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=-9.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable 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 44132C433E1 for ; Fri, 29 May 2020 23:43:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 028B820810 for ; Fri, 29 May 2020 23:43:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="aLSQsNOd" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 028B820810 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=nvidia.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id C5A4B80008; Fri, 29 May 2020 19:43:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A7898800B7; Fri, 29 May 2020 19:43:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8F164800B6; Fri, 29 May 2020 19:43:14 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0118.hostedemail.com [216.40.44.118]) by kanga.kvack.org (Postfix) with ESMTP id 6688C8001A for ; Fri, 29 May 2020 19:43:14 -0400 (EDT) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 20CBE1F0A for ; Fri, 29 May 2020 23:43:14 +0000 (UTC) X-FDA: 76871385108.30.heart23_737860bdd4a2c Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id 049AD180B3C83 for ; Fri, 29 May 2020 23:43:13 +0000 (UTC) X-HE-Tag: heart23_737860bdd4a2c X-Filterd-Recvd-Size: 5339 Received: from hqnvemgate24.nvidia.com (hqnvemgate24.nvidia.com [216.228.121.143]) by imf07.hostedemail.com (Postfix) with ESMTP for ; Fri, 29 May 2020 23:43:13 +0000 (UTC) Received: from hqpgpgate102.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate24.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Fri, 29 May 2020 16:41:39 -0700 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate102.nvidia.com (PGP Universal service); Fri, 29 May 2020 16:43:11 -0700 X-PGP-Universal: processed; by hqpgpgate102.nvidia.com on Fri, 29 May 2020 16:43:11 -0700 Received: from HQMAIL109.nvidia.com (172.20.187.15) by HQMAIL101.nvidia.com (172.20.187.10) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Fri, 29 May 2020 23:43:11 +0000 Received: from hqnvemgw03.nvidia.com (10.124.88.68) by HQMAIL109.nvidia.com (172.20.187.15) with Microsoft SMTP Server (TLS) id 15.0.1473.3 via Frontend Transport; Fri, 29 May 2020 23:43:11 +0000 Received: from sandstorm.nvidia.com (Not Verified[10.2.87.173]) by hqnvemgw03.nvidia.com with Trustwave SEG (v7,5,8,10121) id ; Fri, 29 May 2020 16:43:11 -0700 From: John Hubbard To: Andrew Morton CC: "Michael S . Tsirkin" , Jason Wang , Vlastimil Babka , =?UTF-8?q?J=C3=A9r=C3=B4me=20Glisse?= , Jan Kara , Dave Chinner , Souptick Joarder , Jonathan Corbet , , , , , , LKML , , John Hubbard Subject: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5" Date: Fri, 29 May 2020 16:43:08 -0700 Message-ID: <20200529234309.484480-2-jhubbard@nvidia.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200529234309.484480-1-jhubbard@nvidia.com> References: <20200529234309.484480-1-jhubbard@nvidia.com> MIME-Version: 1.0 X-NVConfidentiality: public Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1590795699; bh=/rPFi6UMeQqIwH0AXw3mk+tI0oUReiPuYz0u4gk+gB4=; h=X-PGP-Universal:From:To:CC:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:X-NVConfidentiality: Content-Type:Content-Transfer-Encoding; b=aLSQsNOdMWIF0LkIlbgwkwE7Tydi/7c5/ubijb8BcaFTIZEmhASKRET9Pca2eBhxG dREVJpcO2ADpj3UkVt0jUxkcnrRTCw4rXg9IKomdo4dQTH1CfYYB8ffwCmWCd9HN3A JIWv3bqa+Gg3EHmOTKcmCEuIvUL/VqJY25SULLKbpwSQ6b1XwGVe3sAYTL/Qwlabih x607bsQ0F1z2uQFX8AL5YzbsWkdWeUT/F0Jb8Ib1qqB0Pn0XVtAfJaEZQDl8GkPKcN TFCHCxL5GeGK/stqfrP64DTXAMPgO0H6TJzbyusWRjhSQfHOqkJ1IE/IVGf3B5XjlI 4QyQIJBypY8RA== X-Rspamd-Queue-Id: 049AD180B3C83 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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: 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: J=C3=A9r=C3=B4me Glisse Cc: Dave Chinner Cc: Jonathan Corbet Cc: linux-doc@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Signed-off-by: John Hubbard --- Documentation/core-api/pin_user_pages.rst | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core= -api/pin_user_pages.rst index 4675b04e8829..b9f2688a2c67 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -171,6 +171,26 @@ If only struct page data (as opposed to the actual mem= ory contents that a page is tracking) is affected, then normal GUP calls are sufficient, and neithe= r flag needs to be set. =20 +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, +access 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 requir= e +FOLL_PIN, for patterns like this: + +Correct (uses FOLL_PIN calls): + pin_user_pages() + access the data within the pages + set_page_dirty_lock() + unpin_user_pages() + +INCORRECT (uses FOLL_GET calls): + get_user_pages() + access the data within the pages + set_page_dirty_lock() + put_page() + page_maybe_dma_pinned(): the whole point of pinning =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D =20 --=20 2.26.2