From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f47.google.com (mail-pa0-f47.google.com [209.85.220.47]) by kanga.kvack.org (Postfix) with ESMTP id 91FC36B0127 for ; Thu, 8 May 2014 19:11:05 -0400 (EDT) Received: by mail-pa0-f47.google.com with SMTP id lf10so967578pab.34 for ; Thu, 08 May 2014 16:11:05 -0700 (PDT) Received: from lgeamrelo01.lge.com (lgeamrelo01.lge.com. [156.147.1.125]) by mx.google.com with ESMTP id vv4si1187677pbc.279.2014.05.08.16.11.03 for ; Thu, 08 May 2014 16:11:04 -0700 (PDT) Date: Fri, 9 May 2014 08:12:59 +0900 From: Minchan Kim Subject: Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas Message-ID: <20140508231259.GA25951@bbox> References: <1398806483-19122-1-git-send-email-john.stultz@linaro.org> <1398806483-19122-3-git-send-email-john.stultz@linaro.org> <20140508012142.GA5282@bbox> <536BB310.1050105@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <536BB310.1050105@linaro.org> Sender: owner-linux-mm@kvack.org List-ID: To: John Stultz 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 Thu, May 08, 2014 at 09:38:40AM -0700, John Stultz wrote: > 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? MADV_FREE is one-shot operation. I mean we should call it again to make them lazyfree while vrange could preserve volatility. Pz, think about thread-sanitizer usecase. They do mmap 70TB once start up and want to mark the range as volatile. If they uses MADV_FREE instead of volatile, they should mark 70TB as lazyfree periodically, which is terrible because MADV_FREE's cost is O(N). > > 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. I know. New vrange syscall which works with existing VMA instead of new vrange interval tree removed big concern from mm folks about duplicating of manage layer(ex, vm_area_struct and vrange inteval tree) and it removed my concern that mmap_sem write-side lock scalability for allocator usecase so we can make the implemenation simple and clear. I like it but zero-page VS SIGBUS is another issue we should make an agreement. > > > > 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. I already explained above why MADV_FREE is not enough. > > thanks > -john > > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org