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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,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 7C87DC43603 for ; Wed, 4 Dec 2019 15:19:38 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 41709206DB for ; Wed, 4 Dec 2019 15:19:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=shipmail.org header.i=@shipmail.org header.b="BIzth5pm" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 41709206DB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=shipmail.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D11DC6B0B32; Wed, 4 Dec 2019 10:19:37 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id CC2C86B0B33; Wed, 4 Dec 2019 10:19:37 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BB2806B0B34; Wed, 4 Dec 2019 10:19:37 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0035.hostedemail.com [216.40.44.35]) by kanga.kvack.org (Postfix) with ESMTP id A65FB6B0B32 for ; Wed, 4 Dec 2019 10:19:37 -0500 (EST) Received: from smtpin05.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id 6495E49960F for ; Wed, 4 Dec 2019 15:19:37 +0000 (UTC) X-FDA: 76227818394.05.knee07_70af105ff5840 X-HE-Tag: knee07_70af105ff5840 X-Filterd-Recvd-Size: 6586 Received: from pio-pvt-msa2.bahnhof.se (pio-pvt-msa2.bahnhof.se [79.136.2.41]) by imf24.hostedemail.com (Postfix) with ESMTP for ; Wed, 4 Dec 2019 15:19:34 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by pio-pvt-msa2.bahnhof.se (Postfix) with ESMTP id 77FAC3F418; Wed, 4 Dec 2019 16:19:32 +0100 (CET) Authentication-Results: pio-pvt-msa2.bahnhof.se; dkim=pass (1024-bit key; unprotected) header.d=shipmail.org header.i=@shipmail.org header.b=BIzth5pm; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at bahnhof.se Received: from pio-pvt-msa2.bahnhof.se ([127.0.0.1]) by localhost (pio-pvt-msa2.bahnhof.se [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id GMt9AI3aLq8w; Wed, 4 Dec 2019 16:19:31 +0100 (CET) Received: from mail1.shipmail.org (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) (Authenticated sender: mb878879) by pio-pvt-msa2.bahnhof.se (Postfix) with ESMTPA id 58EA03F413; Wed, 4 Dec 2019 16:19:27 +0100 (CET) Received: from localhost.localdomain (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) by mail1.shipmail.org (Postfix) with ESMTPSA id 8C0DC360608; Wed, 4 Dec 2019 16:19:27 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=shipmail.org; s=mail; t=1575472767; bh=YgSZucqCNwJPIVx9FkQL9howdBrm5GEYzP1TK7lj8f4=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=BIzth5pmvF4LmI9Ct05xwXwGYvq1ClqzfKX6OlROHfo+dfi3odLmEnYt7wOSE4MBk rzIsPiJ9OxjHKRqUaQbG/9n9uqAIq5sBub4hHmylLYR/UmHlWjKXc7ld9jzNkIqwC7 RhQQzxPlcJxlNKlI4pKtwb3bHoRKJN+FyWGmQBFg= Subject: Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling To: Michal Hocko Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, pv-drivers@vmware.com, linux-graphics-maintainer@vmware.com, Thomas Hellstrom , Andrew Morton , "Matthew Wilcox (Oracle)" , "Kirill A. Shutemov" , Ralph Campbell , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , =?UTF-8?Q?Christian_K=c3=b6nig?= References: <20191203104853.4378-1-thomas_os@shipmail.org> <20191203104853.4378-3-thomas_os@shipmail.org> <20191204135219.GH25242@dhcp22.suse.cz> <20191204143521.GJ25242@dhcp22.suse.cz> <5c2658b6-b5ec-5747-c360-fada54d759ed@shipmail.org> <20191204144248.GK25242@dhcp22.suse.cz> From: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28VMware=29?= Organization: VMware Inc. Message-ID: Date: Wed, 4 Dec 2019 16:19:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20191204144248.GK25242@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 12/4/19 3:42 PM, Michal Hocko wrote: > On Wed 04-12-19 15:36:58, Thomas Hellstr=C3=B6m (VMware) wrote: >> On 12/4/19 3:35 PM, Michal Hocko wrote: >>> On Wed 04-12-19 15:16:09, Thomas Hellstr=C3=B6m (VMware) wrote: >>>> On 12/4/19 2:52 PM, Michal Hocko wrote: >>>>> On Tue 03-12-19 11:48:53, Thomas Hellstr=C3=B6m (VMware) wrote: >>>>>> From: Thomas Hellstrom >>>>>> >>>>>> TTM graphics buffer objects may, transparently to user-space, mov= e >>>>>> between IO and system memory. When that happens, all PTEs pointing= to the >>>>>> old location are zapped before the move and then faulted in again = if >>>>>> needed. When that happens, the page protection caching mode- and >>>>>> encryption bits may change and be different from those of >>>>>> struct vm_area_struct::vm_page_prot. >>>>>> >>>>>> We were using an ugly hack to set the page protection correctly. >>>>>> Fix that and instead use vmf_insert_mixed_prot() and / or >>>>>> vmf_insert_pfn_prot(). >>>>>> Also get the default page protection from >>>>>> struct vm_area_struct::vm_page_prot rather than using vm_get_page_= prot(). >>>>>> This way we catch modifications done by the vm system for drivers = that >>>>>> want write-notification. >>>>> So essentially this should have any new side effect on functionalit= y it >>>>> is just making a hacky/ugly code less so? >>>> Functionality is unchanged. The use of a on-stack vma copy was sever= ely >>>> frowned upon in an earlier thread, which also points to another simi= lar >>>> example using vmf_insert_pfn_prot(). >>>> >>>> https://lore.kernel.org/lkml/20190905103541.4161-2-thomas_os@shipmai= l.org/ >>>> >>>>> In other words what are the >>>>> consequences of having page protection inconsistent from vma's? >>>> During the years, it looks like the caching- and encryption flags of >>>> vma::vm_page_prot have been largely removed from usage. From what I = can >>>> tell, there are no more places left that can affect TTM. We discusse= d >>>> __split_huge_pmd_locked() towards the end of that thread, but that d= oesn't >>>> affect TTM even with huge page-table entries. >>> Please state all those details/assumptions you are operating on in th= e >>> changelog. >> Thanks. I'll update the patchset and add that. > And thinking about that this also begs for a comment in the code to > explain that some (which?) mappings might have a mismatch and the > generic code have to be careful. Because as things stand now this seems > to be really subtle and happen to work _now_ and might break in the fut= ure. > Or what does prevent a generic code to stumble over this discrepancy? Yes we had that discussion in the thread I pointed to. I initially=20 suggested and argued for updating the vma::vm_page_prot using a=20 WRITE_ONCE() (we only have the mmap_sem in read mode), there seems to be=20 other places in generic code that does the same. But I was convinced by Andy that this was the right way and also was=20 used elsewhere. (See also=20 https://elixir.bootlin.com/linux/latest/source/arch/x86/entry/vdso/vma.c#= L116) I guess to have this properly formulated, what's required is that=20 generic code doesn't build page-table entries using vma::vm_page_prot=20 for VM_PFNMAP and VM_MIXEDMAP outside of driver control. /Thomas