Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <1532866319.28585.12.camel@surriel.com>
Date:   Sun, 29 Jul 2018 08:29:27 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Mike Galbraith <efault@gmx.de>,
        Dave Hansen <dave.hansen@intel.com>, will.daecon@arm.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Content-Transfer-Encoding: quoted-printable
Message-Id: <FC4DF151-463C-4CED-8962-9056648BFEDC@amacapital.net>
References: <20180728215357.3249-1-riel@surriel.com> <20180728215357.3249-11-riel@surriel.com> <CALCETrWS4pE5Tv3OErM3AD-E=CGG3U+2CBVecRF3N0c+g42vEg@mail.gmail.com> <1532866319.28585.12.camel@surriel.com>
To:     Rik van Riel <riel@surriel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jul 29, 2018, at 5:11 AM, Rik van Riel <riel@surriel.com> wrote:
>=20
>> On Sat, 2018-07-28 at 21:21 -0700, Andy Lutomirski wrote:
>> On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@surriel.com>
>> wrote:
>>> Conditionally skip lazy TLB mm refcounting. When an architecture
>>> has
>>> CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is used in
>>> lazy TLB mode anywhere will get shot down from exit_mmap, and there
>>> in no need to incur the cache line bouncing overhead of refcounting
>>> a lazy TLB mm.
>>=20
>> Unless I've misunderstood something, this patch results in idle tasks
>> whose active_mm has been freed still having active_mm pointing at
>> freed memory.=20
>=20
> Patch 9/10 is supposed to ensure that the lazy TLB CPUs get
> switched to init_mm before an mm is freed. No CPU should ever
> have its active_mm pointing at a freed mm.
>=20
> Your message made me re-read the code, and now I realize that
> leave_mm does not actually do that.
>=20
> Looking at the other callers of leave_mm, I might not be the
> only one surprised by that; xen_drop_mm_ref comes to mind.
>=20
> I guess I should some code to leave_mm to have it actually
> clear active_mm and call the conditional refcount drop helper
> function.
>=20
> Does that clear up the confusion?
>=20

Kind of. But what=E2=80=99s the point of keeping active_mm?  On architecture=
s that opt in to the new mode, there won=E2=80=99t be any code that cares ab=
out it=E2=80=99s value.  What=E2=80=99s the benefit of keeping it around?  I=
f you ifdef it out, then it can=E2=80=99t possibly point to freed memory, an=
d there=E2=80=99s nothing to worry about.=