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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 7471CC433E1 for ; Tue, 23 Jun 2020 00:02:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 512E42076A for ; Tue, 23 Jun 2020 00:02:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca header.b="jOiz5xhY" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731387AbgFWACg (ORCPT ); Mon, 22 Jun 2020 20:02:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33496 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730994AbgFWACf (ORCPT ); Mon, 22 Jun 2020 20:02:35 -0400 Received: from mail-qt1-x841.google.com (mail-qt1-x841.google.com [IPv6:2607:f8b0:4864:20::841]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4189BC061795 for ; Mon, 22 Jun 2020 17:02:34 -0700 (PDT) Received: by mail-qt1-x841.google.com with SMTP id o38so5445041qtf.6 for ; Mon, 22 Jun 2020 17:02:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ziepe.ca; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=fL1wENgZCNwSv3s/JTNbtGHYfJcNOvZphPgobRgCX+0=; b=jOiz5xhY4H9RrA31ewUoMb6lWoSEJluMNJ4pFLIyuL2C/PAw2cU4NhHZPSbOtneWmY ozUMm9lxePnw9yOjxx4YXvXCK3kITgyY6VI+mhuSl9fkmcyIppyhy/Jz6vVv/4LKzuRn Gb8qhMwxyR4RKm7f3IBCU6P9IiqbHgwNX0yoHpJ3PAkHDvus22DcgqWtCXRMKvuPd80+ GtMIE29iM9uSkrK/mkJM41mZz+r3jtg9ekirUsyx5erxi/e/W0HuUH+SRS8iBijVSbYY gVUrM+bjvjO7k15tkl2NlzpoPMdsvYPWVTdqh0sfLGGUDdoylYfcONvUD2B6N+OO84GZ lj3Q== 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; bh=fL1wENgZCNwSv3s/JTNbtGHYfJcNOvZphPgobRgCX+0=; b=i+ONEsYI4JP6SJky8btKT3QbnjRSILr37Mz+bfkUieh4CTjrABh/b/BXmx+IXIZVfC TmjGVCnG0UR8OVHNV0N8hsruRrO9+b/UKJwt/dk0qt5kCclB2ou6n1BNtfjrgaGeIRc5 lwgp6D1/Jgb0QlNWZJVRwh/TI1L4lYE4nz9KV7G+7dZAi/5N+a93xhBb2/BTYpGiu0WH HTWCWPHqIC4hJrnfbq5TgiOHR0DJkp8SHp9YymnQvIEmCV/DuiKm4ZxAIASYmoFm13Qy 2lDqLcNip/9diJJihFucVX0BA7y1xdsUdd2PT6NAZIPEBPq0R3ZJpC+yOWL/co/mI3zX 5Bjg== X-Gm-Message-State: AOAM532oBeHpqRfPH9tYA5U2th77wFAsLyF++jzs5i52FQpfZEJVlifs yhRSDeayREVSKVzwTYko5/7vfA== X-Google-Smtp-Source: ABdhPJyCyWWM03fNzzwWcrkorPAde8ZwkJyT1NwEy7u+QyW/bvUE+EE8dkCWV5n1HdRyI+PQxXYPRQ== X-Received: by 2002:ac8:70da:: with SMTP id g26mr17916678qtp.333.1592870553204; Mon, 22 Jun 2020 17:02:33 -0700 (PDT) Received: from ziepe.ca ([206.223.160.26]) by smtp.gmail.com with ESMTPSA id d186sm6145604qkb.110.2020.06.22.17.02.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Jun 2020 17:02:32 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.93) (envelope-from ) id 1jnWOV-00CHMM-GL; Mon, 22 Jun 2020 21:02:31 -0300 Date: Mon, 22 Jun 2020 21:02:31 -0300 From: Jason Gunthorpe To: Jerome Glisse Cc: Felix Kuehling , linux-rdma , Thomas =?utf-8?B?SGVsbHN0csO2bSAoSW50ZWwp?= , Maarten Lankhorst , LKML , DRI Development , Christian =?utf-8?B?S8O2bmln?= , "moderated list:DMA BUFFER SHARING FRAMEWORK" , Thomas Hellstrom , amd-gfx list , Daniel Vetter , Daniel Vetter , Mika Kuoppala , Intel Graphics Development , "open list:DMA BUFFER SHARING FRAMEWORK" Subject: Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations Message-ID: <20200623000231.GW6578@ziepe.ca> References: <20200619151551.GP6578@ziepe.ca> <20200619172308.GQ6578@ziepe.ca> <20200619180935.GA10009@redhat.com> <20200619181849.GR6578@ziepe.ca> <56008d64-772d-5757-6136-f20591ef71d2@amd.com> <20200619195538.GT6578@ziepe.ca> <20200619203147.GC13117@redhat.com> <20200622114617.GU6578@ziepe.ca> <20200622201540.GB9708@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200622201540.GB9708@redhat.com> Sender: linux-rdma-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org On Mon, Jun 22, 2020 at 04:15:40PM -0400, Jerome Glisse wrote: > On Mon, Jun 22, 2020 at 08:46:17AM -0300, Jason Gunthorpe wrote: > > On Fri, Jun 19, 2020 at 04:31:47PM -0400, Jerome Glisse wrote: > > > Not doable as page refcount can change for things unrelated to GUP, with > > > John changes we can identify GUP and we could potentialy copy GUPed page > > > instead of COW but this can potentialy slow down fork() and i am not sure > > > how acceptable this would be. Also this does not solve GUP against page > > > that are already in fork tree ie page P0 is in process A which forks, > > > we now have page P0 in process A and B. Now we have process A which forks > > > again and we have page P0 in A, B, and C. Here B and C are two branches > > > with root in A. B and/or C can keep forking and grow the fork tree. > > > > For a long time now RDMA has broken COW pages when creating user DMA > > regions. > > > > The problem has been that fork re-COW's regions that had their COW > > broken. > > > > So, if you break the COW upon mapping and prevent fork (and others) > > from copying DMA pinned then you'd cover the cases. > > I am not sure we want to prevent COW for pinned GUP pages, this would > change current semantic and potentialy break/slow down existing apps. Isn't that basically exactly what 17839856fd588 does? It looks like it uses the same approach RDMA does by sticking FOLL_WRITE even though it is a read action. After that change the reamining bug is that fork can re-establish a COW./ > Anyway i think we focus too much on fork/COW, it is just an unfixable > broken corner cases, mmu notifier allows you to avoid it. Forcing real > copy on fork would likely be seen as regression by most people. If you don't copy the there are data corruption bugs though. Real apps probably don't hit a problem here as they are not forking while GUP's are active (RDMA excluded, which does do this) I think that implementing page pinning by blocking mmu notifiers for the duration of the pin is a particularly good idea either, that actually seems a lot worse than just having the pin in the first place. Particularly if it is only being done to avoid corner case bugs that already afflict other GUP cases :( > > What do you mean 'GUP fast is still succeptible to this' ? > > Not all GUP fast path are updated (intentionaly) __get_user_pages_fast() Sure, that is is the 'raw' accessor Jason