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=-2.3 required=3.0 tests=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 3A8BAC2BA19 for ; Tue, 14 Apr 2020 07:00:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id F1BF0206E9 for ; Tue, 14 Apr 2020 07:00:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F1BF0206E9 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lst.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 85A1E8E0003; Tue, 14 Apr 2020 03:00:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 80AFA8E0001; Tue, 14 Apr 2020 03:00:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 748168E0003; Tue, 14 Apr 2020 03:00:19 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0064.hostedemail.com [216.40.44.64]) by kanga.kvack.org (Postfix) with ESMTP id 6102B8E0001 for ; Tue, 14 Apr 2020 03:00:19 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 128E3181AEF09 for ; Tue, 14 Apr 2020 07:00:19 +0000 (UTC) X-FDA: 76705561758.18.rose43_5a011d79d5754 X-HE-Tag: rose43_5a011d79d5754 X-Filterd-Recvd-Size: 2308 Received: from verein.lst.de (verein.lst.de [213.95.11.211]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Tue, 14 Apr 2020 07:00:18 +0000 (UTC) Received: by verein.lst.de (Postfix, from userid 2407) id 024D068BEB; Tue, 14 Apr 2020 09:00:13 +0200 (CEST) Date: Tue, 14 Apr 2020 09:00:13 +0200 From: Christoph Hellwig To: Yan Zhao Cc: Christoph Hellwig , Jens Axboe , Felipe Balbi , "Michael S. Tsirkin" , Jason Wang , intel-gvt-dev@lists.freedesktop.org, Felix Kuehling , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, io-uring@vger.kernel.org, linux-mm@kvack.org, Zhenyu Wang , intel-gfx@lists.freedesktop.org, linux-fsdevel@vger.kernel.org, Alex Deucher , Andrew Morton , virtualization@lists.linux-foundation.org, Linus Torvalds , Zhi Wang , Al Viro Subject: Re: [PATCH 2/6] i915/gvt/kvm: a NULL ->mm does not mean a thread is a kthread Message-ID: <20200414070013.GA23680@lst.de> References: <20200404094101.672954-1-hch@lst.de> <20200404094101.672954-3-hch@lst.de> <20200407030845.GA10586@joy-OptiPlex-7040> <20200413132730.GB14455@lst.de> <20200414000410.GE10586@joy-OptiPlex-7040> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200414000410.GE10586@joy-OptiPlex-7040> User-Agent: Mutt/1.5.17 (2007-11-01) 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 Mon, Apr 13, 2020 at 08:04:10PM -0400, Yan Zhao wrote: > > I can't think of another way for a kernel thread to have a mm indeed. > for example, before calling to vfio_dma_rw(), a kernel thread has already > called use_mm(), then its current->mm is not null, and it has flag > PF_KTHREAD. > in this case, we just want to allow the copy_to_user() directly if > current->mm == mm, rather than call another use_mm() again. > > do you think it makes sense? I mean no other way than using use_mm. That being said nesting potentional use_mm callers sounds like a rather bad idea, and we should avoid that.