From patchwork Fri Jul 22 16:29:34 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Luis Chamberlain X-Patchwork-Id: 12926594 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F07CBCCA48C for ; Fri, 22 Jul 2022 16:29:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235516AbiGVQ35 (ORCPT ); Fri, 22 Jul 2022 12:29:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57698 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229850AbiGVQ3y (ORCPT ); Fri, 22 Jul 2022 12:29:54 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:3::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D8DE691CD5; Fri, 22 Jul 2022 09:29:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Sender:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description; bh=dfsvCtbqj1aTabifZLD8rO8Jn7E44Br7CNa+F9C5WOo=; b=Eh0R/EMH/rCoU82UJQhON4r0qZ qqmUefp4at7/z2HepFQw3gDMVCE719zc+a/allPpuCx/8Mi5FyLndHi8qs+tkuUJMog80gIvoc/a6 TKzO7Qf23pJPkOncSRAcwZfP6QOs1W28Vr68g8EmOc1RgQO0xuPY1XFE9MNhgCKU57JSzDRyEs9Q1 +65qpuaaWRCVcjTxI4rU/nUdY8ESPq6vi8fgrkw6liUVeh7UTJA9qbYr2bQ6DrpiHoAGh5toS5k3M 5HWKNJOoIo+81NOoC0QqXdZ3RR45Bn+MA1JORBKDGG2e7r9qUVvMx8RzV30FP3wk7Yr9MIN4f3w2j eNfN8liQ==; Received: from mcgrof by bombadil.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1oEvX1-007vOh-CK; Fri, 22 Jul 2022 16:29:39 +0000 From: Luis Chamberlain To: ebiederm@xmission.com, corbet@lwn.net, keescook@chromium.org, yzaikin@google.com Cc: songmuchun@bytedance.com, zhangyuchen.lcr@bytedance.com, dhowells@redhat.com, deepa.kernel@gmail.com, hch@lst.de, mcgrof@kernel.org, linux-doc@vger.kernel.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 2/2] Documentation/filesystems/proc.rst: document procfs inode timestamps Date: Fri, 22 Jul 2022 09:29:34 -0700 Message-Id: <20220722162934.1888835-3-mcgrof@kernel.org> X-Mailer: git-send-email 2.35.3 In-Reply-To: <20220722162934.1888835-1-mcgrof@kernel.org> References: <20220722162934.1888835-1-mcgrof@kernel.org> MIME-Version: 1.0 Sender: Luis Chamberlain Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org The timestamps for procfs files are not well understood and can confuse users and developers [0] in particular for the timestamp for the start time or a process. Clarify what they mean and that they are a reflection of the ephemeral nature of the filesystem inodes. The procfs inodes are created when you first read them and then stuffed in the page cache. If the page cache and indodes are reclaimed they can be removed, and re-created with a new timestamp after read again. Document this little bit of tribal knowledge. [0] https://lkml.kernel.org/r/20220721081617.36103-1-zhangyuchen.lcr@bytedance.com Reported-by: Zhang Yuchen Signed-off-by: Luis Chamberlain --- Documentation/filesystems/proc.rst | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst index 9fd5249f1a5f..9defe9af683a 100644 --- a/Documentation/filesystems/proc.rst +++ b/Documentation/filesystems/proc.rst @@ -59,6 +59,15 @@ The proc file system acts as an interface to internal data structures in the kernel. It can be used to obtain information about the system and to change certain kernel parameters at runtime (sysctl). +The proc files are dynamic in nature and allow for developers to make the +content to be changed each time a file is read. The proc files and directories +inodes are created when someone first reads a respective proc file or directory, +as such the timestamps of the proc files reflect this time. As with other +filesystems, these proc inodes can be removed through reclaim under memory +pressure and so the timestamps of the proc files can change if the proc files +are destroyed and re-created (echo 3 > /proc/sys/vm/drop_caches forces and +illustrate the reclaim of inodes and page cache). + First, we'll take a look at the read-only parts of /proc. In Chapter 2, we show you how you can use /proc/sys to change settings. @@ -328,6 +337,13 @@ It's slow but very precise. system call ============= =============================================================== +Note that the start_time inside the stat file is different than the timestamp +of the stat file itself. The timestamp of the stat file simply reflects the +first time the stat file was read. The proc inode for this file can be reclaimed +under memory pressure and be recreated after this and so the timestamp can +change. Userspace should rely on the start_time entry in the the stat file to +get a process start time. + The /proc/PID/maps file contains the currently mapped memory regions and their access permissions.