From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 77B39C48BE8 for ; Mon, 14 Jun 2021 04:47:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 59F95613AD for ; Mon, 14 Jun 2021 04:47:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232251AbhFNEtp (ORCPT ); Mon, 14 Jun 2021 00:49:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229596AbhFNEto (ORCPT ); Mon, 14 Jun 2021 00:49:44 -0400 Received: from mail-pf1-x42c.google.com (mail-pf1-x42c.google.com [IPv6:2607:f8b0:4864:20::42c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 99E1CC061574; Sun, 13 Jun 2021 21:47:27 -0700 (PDT) Received: by mail-pf1-x42c.google.com with SMTP id k15so9639753pfp.6; Sun, 13 Jun 2021 21:47:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:subject:to:cc:references:in-reply-to:mime-version :message-id:content-transfer-encoding; bh=GgivP6wfXR9rSJg48GusfXouXjNtu1u2kxviIHXQuek=; b=GZiYNnrjLROl4HyLAb1Mi2k99nTMkX9k32Y7sI+jUqtOAjqoW3wdE0Qdbt4QA71LiF C+FQqQ6cmSI5/FCaehqw9Arzp2qtNurvxDjI8iXd3w9Ug18cI2wMQDoKuUPwD0BAjmp7 NCphqgGDRD5VVe9OBOuEDw5GoRYIu80f1T9t/hdYYZ3kXxuvwNkL+d7ehrI8xkZS1DJB N4nbeqsst9z1BsNQkALQgBUw0Baf8ya8r3IQNlc2irR8dPByGvZEzBWy1MV9SHmpCtLv sY6INd/ayKtGKD5s/pw4gSseUZdOiY+gBCyVYUMhNYleCr5gj9Vm85IWBIDzT4sNVX8N 4vcw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:subject:to:cc:references:in-reply-to :mime-version:message-id:content-transfer-encoding; bh=GgivP6wfXR9rSJg48GusfXouXjNtu1u2kxviIHXQuek=; b=Rpgsv/BT4zYgSM+yReDHc31GF7jBEeM6OtkOE/K7cEvteqG/mF2uYrsa7zGVV0JFCg OaI5CIiPsA6SM1sGJNSXGBGoPwM7ZIgst8NRBSRj2CdmL1ypeB/raQPGcKOxIUA2Knxh lr62envknLFC1GS7qrprKyfz77chSzDtx0x1Uazz4jh7LzvOrjfC0FKx5/VhJCFla+ZW 4BbsRuydVt4UecCfUFDjImkxxBmEgT022I9EX+FskMQPwxqHG0TgJyIZWxnZ6zy9GcDl WU79kj++3Tj3vg5oMJPLxdx8Mz+P9yIQ6rqCNUDKqu0tEdKhUGwU9qAhFStfnW+tNQL6 zm+Q== X-Gm-Message-State: AOAM531mC7Ci3ebl3p7NeluSaM5dUvHYU2rK2GTmKoRCja4iQB90KwFh pidkuU8YeDscdmyoeGOk//8= X-Google-Smtp-Source: ABdhPJyIN740d4qJI/bTT+yC6qUfvPgxGu4p98oZQFvuxVV+BViUMjiMYqB8otWkz+zKlzMhlykc6Q== X-Received: by 2002:a63:5d65:: with SMTP id o37mr1063970pgm.79.1623646047034; Sun, 13 Jun 2021 21:47:27 -0700 (PDT) Received: from localhost (60-242-147-73.tpgi.com.au. [60.242.147.73]) by smtp.gmail.com with ESMTPSA id mr23sm11061887pjb.12.2021.06.13.21.47.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Jun 2021 21:47:26 -0700 (PDT) Date: Mon, 14 Jun 2021 14:47:21 +1000 From: Nicholas Piggin Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable To: Andrew Morton , Andy Lutomirski Cc: Anton Blanchard , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, Randy Dunlap , Linus Torvalds References: <20210605014216.446867-1-npiggin@gmail.com> <20210605014216.446867-3-npiggin@gmail.com> <8ac1d420-b861-f586-bacf-8c3949e9b5c4@kernel.org> <1623629185.fxzl5xdab6.astroid@bobo.none> <02e16a2f-2f58-b4f2-d335-065e007bcea2@kernel.org> <1623643443.b9twp3txmw.astroid@bobo.none> In-Reply-To: <1623643443.b9twp3txmw.astroid@bobo.none> MIME-Version: 1.0 Message-Id: <1623645385.u2cqbcn3co.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org Excerpts from Nicholas Piggin's message of June 14, 2021 2:14 pm: > Excerpts from Andy Lutomirski's message of June 14, 2021 1:52 pm: >> On 6/13/21 5:45 PM, Nicholas Piggin wrote: >>> Excerpts from Andy Lutomirski's message of June 9, 2021 2:20 am: >>>> On 6/4/21 6:42 PM, Nicholas Piggin wrote: >>>>> Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb= mm >>>>> when it is context switched. This can be disabled by architectures th= at >>>>> don't require this refcounting if they clean up lazy tlb mms when the >>>>> last refcount is dropped. Currently