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=-6.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,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 8E73BC2D0E2 for ; Tue, 22 Sep 2020 20:01:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2BE08221E8 for ; Tue, 22 Sep 2020 20:01:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="Tde9CBw9" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726714AbgIVUBP (ORCPT ); Tue, 22 Sep 2020 16:01:15 -0400 Received: from hqnvemgate24.nvidia.com ([216.228.121.143]:15118 "EHLO hqnvemgate24.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726563AbgIVUBO (ORCPT ); Tue, 22 Sep 2020 16:01:14 -0400 Received: from hqmail.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate24.nvidia.com (using TLS: TLSv1.2, AES256-SHA) id ; Tue, 22 Sep 2020 12:59:40 -0700 Received: from [10.2.52.174] (172.20.13.39) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Tue, 22 Sep 2020 20:01:14 +0000 Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork() To: Jan Kara CC: Peter Xu , , , Linus Torvalds , Michal Hocko , "Kirill Shutemov" , Jann Horn , Oleg Nesterov , Kirill Tkhai , Hugh Dickins , Leon Romanovsky , Christoph Hellwig , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli References: <20200921211744.24758-1-peterx@redhat.com> <20200921212031.25233-1-peterx@redhat.com> <5e594e71-537f-3e9f-85b6-034b7f5fedbe@nvidia.com> <20200922103315.GD15112@quack2.suse.cz> From: John Hubbard Message-ID: <4a65586e-9282-beb0-1880-1ef8da03727c@nvidia.com> Date: Tue, 22 Sep 2020 13:01:13 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <20200922103315.GD15112@quack2.suse.cz> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [172.20.13.39] X-ClientProxiedBy: HQMAIL107.nvidia.com (172.20.187.13) To HQMAIL107.nvidia.com (172.20.187.13) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1600804780; bh=YRzr4MXKIVqdNO7ktu5glQ0JCVg8EzQ8Vf8cqhDwAPQ=; h=Subject:To:CC:References:From:Message-ID:Date:User-Agent: MIME-Version:In-Reply-To:Content-Type:Content-Language: Content-Transfer-Encoding:X-Originating-IP:X-ClientProxiedBy; b=Tde9CBw9i4FTYZdoQ+BZOyvSSyyK9tfAUY6tej1s7YdKStE5rjEv2ZbcpO3mWq8og 6G4oyPTK3tDtwaUmke4V/h1xPiyKzeoa/sCr9IzKhnUnbv1rUe5hxEnJIqJyRt0Bp5 SBHW4exjcXhLgSLey6guHt0tb+QoFNS1zm2ZFcwHz40DAWTeKM7t7XfI4lSQWX02Jv G7S3Tw8swWSWRqSbDjsEYlNsRRevJfQyz+HnfeuHk1Fv98R+mlUQMBq3lJJIgkFToR +OOJ4Tc5lHycHJ7sJpi0Cpt8x8a6LtUm6G97vw9aBuPFBgdE2v9J4JOKaCg67SWI5N lqCJqEZy+wcfQ== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/22/20 3:33 AM, Jan Kara wrote: > On Mon 21-09-20 23:41:16, John Hubbard wrote: >> On 9/21/20 2:20 PM, Peter Xu wrote: >> ... >>> + if (unlikely(READ_ONCE(src_mm->has_pinned) && >>> + page_maybe_dma_pinned(src_page))) { >> >> This condition would make a good static inline function. It's used in 3 >> places, and the condition is quite special and worth documenting, and >> having a separate function helps with that, because the function name >> adds to the story. I'd suggest approximately: >> >> page_likely_dma_pinned() >> >> for the name. > > Well, but we should also capture that this really only works for anonymous > pages. For file pages mm->has_pinned does not work because the page may be > still pinned by completely unrelated process as Jann already properly > pointed out earlier in the thread. So maybe anon_page_likely_pinned()? > Possibly also assert PageAnon(page) in it if we want to be paranoid... > > Honza The file-backed case doesn't really change anything, though: page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you can get a false positive. Just like here, with an mm->has_pinned that could be a false positive for a process. And for that reason, I'm also not sure an "assert PageAnon(page)" is desirable. That assertion would prevent file-backed callers from being able to call a function that provides a fuzzy answer, but I don't see why you'd want or need to do that. The goal here is to make the fuzzy answer a little bit more definite, but it's not "broken" just because the result is still fuzzy, right? Apologies if I'm missing a huge point here... :) thanks, -- John Hubbard NVIDIA