From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754973AbaEHQir (ORCPT ); Thu, 8 May 2014 12:38:47 -0400 Received: from mail-pd0-f178.google.com ([209.85.192.178]:34984 "EHLO mail-pd0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754159AbaEHQip (ORCPT ); Thu, 8 May 2014 12:38:45 -0400 Message-ID: <536BB310.1050105@linaro.org> Date: Thu, 08 May 2014 09:38:40 -0700 From: John Stultz User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Minchan Kim CC: LKML , Andrew Morton , Android Kernel Team , Johannes Weiner , Robert Love , Mel Gorman , Hugh Dickins , Dave Hansen , Rik van Riel , Dmitry Adamushko , Neil Brown , Andrea Arcangeli , Mike Hommey , Taras Glek , Jan Kara , KOSAKI Motohiro , Michel Lespinasse , Keith Packard , "linux-mm@kvack.org" Subject: Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas References: <1398806483-19122-1-git-send-email-john.stultz@linaro.org> <1398806483-19122-3-git-send-email-john.stultz@linaro.org> <20140508012142.GA5282@bbox> In-Reply-To: <20140508012142.GA5282@bbox> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/07/2014 06:21 PM, Minchan Kim wrote: > Hey John, > > On Tue, Apr 29, 2014 at 02:21:21PM -0700, John Stultz wrote: >> This patch introduces MADV_VOLATILE/NONVOLATILE flags to madvise(), >> which allows for specifying ranges of memory as volatile, and able >> to be discarded by the system. >> >> This initial patch simply adds flag handling to madvise, and the >> vma handling, splitting and merging the vmas as needed, and marking >> them with VM_VOLATILE. >> >> No purging or discarding of volatile ranges is done at this point. >> >> This a simplified implementation which reuses some of the logic >> from Minchan's earlier efforts. So credit to Minchan for his work. > Remove purged argument is really good thing but I'm not sure merging > the feature into madvise syscall is good idea. > My concern is how we support user who don't want SIGBUS. > I believe we should support them because someuser(ex, sanitizer) really > want to avoid MADV_NONVOLATILE call right before overwriting their cache > (ex, If there was purged page for cyclic cache, user should call NONVOLATILE > right before overwriting to avoid SIGBUS). So... Why not use MADV_FREE then for this case? Just to be clear, by moving back to madvise, I'm not trying to replace MADV_FREE. I think you're work there is still useful and splitting the semantics between the two is cleaner. > Moreover, this changes made unmarking cost O(N) so I'd like to avoid > NOVOLATILE syscall if possible. Well, I think that was made in v13, but yes. NONVOLATILE is currently an expensive operation in order to keep the semantics simpler, as requested by Johannes and Kosaki-san. > For me, SIGBUS is more special usecase for code pages but I believe > both are reasonable for each usecase so my preference is MADV_VOLATILE > is just zero-filled page and MADV_VOLATILE_SIGBUS, another new advise > if you really want to merge volatile range feature with madvise. This I disagree with. Even for non-code page cases, SIGBUS on volatile page access is important for normal users who might accidentally touch volatile data, so they know they are corrupting their data. I know Johannes suggested this is simply a use-after-free issue, but I really feel it results in having very strange semantics. And for those cases where there is a benefit to zero-fill, MADV_FREE seems more appropriate. thanks -john From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f41.google.com (mail-pa0-f41.google.com [209.85.220.41]) by kanga.kvack.org (Postfix) with ESMTP id 02E6E6B0101 for ; Thu, 8 May 2014 12:38:46 -0400 (EDT) Received: by mail-pa0-f41.google.com with SMTP id lj1so3089075pab.28 for ; Thu, 08 May 2014 09:38:46 -0700 (PDT) Received: from mail-pa0-f52.google.com (mail-pa0-f52.google.com [209.85.220.52]) by mx.google.com with ESMTPS id pb4si764154pac.441.2014.05.08.09.38.45 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 08 May 2014 09:38:46 -0700 (PDT) Received: by mail-pa0-f52.google.com with SMTP id kx10so3134334pab.11 for ; Thu, 08 May 2014 09:38:45 -0700 (PDT) Message-ID: <536BB310.1050105@linaro.org> Date: Thu, 08 May 2014 09:38:40 -0700 From: John Stultz MIME-Version: 1.0 Subject: Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas References: <1398806483-19122-1-git-send-email-john.stultz@linaro.org> <1398806483-19122-3-git-send-email-john.stultz@linaro.org> <20140508012142.GA5282@bbox> In-Reply-To: <20140508012142.GA5282@bbox> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: LKML , Andrew Morton , Android Kernel Team , Johannes Weiner , Robert Love , Mel Gorman , Hugh Dickins , Dave Hansen , Rik van Riel , Dmitry Adamushko , Neil Brown , Andrea Arcangeli , Mike Hommey , Taras Glek , Jan Kara , KOSAKI Motohiro , Michel Lespinasse , Keith Packard , "linux-mm@kvack.org" On 05/07/2014 06:21 PM, Minchan Kim wrote: > Hey John, > > On Tue, Apr 29, 2014 at 02:21:21PM -0700, John Stultz wrote: >> This patch introduces MADV_VOLATILE/NONVOLATILE flags to madvise(), >> which allows for specifying ranges of memory as volatile, and able >> to be discarded by the system. >> >> This initial patch simply adds flag handling to madvise, and the >> vma handling, splitting and merging the vmas as needed, and marking >> them with VM_VOLATILE. >> >> No purging or discarding of volatile ranges is done at this point. >> >> This a simplified implementation which reuses some of the logic >> from Minchan's earlier efforts. So credit to Minchan for his work. > Remove purged argument is really good thing but I'm not sure merging > the feature into madvise syscall is good idea. > My concern is how we support user who don't want SIGBUS. > I believe we should support them because someuser(ex, sanitizer) really > want to avoid MADV_NONVOLATILE call right before overwriting their cache > (ex, If there was purged page for cyclic cache, user should call NONVOLATILE > right before overwriting to avoid SIGBUS). So... Why not use MADV_FREE then for this case? Just to be clear, by moving back to madvise, I'm not trying to replace MADV_FREE. I think you're work there is still useful and splitting the semantics between the two is cleaner. > Moreover, this changes made unmarking cost O(N) so I'd like to avoid > NOVOLATILE syscall if possible. Well, I think that was made in v13, but yes. NONVOLATILE is currently an expensive operation in order to keep the semantics simpler, as requested by Johannes and Kosaki-san. > For me, SIGBUS is more special usecase for code pages but I believe > both are reasonable for each usecase so my preference is MADV_VOLATILE > is just zero-filled page and MADV_VOLATILE_SIGBUS, another new advise > if you really want to merge volatile range feature with madvise. This I disagree with. Even for non-code page cases, SIGBUS on volatile page access is important for normal users who might accidentally touch volatile data, so they know they are corrupting their data. I know Johannes suggested this is simply a use-after-free issue, but I really feel it results in having very strange semantics. And for those cases where there is a benefit to zero-fill, MADV_FREE seems more appropriate. thanks -john -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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