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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 22028C3A5A6 for ; Fri, 30 Aug 2019 16:42:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EBB6B2087F for ; Fri, 30 Aug 2019 16:42:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728088AbfH3QmN (ORCPT ); Fri, 30 Aug 2019 12:42:13 -0400 Received: from mx2.suse.de ([195.135.220.15]:41120 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727976AbfH3QmM (ORCPT ); Fri, 30 Aug 2019 12:42:12 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A5864B696; Fri, 30 Aug 2019 16:42:11 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 6CA5F1E099F; Fri, 30 Aug 2019 18:42:11 +0200 (CEST) Date: Fri, 30 Aug 2019 18:42:11 +0200 From: Jan Kara To: Tejun Heo Cc: Jan Kara , Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH block/for-next] writeback: add tracepoints for cgroup foreign writebacks Message-ID: <20190830164211.GD25069@quack2.suse.cz> References: <20190829224701.GX2263813@devbig004.ftw2.facebook.com> <20190830154023.GC25069@quack2.suse.cz> <20190830154921.GZ2263813@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190830154921.GZ2263813@devbig004.ftw2.facebook.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 Fri 30-08-19 08:49:21, Tejun Heo wrote: > Hello, Jan. > > On Fri, Aug 30, 2019 at 05:40:23PM +0200, Jan Kara wrote: > > > + TP_fast_assign( > > > + strncpy(__entry->name, dev_name(wb->bdi->dev), 32); > > > + __entry->bdi_id = wb->bdi->id; > > > + __entry->ino = page->mapping->host->i_ino; > > > + __entry->memcg_id = wb->memcg_css->id; > > > + __entry->cgroup_ino = __trace_wb_assign_cgroup(wb); > > > + __entry->page_cgroup_ino = page->mem_cgroup->css.cgroup->kn->id.ino; > > > + ), > > > > Are the page dereferences above safe? I suppose lock_page_memcg() protects > > the page->mem_cgroup->css.cgroup->kn->id dereference? But page->mapping > > does not seem to be protected by page lock? > > Hah, I assumed it would work because there are preceding if > (page_mapping()) tests in the dirty paths - > e.g. __set_page_dirty_nobuffers(). Oh, regardless of that assumption, > I should have used page_mapping(). Well, but if you look at __set_page_dirty_nobuffers() it is careful. It does: struct address_space *mapping = page_mapping(page); if (!mapping) { bail } ... use mapping Exactly because page->mapping can become NULL under your hands if you don't hold page lock. So I think you either need something similar in your tracepoint or handle this in the caller. Honza -- Jan Kara SUSE Labs, CR