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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 A4BE1C33CB1 for ; Mon, 20 Jan 2020 08:03:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 795402077C for ; Mon, 20 Jan 2020 08:03:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579507414; bh=7IhXPbsNriUrUDNvgKxZRuAQJLo8U2+A6F86F1CLA4g=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=tIc7NYRcpdwA6nQOQszfCz6ir2WrayM7LO7wwYHvKbMuU96MmsUeEQ0sO7qom4Qi3 yK6AQvLGhBgw9tXvwO+GuxjC/0FkHU8/WXs0yXL5y1+SuETYMtIIQWH5jK8F7CGSez 5LqtAoLGx21/QycqPT1Xk+xukCeSKiYvAIAcxStc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726728AbgATIDd (ORCPT ); Mon, 20 Jan 2020 03:03:33 -0500 Received: from mail-wr1-f65.google.com ([209.85.221.65]:43679 "EHLO mail-wr1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726130AbgATIDd (ORCPT ); Mon, 20 Jan 2020 03:03:33 -0500 Received: by mail-wr1-f65.google.com with SMTP id d16so28429953wre.10; Mon, 20 Jan 2020 00:03:30 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=lDmezBsJvvXTKgPzl7Xvo58o6+oumRfpkAIXFK+J8DU=; b=SPQwoc6nrZpw4pSUqZfB7ptf7WFMx6MpIaUm1UgqLErbCylB9sJ2UuSBQkDvo01dYH uAU2q7FOZhhWogeMKKp3feHlGGuGegyYmIZPBWWphkFtfXwPgMqw6DaEsl6Gyi0etPg/ /jleXbpvMgY3fHQGTw9+06dcNUJPmXF5C5yfyQ3+9gDWOOeQrOd1B5BFWn7YdJBLwgPN DTsW8uDUkwNPNgV93pS+kNnBLpYbnSXt2svcqiZfJ02T0he6BWRH+f37Tb6suZ3iYcL8 bdptRijVkoPstlVLW8QKksLGVD0Buh5zw9GoXnbUzrq30WhkySpQ4WfmP2E2FdPafldm qv3w== X-Gm-Message-State: APjAAAWu2GSy3AHmNPuzbEYt00wRDaVCtm4ZLAKZDhoX47Mrxei1W+tR sbCMEwnfEzhmM6sp2sbruD0= X-Google-Smtp-Source: APXvYqw9SSSto7+XdEORAAN2GaIZ7SGXrUkE5SOTrMmMO40hITkY79xhN1nH/PfkvbFscm8Z/vIAcQ== X-Received: by 2002:adf:e290:: with SMTP id v16mr17398000wri.16.1579507410211; Mon, 20 Jan 2020 00:03:30 -0800 (PST) Received: from localhost (ip-37-188-138-155.eurotel.cz. [37.188.138.155]) by smtp.gmail.com with ESMTPSA id t5sm45407177wrr.35.2020.01.20.00.03.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Jan 2020 00:03:29 -0800 (PST) Date: Mon, 20 Jan 2020 09:03:26 +0100 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , LKML , linux-mm , linux-api@vger.kernel.org, oleksandr@redhat.com, Suren Baghdasaryan , Tim Murray , Daniel Colascione , Sandeep Patil , Sonny Rao , Brian Geffon , Johannes Weiner , Shakeel Butt , John Dias , ktkhai@virtuozzo.com, christian.brauner@ubuntu.com, sjpark@amazon.de Subject: Re: [PATCH v2 2/5] mm: introduce external memory hinting API Message-ID: <20200120080326.GI18451@dhcp22.suse.cz> References: <20200116235953.163318-1-minchan@kernel.org> <20200116235953.163318-3-minchan@kernel.org> <20200117115225.GV19428@dhcp22.suse.cz> <20200117172542.GA140922@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200117172542.GA140922@google.com> User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 17-01-20 09:25:42, Minchan Kim wrote: > On Fri, Jan 17, 2020 at 12:52:25PM +0100, Michal Hocko wrote: > > On Thu 16-01-20 15:59:50, Minchan Kim wrote: > > > There is usecase that System Management Software(SMS) want to give > > > a memory hint like MADV_[COLD|PAGEEOUT] to other processes and > > > in the case of Android, it is the ActivityManagerService. > > > > > > It's similar in spirit to madvise(MADV_WONTNEED), but the information > > > required to make the reclaim decision is not known to the app. Instead, > > > it is known to the centralized userspace daemon(ActivityManagerService), > > > and that daemon must be able to initiate reclaim on its own without > > > any app involvement. > > > > > > To solve the issue, this patch introduces new syscall process_madvise(2). > > > It uses pidfd of an external processs to give the hint. > > > > > > int process_madvise(int pidfd, void *addr, size_t length, int advise, > > > unsigned long flag); > > > > > > Since it could affect other process's address range, only privileged > > > process(CAP_SYS_PTRACE) or something else(e.g., being the same UID) > > > gives it the right to ptrace the process could use it successfully. > > > The flag argument is reserved for future use if we need to extend the > > > API. > > > > > > I think