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=-5.2 required=3.0 tests=BAYES_00, 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 7B0C6C43464 for ; Mon, 21 Sep 2020 08:35:09 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id CC451207DE for ; Mon, 21 Sep 2020 08:35:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CC451207DE 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 22C46900047; Mon, 21 Sep 2020 04:35:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1DE8F900046; Mon, 21 Sep 2020 04:35:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0CC90900047; Mon, 21 Sep 2020 04:35:08 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0117.hostedemail.com [216.40.44.117]) by kanga.kvack.org (Postfix) with ESMTP id EC58B900046 for ; Mon, 21 Sep 2020 04:35:07 -0400 (EDT) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id AD6BD180AD807 for ; Mon, 21 Sep 2020 08:35:07 +0000 (UTC) X-FDA: 77286408654.06.cat18_1b0e6ab27143 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin06.hostedemail.com (Postfix) with ESMTP id 88EE810062D0D for ; Mon, 21 Sep 2020 08:35:07 +0000 (UTC) X-HE-Tag: cat18_1b0e6ab27143 X-Filterd-Recvd-Size: 4567 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf27.hostedemail.com (Postfix) with ESMTP for ; Mon, 21 Sep 2020 08:35:06 +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 B050AB50F; Mon, 21 Sep 2020 08:35:41 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 0ACAE1E12E1; Mon, 21 Sep 2020 10:35:05 +0200 (CEST) Date: Mon, 21 Sep 2020 10:35:05 +0200 From: Jan Kara To: Jason Gunthorpe Cc: John Hubbard , Peter Xu , Linus Torvalds , Leon Romanovsky , Linux-MM , Linux Kernel Mailing List , "Maya B . Gokhale" , Yang Shi , Marty Mcfadden , Kirill Shutemov , Oleg Nesterov , Jann Horn , Jan Kara , Kirill Tkhai , Andrea Arcangeli , Christoph Hellwig , Andrew Morton Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification Message-ID: <20200921083505.GA5862@quack2.suse.cz> References: <20200916184619.GB40154@xz-x1> <20200917112538.GD8409@ziepe.ca> <20200917193824.GL8409@ziepe.ca> <20200918164032.GA5962@xz-x1> <20200918173240.GY8409@ziepe.ca> <20200918204048.GC5962@xz-x1> <0af8c77e-ff60-cada-7d22-c7cfcf859b19@nvidia.com> <20200919000153.GZ8409@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200919000153.GZ8409@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 Fri 18-09-20 21:01:53, Jason Gunthorpe wrote: > On Fri, Sep 18, 2020 at 02:06:23PM -0700, John Hubbard wrote: > > On 9/18/20 1:40 PM, Peter Xu wrote: > > > On Fri, Sep 18, 2020 at 02:32:40PM -0300, Jason Gunthorpe wrote: > > > > On Fri, Sep 18, 2020 at 12:40:32PM -0400, Peter Xu wrote: > > > > > > > > > Firstly in the draft patch mm->has_pinned is introduced and it's written to 1 > > > > > as long as FOLL_GUP is called once. It's never reset after set. > > > > > > > > Worth thinking about also adding FOLL_LONGTERM here, at last as long > > > > as it is not a counter. That further limits the impact. > > > > > > But theoritically we should also trigger COW here for pages even with PIN && > > > !LONGTERM, am I right? Assuming that FOLL_PIN is already a corner case. > > > > > > > This note, plus Linus' comment about "I'm a normal process, I've never > > done any special rdma page pinning", has me a little worried. Because > > page_maybe_dma_pinned() is counting both short- and long-term pins, > > actually. And that includes O_DIRECT callers. > > > > O_DIRECT pins are short-term, and RDMA systems are long-term (and should > > be setting FOLL_LONGTERM). But there's no way right now to discern > > between them, once the initial pin_user_pages*() call is complete. All > > we can do today is to count the number of FOLL_PIN calls, not the number > > of FOLL_PIN | FOLL_LONGTERM calls. > > My thinking is to hit this issue you have to already be doing > FOLL_LONGTERM, and if some driver hasn't been properly marked and > regresses, the fix is to mark it. > > Remember, this use case requires the pin to extend after a system > call, past another fork() system call, and still have data-coherence. > > IMHO that can only happen in the FOLL_LONGTERM case as it inhernetly > means the lifetime of the pin is being controlled by userspace, not by > the kernel. Otherwise userspace could not cause new DMA touches after > fork. I agree that the new aggressive COW behavior is probably causing issues only for FOLL_LONGTERM users. That being said it would be nice if even ordinary threaded FOLL_PIN users would not have to be that careful about fork(2) and possible data loss due to COW - we had certainly reports of O_DIRECT IO loosing data due to fork(2) and COW exactly because it is very subtle how it behaves... But as I wrote above this is not urgent since that problematic behavior exists since the beginning of O_DIRECT IO in Linux. Honza -- Jan Kara SUSE Labs, CR