this is always enabled, which is >>>>> what existing code does, so the patch is effectively a no-op. >>>>> >>>>> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is. >>>> >>>> I am in favor of this approach, but I would be a lot more comfortable >>>> with the resulting code if task->active_mm were at least better >>>> documented and possibly even guarded by ifdefs. >>>=20 >>> active_mm is fairly well documented in Documentation/active_mm.rst IMO. >>> I don't think anything has changed in 20 years, I don't know what more >>> is needed, but if you can add to documentation that would be nice. Mayb= e >>> moving a bit of that into .c and .h files? >>>=20 >>=20 >> Quoting from that file: >>=20 >> - however, we obviously need to keep track of which address space we >> "stole" for such an anonymous user. For that, we have "tsk->active_m= m", >> which shows what the currently active address space is. >>=20 >> This isn't even true right now on x86. >=20 > From the perspective of core code, it is. x86 might do something crazy=20 > with it, but it has to make it appear this way to non-arch code that > uses active_mm. >=20 > Is x86's scheme documented? >=20 >> With your patch applied: >>=20 >> To support all that, the "struct mm_struct" now has two counters: a >> "mm_users" counter that is how many "real address space users" there ar= e, >> and a "mm_count" counter that is the number of "lazy" users (ie anonymo= us >> users) plus one if there are any real users. >>=20 >> isn't even true any more. >=20 > Well yeah but the active_mm concept hasn't changed. The refcounting=20 > change is hopefully reasonably documented? >=20 >>=20 >>=20 >>>> x86 bare metal currently does not need the core lazy mm refcounting, a= nd >>>> x86 bare metal *also* does not need ->active_mm. Under the x86 scheme= , >>>> if lazy mm refcounting were configured out, ->active_mm could become a >>>> dangling pointer, and this makes me extremely uncomfortable. >>>> >>>> So I tend to think that, depending on config, the core code should >>>> either keep ->active_mm [1] alive or get rid of it entirely. >>>=20 >>> I don't actually know what you mean. >>>=20 >>> core code needs the concept of an "active_mm". This is the mm that your= =20 >>> kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=3Dn patc= h, >>> active_mm still points to init_mm for kernel threads. >>=20 >> Core code does *not* need this concept. First, it's wrong on x86 since >> at least 4.15. Any core code that actually assumes that ->active_mm is >> "active" for any sensible definition of the word active is wrong. >> Fortunately there is no such code. >>=20 >> I looked through all active_mm references in core code. We have: >>=20 >> kernel/sched/core.c: it's all refcounting, although it's a bit tangled >> with membarrier. >>=20 >> kernel/kthread.c: same. refcounting and membarrier stuff. >>=20 >> kernel/exit.c: exit_mm() a BUG_ON(). >>=20 >> kernel/fork.c: initialization code and a warning. >>=20 >> kernel/cpu.c: cpu offline stuff. wouldn't be needed if active_mm went a= way. >>=20 >> fs/exec.c: nothing of interest >=20 > I might not have been clear. Core code doesn't need active_mm if=20 > active_mm somehow goes away. I'm saying active_mm can't go away because > it's needed to support (most) archs that do lazy tlb mm switching. >=20 > The part I don't understand is when you say it can just go away. How?