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([2600:1010:b066:ba1f:d10d:1ec8:df2a:b975]) by smtp.gmail.com with ESMTPSA id s20-v6sm11413696pga.37.2018.07.29.08.29.28 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 29 Jul 2018 08:29:29 -0700 (PDT) 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 X-Mailer: iPhone Mail (15G77) In-Reply-To: <1532866319.28585.12.camel@surriel.com> Date: Sun, 29 Jul 2018 08:29:27 -0700 Cc: Andy Lutomirski , LKML , kernel-team , Peter Zijlstra , X86 ML , Vitaly Kuznetsov , Ingo Molnar , Mike Galbraith , Dave Hansen , will.daecon@arm.com, Catalin Marinas , Benjamin Herrenschmidt Content-Transfer-Encoding: quoted-printable Message-Id: References: <20180728215357.3249-1-riel@surriel.com> <20180728215357.3249-11-riel@surriel.com> <1532866319.28585.12.camel@surriel.com> To: Rik van Riel Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Jul 29, 2018, at 5:11 AM, Rik van Riel 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 >> 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.=