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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 B4A2EC433B4 for ; Wed, 19 May 2021 14:04:52 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 74CF161074 for ; Wed, 19 May 2021 14:04:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 74CF161074 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id CA8436E1CE; Wed, 19 May 2021 14:04:51 +0000 (UTC) Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by gabe.freedesktop.org (Postfix) with ESMTPS id 355296E1CE for ; Wed, 19 May 2021 14:04:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621433089; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=q0X6ufyJH5fadmIknxn4nGcWs1xnqbL6wQGkTAtdW1c=; b=YJ9Sl+UpqDhjLciNBRI3ns8McCD2kWDjznAOgNh1Zr7HeAjoLj2REOkLLmsVAXAIcReH+L 23Oygl8Ahgdt5OI2riy467QCWxoEDJi+6dddMhwiQCpf/dYFuWK3yMwfyhCLdfFiC1l2JZ LDxSQ27MewB2MRLysdCr8IaM+RFt3i0= Received: from mail-qv1-f71.google.com (mail-qv1-f71.google.com [209.85.219.71]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-9-2Nj8MUucN0aUFV8s0v2Ytw-1; Wed, 19 May 2021 10:04:34 -0400 X-MC-Unique: 2Nj8MUucN0aUFV8s0v2Ytw-1 Received: by mail-qv1-f71.google.com with SMTP id fi6-20020a0562141a46b02901f064172b74so5522587qvb.3 for ; Wed, 19 May 2021 07:04:34 -0700 (PDT) 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=q0X6ufyJH5fadmIknxn4nGcWs1xnqbL6wQGkTAtdW1c=; b=Nu5QhcugeII1Ea+uxm+3an+Hyozkg4Ane1VBDgrcX/3zCp13No+UbMva+Mb7DXA20y ZTpEDI7bmZSVBG1R5bb6jDXEnKh57aC5dqAjZr6w4B9Gs3cxgtMa+IGaU+OB0yNI3p8M 9c2AEm11u4d2yFGmC+xd5C87Byml/rHW1d2P4ORTvTbQ7Ws4J2Fl3hRMBMa5A9Bq3+zr M6HtF2ED5TkVA4JHooy/b4vt1CCHBTDaSznoyvR72OzBnPOFYvno1n67AhgEu0/0KX+J 9INmaffiJti8CFA8L6cGo28A1Pb2GazGKgRUQhQXPpd4nBc93SVy0j3AQ77wJZJsH65f XVCw== X-Gm-Message-State: AOAM532QVdz06LDO+cm9lpwpOcyWH72Gd/puUfLs1khwghnIUrEiM92p WlJiZyUau+WqZPEdrvb5odY5Y3Il+NK6L8Zmir18f8mpTdHjrdKnfFKuj4+Z6sqa6JhrDSsX6Z0 1PbqXmrKf8Ip0iSgyUoafrz+cFTgw X-Received: by 2002:a05:6214:d87:: with SMTP id e7mr13037375qve.53.1621433074137; Wed, 19 May 2021 07:04:34 -0700 (PDT) X-Google-Smtp-Source: ABdhPJw6J2iQWttFbTggjwn9zrUjjmVN+9VOim38VY5Z448bl6PpUs3xqQHJcSoYTLuhDbSvYY+iNA== X-Received: by 2002:a05:6214:d87:: with SMTP id e7mr13037337qve.53.1621433073770; Wed, 19 May 2021 07:04:33 -0700 (PDT) Received: from t490s (bras-base-toroon474qw-grc-72-184-145-4-219.dsl.bell.ca. [184.145.4.219]) by smtp.gmail.com with ESMTPSA id b13sm802748qkl.16.2021.05.19.07.04.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 May 2021 07:04:33 -0700 (PDT) Date: Wed, 19 May 2021 10:04:32 -0400 From: Peter Xu To: Alistair Popple Subject: Re: [PATCH v8 5/8] mm: Device exclusive memory access Message-ID: References: <20210407084238.20443-1-apopple@nvidia.com> <2235357.HsqDk0zIjc@nvdebian> <2569629.VzlulnA7BY@nvdebian> MIME-Version: 1.0 In-Reply-To: <2569629.VzlulnA7BY@nvdebian> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rcampbell@nvidia.com, willy@infradead.org, linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, bsingharora@gmail.com, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, hch@infradead.org, linux-mm@kvack.org, jglisse@redhat.com, bskeggs@redhat.com, Jason Gunthorpe , jhubbard@nvidia.com, akpm@linux-foundation.org, Christoph Hellwig Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, May 19, 2021 at 11:11:55PM +1000, Alistair Popple wrote: > On Wednesday, 19 May 2021 10:15:41 PM AEST Peter Xu wrote: > > External email: Use caution opening links or attachments > > > > On Wed, May 19, 2021 at 09:04:53PM +1000, Alistair Popple wrote: > > > Failing fork() because we couldn't take a lock doesn't seem like the right > > > approach though, especially as there is already existing code that > > > retries. I get this adds complexity though, so would be happy to take a > > > look at cleaning copy_pte_range() up in future. > > > > Yes, I proposed that as this one won't affect any existing applications > > (unlike the existing ones) but only new userspace driver apps that will use > > this new atomic feature. > > > > IMHO it'll be a pity to add extra complexity and maintainance burden into > > fork() if only for keeping the "logical correctness of fork()" however the > > code never triggers. If we start with trylock we'll know whether people > > will use it, since people will complain with a reason when needed; however > > I still doubt whether a sane userspace device driver should fork() within > > busy interaction with the device underneath.. > > I will refrain from commenting on the sanity or otherwise of doing that :-) > > Agree such a scenario seems unlikely in practice (and possibly unreasonable). > Keeping the "logical correctness of fork()" still seems worthwhile to me, but > if the added complexity/maintenance burden for an admittedly fairly specific > feature is going to stop progress here I am happy to take the fail fork > approach. I could then possibly fix it up as a future clean up to > copy_pte_range(). Perhaps others have thoughts? Yes, it's more about making this series easier to be accepted, and it'll be great to have others' input. Btw, just to mention that I don't even think fail fork() on failed trylock() is against "logical correctness of fork()": IMHO it's still 100% correct just like most syscalls can return with -EAGAIN, that suggests the userspace to try again the syscall, and I hope that also works for fork(). I'd be more than glad to be corrected too. -- Peter Xu