=20 >=20 >> I didn't go through drivers, but I maintain my point. active_mm is >> there for refcounting. So please don't just make it even more confusing >> -- do your performance improvement, but improve the code at the same >> time: get rid of active_mm, at least on architectures that opt out of >> the refcounting. >=20 > powerpc opts out of the refcounting and can not "get rid of active_mm". > Not even in theory. That is to say, it does do a type of reference management that requires=20 active_mm so you can argue it has not entirely opted out of refcounting. But we're not just doing refcounting for the sake of refcounting! That would make no sense. active_mm is required because that's the mm that we have switched to=20 (from core code's perspective), and it is integral to know when to=20 switch to a different mm. See how active_mm is a fundamental concept in core code? It's part of the contract between core code and the arch mm context management calls. reference counting follows from there but it's not the _reason_ for this code. Pretend the reference problem does not exit (whether by refcounting or=20 shootdown or garbage collection or whatever). We still can't remove=20 active_mm! We need it to know how to call into arch functions like=20 switch_mm. I don't know if you just forgot that critical requirement in your above=20 list, or you actually are entirely using x86's mental model for this=20 code which is doing something entirely different that does not need it=20 at all. If that is the case I really don't mind some cleanup or wrapper=20 functions for x86 do entirely do its own thing, but if that's the case you can't criticize core code's use of active_mm due to the current state of x86. It's x86 that needs documentation and cleaning up. Thanks, Nick From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.5 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, DKIM_INVALID,DKIM_SIGNED,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EB9D3C48BE6 for ; Mon, 14 Jun 2021 04:48:00 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0CEC461283 for ; Mon, 14 Jun 2021 04:47:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0CEC461283 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4G3Jq30F4Kz30B3 for ; Mon, 14 Jun 2021 14:47:59 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256 header.s=20161025 header.b=GZiYNnrj; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=gmail.com (client-ip=2607:f8b0:4864:20::429; helo=mail-pf1-x429.google.com; envelope-from=npiggin@gmail.com; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256 header.s=20161025 header.b=GZiYNnrj; dkim-atps=neutral Received: from mail-pf1-x429.google.com (mail-pf1-x429.google.com [IPv6:2607:f8b0:4864:20::429]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4G3JpX1G9Rz2y0B for ; Mon, 14 Jun 2021 14:47:31 +1000 (AEST) Received: by mail-pf1-x429.google.com with SMTP id x16so5424429pfa.13 for ; Sun, 13 Jun 2021 21:47:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:subject:to:cc:references:in-reply-to:mime-version :message-id:content-transfer-encoding; bh=GgivP6wfXR9rSJg48GusfXouXjNtu1u2kxviIHXQuek=; b=GZiYNnrjLROl4HyLAb1Mi2k99nTMkX9k32Y7sI+jUqtOAjqoW3wdE0Qdbt4QA71LiF C+FQqQ6cmSI5/FCaehqw9Arzp2qtNurvxDjI8iXd3w9Ug18cI2wMQDoKuUPwD0BAjmp7 NCphqgGDRD5VVe9OBOuEDw5GoRYIu80f1T9t/hdYYZ3kXxuvwNkL+d7ehrI8xkZS1DJB N4nbeqsst9z1BsNQkALQgBUw0Baf8ya8r3IQNlc2irR8dPByGvZEzBWy1MV9SHmpCtLv sY6INd/ayKtGKD5s/pw4gSseUZdOiY+gBCyVYUMhNYleCr5gj9Vm85IWBIDzT4sNVX8N 4vcw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:subject:to:cc:references:in-reply-to :mime-version:message-id:content-transfer-encoding; bh=GgivP6wfXR9rSJg48GusfXouXjNtu1u2kxviIHXQuek=; b=fsXKug+j6lC/AN2Kg9gPIcz5ajRClYbemKXfhW3C5sz5EJC8qOFNDgHiWdmCH87OHt JvCH8YbvJk+Tha7MwM4Xjf1nJj/DDXP8lTZvm8bBSy09516qnhuDQXrZZ3Od0CmHBkqd 