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Subject: Re: [PATCH] x86/mm/pat: Fix missing preemption disable for __native_flush_tlb()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAPcyv4hXzBT1LkEdBYw_mkrKV+SK1LRE2PwJ5=T+e7GxWbXaNQ@mail.gmail.com>
Date:   Sat, 10 Nov 2018 16:19:08 -0800
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        stable <stable@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Message-Id: <9DFD717D-857D-493D-A606-B635D72BAC21@amacapital.net>
References: <154180834787.2060925.7738215365584115230.stgit@dwillia2-desk3.amr.corp.intel.com> <7590EF40-B0CF-40BD-9D29-FB731A2A2E3A@amacapital.net> <CAPcyv4hXzBT1LkEdBYw_mkrKV+SK1LRE2PwJ5=T+e7GxWbXaNQ@mail.gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 10, 2018, at 3:57 PM, Dan Williams <dan.j.williams@intel.com> wrote=
:
>=20
>> On Fri, Nov 9, 2018 at 4:22 PM Andy Lutomirski <luto@amacapital.net> wrot=
e:
>>=20
>>=20
>>=20
>>> On Nov 9, 2018, at 4:05 PM, Dan Williams <dan.j.williams@intel.com> wrot=
e:
>>>=20
>>> Commit f77084d96355 "x86/mm/pat: Disable preemption around
>>> __flush_tlb_all()" addressed a case where __flush_tlb_all() is called
>>> without preemption being disabled. It also left a warning to catch other=

>>> cases where preemption is not disabled. That warning triggers for the
>>> memory hotplug path which is also used for persistent memory enabling:
>>=20
>> I don=E2=80=99t think I agree with the patch. If you call __flush_tlb_all=
() in a context where you might be *migrated*, then there=E2=80=99s a bug. W=
e could change the code to allow this particular use by checking that we hav=
en=E2=80=99t done SMP init yet, perhaps.
>=20
> Hmm, are saying the entire kernel_physical_mapping_init() sequence
> needs to run with pre-emption disabled?

If it indeed can run late in boot or after boot, then it sure looks buggy. E=
ither the __flush_tlb_all() should be removed or it should be replaced with f=
lush_tlb_kernel_range(). It=E2=80=99s unclear to me why a flush is needed at=
 all, but if it=E2=80=99s needed, surely all CPUs need flushing.=