supporting all hints madvise has/will supported/support to > > > process_madvise is rather risky. Because we are not sure all hints make > > > sense from external process and implementation for the hint may rely on > > > the caller being in the current context so it could be error-prone. > > > Thus, I just limited hints as MADV_[COLD|PAGEOUT] in this patch. > > > > > > If someone want to add other hints, we could hear hear the usecase and > > > review it for each hint. It's more safe for maintainace rather than > > > introducing a buggy syscall but hard to fix it later. > > > > I have brought this up when we discussed this in the past but there is > > no reflection on that here so let me bring that up again. > > > > I believe that the interface has an inherent problem that it is racy. > > The external entity needs to know the address space layout of the target > > process to do anyhing useful on it. The address space is however under > > the full control of the target process though and the external entity > > has no means to find out that the layout has changed. So > > time-to-check-time-to-act is an inherent problem. > > > > This is a serious design flaw and it should be explained why it doesn't > > matter or how to use the interface properly to prevent that problem. > > Sorry for the missing that part. > > It's not a particular problem of this API because other APIs already have > done with that(e.g., move_pages, process_vm_writev). I am sorry but this is not really an argument. > Point is userspace has several ways for the control of target process > like SIGSTOP, cgroup freezer or even no need to control since platform > is already aware of that the process will never run until he grant it > or it's resilient even though the race happens. If you have that level of control then you can simply inject the code via ptrace and you do not need a new syscall in the first place. > In future, if we want to support more fine-grained consistency model > like memory layout, we could provide some API to get cookie(e.g., > seq count which is updated whenever vma of the process changes). And then > we could feed the cookie to process_madvise's last argument so that > it can fail if founds it's not matched. > For that API, Daniel already posted RFC - process_getinfo[1]. > https://lore.kernel.org/lkml/20190520035254.57579-1-minchan@kernel.org/T/#m7694416fd179b2066a2c62b5b139b14e3894e224 So why do not we start with a clean API since the beginning? I do agree that a remote madvise is an interesting feature and it opens gates to all sorts of userspace memory management which is not possible this days. But the syscall has to have a reasonable semantic to allow that. We cannot simply start with a half proken symantic first based on an Android usecase and then hit the wall as soon as others with a different user space model want to use it as well. -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH v2 2/5] mm: introduce external memory hinting API Date: Mon, 20 Jan 2020 09:03:26 +0100 Message-ID: <20200120080326.GI18451@dhcp22.suse.cz> References: <20200116235953.163318-1-minchan@kernel.org> <20200116235953.163318-3-minchan@kernel.org> <20200117115225.GV19428@dhcp22.suse.cz> <20200117172542.GA140922@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200117172542.GA140922-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Minchan Kim Cc: Andrew Morton , LKML , linux-mm , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, oleksandr-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, Suren Baghdasaryan , Tim Murray , Daniel Colascione , Sandeep Patil , Sonny Rao , Brian Geffon , Johannes Weiner , Shakeel Butt , John Dias , ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org, christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org, sjpark-ebkRAfMGSJGzQB+pC5nmwQ@public.gmane.org List-Id: linux-api@vger.kernel.org On Fri 17-01-20 09:25:42, Minchan Kim wrote: > On Fri, Jan 17, 2020 at 12:52:25PM +0100, Michal Hocko wrote: > > On Thu 16-01-20 15:59:50, Minchan Kim wrote: > > > There is usecase