2gdWAzsbKND6qiC5jm/x8/CiAs5p3sbogUrPeE5dl2zGnIza+OUQmErBFDRCN+1Zphxo NBm0Lb4PoULbSOLZoYegc7eAMfq9WuJtirgvcmKC2CSeosDdtuskytpq1z07f4j9ns4N GyRSU/X2GVzcbXzT7kwhow8zl/PvS6s/YNwJ++eu6SlxjPlAme7zJydiOucSju3r5sua idiQ== X-Gm-Message-State: AOAM531327kgUGjPpOpeG7oQxoc7xxVwEa2xX5NDEgWfkr4iSDkdJ4uZ ThQ/KU6+5GZzlyaCC+AcU0s= X-Google-Smtp-Source: ABdhPJyIN740d4qJI/bTT+yC6qUfvPgxGu4p98oZQFvuxVV+BViUMjiMYqB8otWkz+zKlzMhlykc6Q== X-Received: by 2002:a63:5d65:: with SMTP id o37mr1063970pgm.79.1623646047034; Sun, 13 Jun 2021 21:47:27 -0700 (PDT) Received: from localhost (60-242-147-73.tpgi.com.au. [60.242.147.73]) by smtp.gmail.com with ESMTPSA id mr23sm11061887pjb.12.2021.06.13.21.47.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Jun 2021 21:47:26 -0700 (PDT) Date: Mon, 14 Jun 2021 14:47:21 +1000 From: Nicholas Piggin Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable To: Andrew Morton , Andy Lutomirski References: <20210605014216.446867-1-npiggin@gmail.com> <20210605014216.446867-3-npiggin@gmail.com> <8ac1d420-b861-f586-bacf-8c3949e9b5c4@kernel.org> <1623629185.fxzl5xdab6.astroid@bobo.none> <02e16a2f-2f58-b4f2-d335-065e007bcea2@kernel.org> <1623643443.b9twp3txmw.astroid@bobo.none> In-Reply-To: <1623643443.b9twp3txmw.astroid@bobo.none> MIME-Version: 1.0 Message-Id: <1623645385.u2cqbcn3co.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Linus Torvalds , Randy Dunlap , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Nicholas Piggin's message of June 14, 2021 2:14 pm: > Excerpts from Andy Lutomirski's message of June 14, 2021 1:52 pm: >> On 6/13/21 5:45 PM, Nicholas Piggin wrote: >>> Excerpts from Andy Lutomirski's message of June 9, 2021 2:20 am: >>>> On 6/4/21 6:42 PM, Nicholas Piggin wrote: >>>>> Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb= mm >>>>> when it is context switched. This can be disabled by architectures th= at >>>>> don't require this refcounting if they clean up lazy tlb mms when the >>>>> last refcount is dropped. Currently this is always enabled, which is >>>>> what existing code does, so the patch is effectively a no-op. >>>>> >>>>> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is. >>>> >>>> I am in favor of this approach, but I would be a lot more comfortable >>>> with the resulting code if task->active_mm were at least better >>>> documented and possibly even guarded by ifdefs. >>>=20 >>> active_mm is fairly well documented in Documentation/active_mm.rst IMO. >>> I don't think anything has changed in 20 years, I don't know what more >>> is needed, but if you can add to documentation that would be nice. Mayb= e >>> moving a bit of that into .c and .h files? >>>=20 >>=20 >> Quoting from that file: >>=20 >> - however, we obviously need to keep track of which address space we >> "stole" for such an anonymous user. For that, we have "tsk->active_m= m", >> which shows what the currently active address space is. >>=20 >> This isn't even true right now on x86. >=20 > From the perspective of core code, it is. x86 might do something crazy=20 > with it, but it has to make it appear this way to non-arch code that > uses active_mm. >=20 > Is x86's scheme documented? >=20 >> With your patch applied: >>=20 >> To support all that, the "struct mm_struct" now has two counters: a >> "mm_users" counter that is how many "real address space users" there ar= e, >> and a "mm_count" counter that is the number of "lazy" users (ie anonymo= us >> users) plus one if there are any real users. >>=20 >> isn't even true any more. >=20 > Well yeah but the active_mm concept hasn't changed. The