Peter's series to rework the ftrace code patching ...

Thanks Thomas.=20

Yes, in summary, enabling ftrace or kprobe-on-ftrace causes the kernel
to split PMDs in kernel text mapping.=20

Related question: while Peter's patches fix it for 5.3 kernel, they don't=20
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