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=-3.7 required=3.0 tests=BAYES_00,DATE_IN_PAST_03_06, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 75F44C4363D for ; Thu, 24 Sep 2020 13:40:42 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id AF2482344C for ; Thu, 24 Sep 2020 13:40:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AF2482344C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 2EB24900036; Thu, 24 Sep 2020 09:40:41 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 2CD61900035; Thu, 24 Sep 2020 09:40:41 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1D6D6900035; Thu, 24 Sep 2020 09:40:41 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0047.hostedemail.com [216.40.44.47]) by kanga.kvack.org (Postfix) with ESMTP id 0884490002C for ; Thu, 24 Sep 2020 09:40:41 -0400 (EDT) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id C874E8249980 for ; Thu, 24 Sep 2020 13:40:40 +0000 (UTC) X-FDA: 77298065040.30.pigs72_54147ad2715f Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id A37E6180B3C83 for ; Thu, 24 Sep 2020 13:40:40 +0000 (UTC) X-HE-Tag: pigs72_54147ad2715f X-Filterd-Recvd-Size: 3514 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf23.hostedemail.com (Postfix) with ESMTP for ; Thu, 24 Sep 2020 13:40:40 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A9297AE44; Thu, 24 Sep 2020 13:40:38 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 667671E12E9; Thu, 24 Sep 2020 09:44:09 +0200 (CEST) Date: Thu, 24 Sep 2020 09:44:09 +0200 From: Jan Kara To: Jason Gunthorpe Cc: Jan Kara , Peter Xu , John Hubbard , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Michal Hocko , Kirill Tkhai , Kirill Shutemov , Hugh Dickins , Christoph Hellwig , Andrea Arcangeli , Oleg Nesterov , Leon Romanovsky , Linus Torvalds , Jann Horn Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned Message-ID: <20200924074409.GB27019@quack2.suse.cz> References: <20200921211744.24758-2-peterx@redhat.com> <224908c1-5d0f-8e01-baa9-94ec2374971f@nvidia.com> <20200922151736.GD19098@xz-x1> <20200922161046.GB731578@ziepe.ca> <20200922175415.GI19098@xz-x1> <20200922191116.GK8409@ziepe.ca> <20200923002735.GN19098@xz-x1> <20200923131043.GA59978@xz-x1> <20200923142003.GB15875@quack2.suse.cz> <20200923171207.GB9916@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200923171207.GB9916@ziepe.ca> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Wed 23-09-20 14:12:07, Jason Gunthorpe wrote: > On Wed, Sep 23, 2020 at 04:20:03PM +0200, Jan Kara wrote: > > > I'd hate to take spinlock in the GUP-fast path. Also I don't think this is > > quite correct because GUP-fast-only can be called from interrupt context > > and page table locks are not interrupt safe. > > Yes, IIRC, that is a key element of GUP-fast. Was it something to do > with futexes? Honestly, I'm not sure. > > and then checking page_may_be_dma_pinned() during fork(). That should work > > just fine AFAICT... BTW note that GUP-fast code is (and this is deliberated > > because e.g. DAX depends on this) first updating page->_refcount and then > > rechecking PTE didn't change and the page->_refcount update is actually > > done using atomic_add_unless() so that it cannot be reordered wrt the PTE > > check. So the fork() code only needs to add barriers to pair with this. > > It is not just DAX, everything needs this check. > > After the page is pinned it is prevented from being freed and > recycled. After GUP has the pin it must check that the PTE still > points at the same page, otherwise it might have pinned a page that is > alreay free'd - and that would be a use-after-free issue. I don't think a page use-after-free is really the reason - we add page reference through page_ref_add_unless(page, x, 0) - i.e., it will fail for already freed page. It's more about being able to make sure page is not accessible anymore - and for that modifying pte and then checking page refcount it *reliable* way to synchronize with GUP-fast... Honza -- Jan Kara SUSE Labs, CR