that System Management Software(SMS) want to give > > > a memory hint like MADV_[COLD|PAGEEOUT] to other processes and > > > in the case of Android, it is the ActivityManagerService. > > > > > > It's similar in spirit to madvise(MADV_WONTNEED), but the information > > > required to make the reclaim decision is not known to the app. Instead, > > > it is known to the centralized userspace daemon(ActivityManagerService), > > > and that daemon must be able to initiate reclaim on its own without > > > any app involvement. > > > > > > To solve the issue, this patch introduces new syscall process_madvise(2). > > > It uses pidfd of an external processs to give the hint. > > > > > > int process_madvise(int pidfd, void *addr, size_t length, int advise, > > > unsigned long flag); > > > > > > Since it could affect other process's address range, only privileged > > > process(CAP_SYS_PTRACE) or something else(e.g., being the same UID) > > > gives it the right to ptrace the process could use it successfully. > > > The flag argument is reserved for future use if we need to extend the > > > API. > > > > > > I think supporting all hints madvise has/will supported/support to > > > process_madvise is rather risky. Because we are not sure all hints make > > > sense from external process and implementation for the hint may rely on > > > the caller being in the current context so it could be error-prone. > > > Thus, I just limited hints as MADV_[COLD|PAGEOUT] in this patch. > > > > > > If someone want to add other hints, we could hear hear the usecase and > > > review it for each hint. It's more safe for maintainace rather than > > > introducing a buggy syscall but hard to fix it later. > > > > I have brought this up when we discussed this in the past but there is > > no reflection on that here so let me bring that up again. > > > > I believe that the interface has an inherent problem that it is racy. > > The external entity needs to know the address space layout of the target > > process to do anyhing useful on it. The address space is however under > > the full control of the target process though and the external entity > > has no means to find out that the layout has changed. So > > time-to-check-time-to-act is an inherent problem. > > > > This is a serious design flaw and it should be explained why it doesn't > > matter or how to use the interface properly to prevent that problem. > > Sorry for the missing that part. > > It's not a particular problem of this API because other APIs already have > done with that(e.g., move_pages, process_vm_writev). I am sorry but this is not really an argument. > Point is userspace has several ways for the control of target process > like SIGSTOP, cgroup freezer or even no need to control since platform > is already aware of that the process will never run until he grant it > or it's resilient even though the race happens. If you have that level of control then you can simply inject the code via ptrace and you do not need a new syscall in the first place. > In future, if we want to support more fine-grained consistency model > like memory layout, we could provide some API to get cookie(e.g., > seq count which is updated whenever vma of the process changes). And then > we could feed the cookie to process_madvise's last argument so that > it can fail if founds it's not matched. > For that API, Daniel already posted RFC - process_getinfo[1]. > https://lore.kernel.org/lkml/20190520035254.57579-1-minchan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org/T/#m7694416fd179b2066a2c62b5b139b14e3894e224 So why do not we start with a clean API since the beginning? I do agree that a remote madvise is an interesting feature and it opens gates to all sorts of userspace memory management which is not possible this days. But the syscall has to have a reasonable semantic to allow that. We cannot simply start with a half proken symantic first based on an Android usecase and then hit the wall as soon as others with a different user space model want to use it as well. -- Michal Hocko SUSE Labs