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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 0A647C433E0 for ; Mon, 1 Jun 2020 11:34:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DDFB820738 for ; Mon, 1 Jun 2020 11:34:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726968AbgFALe1 (ORCPT ); Mon, 1 Jun 2020 07:34:27 -0400 Received: from mx2.suse.de ([195.135.220.15]:56316 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725886AbgFALe0 (ORCPT ); Mon, 1 Jun 2020 07:34:26 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 54BD1AECB; Mon, 1 Jun 2020 11:34:26 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 1217B1E0948; Mon, 1 Jun 2020 13:34:24 +0200 (CEST) Date: Mon, 1 Jun 2020 13:34:24 +0200 From: Jan Kara To: John Hubbard Cc: Andrew Morton , "Michael S . Tsirkin" , Jason Wang , Vlastimil Babka , =?iso-8859-1?B?Suly9G1l?= Glisse , Jan Kara , Dave Chinner , Souptick Joarder , Jonathan Corbet , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, LKML , linux-mm@kvack.org Subject: Re: [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5" Message-ID: <20200601113424.GF3960@quack2.suse.cz> References: <20200601052633.853874-1-jhubbard@nvidia.com> <20200601052633.853874-2-jhubbard@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200601052633.853874-2-jhubbard@nvidia.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 31-05-20 22:26:32, John Hubbard wrote: > There are four cases listed in pin_user_pages.rst. These are > intended to help developers figure out whether to use > get_user_pages*(), or pin_user_pages*(). However, the four cases > do not cover all the situations. For example, drivers/vhost/vhost.c > has a "pin, write to page, set page dirty, unpin" case. > > Add a fifth case, to help explain that there is a general pattern > that requires pin_user_pages*() API calls. > > Cc: Vlastimil Babka > Cc: Jan Kara > Cc: Jérôme Glisse > Cc: Dave Chinner > Cc: Jonathan Corbet > Cc: linux-doc@vger.kernel.org > Cc: linux-fsdevel@vger.kernel.org > Signed-off-by: John Hubbard Looks good to me. You can add: Reviewed-by: Jan Kara Honza > --- > Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst > index 4675b04e8829..6068266dd303 100644 > --- a/Documentation/core-api/pin_user_pages.rst > +++ b/Documentation/core-api/pin_user_pages.rst > @@ -171,6 +171,24 @@ If only struct page data (as opposed to the actual memory contents that a page > is tracking) is affected, then normal GUP calls are sufficient, and neither flag > needs to be set. > > +CASE 5: Pinning in order to write to the data within the page > +------------------------------------------------------------- > +Even though neither DMA nor Direct IO is involved, just a simple case of "pin, > +write to a page's data, unpin" can cause a problem. Case 5 may be considered a > +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In > +other words, if the code is neither Case 1 nor Case 2, it may still require > +FOLL_PIN, for patterns like this: > + > +Correct (uses FOLL_PIN calls): > + pin_user_pages() > + write to the data within the pages > + unpin_user_pages() > + > +INCORRECT (uses FOLL_GET calls): > + get_user_pages() > + write to the data within the pages > + put_page() > + > page_maybe_dma_pinned(): the whole point of pinning > =================================================== > > -- > 2.26.2 > -- Jan Kara SUSE Labs, CR