refcounting=20 > change is hopefully reasonably documented? >=20 >>=20 >>=20 >>>> x86 bare metal currently does not need the core lazy mm refcounting, a= nd >>>> x86 bare metal *also* does not need ->active_mm. Under the x86 scheme= , >>>> if lazy mm refcounting were configured out, ->active_mm could become a >>>> dangling pointer, and this makes me extremely uncomfortable. >>>> >>>> So I tend to think that, depending on config, the core code should >>>> either keep ->active_mm [1] alive or get rid of it entirely. >>>=20 >>> I don't actually know what you mean. >>>=20 >>> core code needs the concept of an "active_mm". This is the mm that your= =20 >>> kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=3Dn patc= h, >>> active_mm still points to init_mm for kernel threads. >>=20 >> Core code does *not* need this concept. First, it's wrong on x86 since >> at least 4.15. Any core code that actually assumes that ->active_mm is >> "active" for any sensible definition of the word active is wrong. >> Fortunately there is no such code. >>=20 >> I looked through all active_mm references in core code. We have: >>=20 >> kernel/sched/core.c: it's all refcounting, although it's a bit tangled >> with membarrier. >>=20 >> kernel/kthread.c: same. refcounting and membarrier stuff. >>=20 >> kernel/exit.c: exit_mm() a BUG_ON(). >>=20 >> kernel/fork.c: initialization code and a warning. >>=20 >> kernel/cpu.c: cpu offline stuff. wouldn't be needed if active_mm went a= way. >>=20 >> fs/exec.c: nothing of interest >=20 > I might not have been clear. Core code doesn't need active_mm if=20 > active_mm somehow goes away. I'm saying active_mm can't go away because > it's needed to support (most) archs that do lazy tlb mm switching. >=20 > The part I don't understand is when you say it can just go away. How?=20 >=20 >> I didn't go through drivers, but I maintain my point. active_mm is >> there for refcounting. So please don't just make it even more confusing >> -- do your performance improvement, but improve the code at the same >> time: get rid of active_mm, at least on architectures that opt out of >> the refcounting. >=20 > powerpc opts out of the refcounting and can not "get rid of active_mm". > Not even in theory. That is to say, it does do a type of reference management that requires=20 active_mm so you can argue it has not entirely opted out of refcounting. But we're not just doing refcounting for the sake of refcounting! That would make no sense. active_mm is required because that's the mm that we have switched to=20 (from core code's perspective), and it is integral to know when to=20 switch to a different mm. See how active_mm is a fundamental concept in core code? It's part of the contract between core code and the arch mm context management calls. reference counting follows from there but it's not the _reason_ for this code. Pretend the reference problem does not exit (whether by refcounting or=20 shootdown or garbage collection or whatever). We still can't remove=20 active_mm! We need it to know how to call into arch functions like=20 switch_mm. I don't know if you just forgot that critical requirement in your above=20 list, or you actually are entirely using x86's mental model for this=20 code which is doing something entirely different that does not need it=20 at all. If that is the case I really don't mind some cleanup or wrapper=20 functions for x86 do entirely do its own thing, but if that's the case you can't criticize core code's use of active_mm due to the current state of x86. It's x86 that needs documentation and cleaning up. Thanks, Nick