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 758F2C41514 for ; Mon, 2 Sep 2019 08:38:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5720321881 for ; Mon, 2 Sep 2019 08:38:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730559AbfIBIiR (ORCPT ); Mon, 2 Sep 2019 04:38:17 -0400 Received: from mx2.suse.de ([195.135.220.15]:34852 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729408AbfIBIiR (ORCPT ); Mon, 2 Sep 2019 04:38:17 -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 CD795B7A8; Mon, 2 Sep 2019 08:38:14 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 011111E406C; Mon, 2 Sep 2019 10:38:12 +0200 (CEST) Date: Mon, 2 Sep 2019 10:38:12 +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: <20190902083812.GA14207@quack2.suse.cz> References: <20190829224701.GX2263813@devbig004.ftw2.facebook.com> <20190830154023.GC25069@quack2.suse.cz> <20190830154921.GZ2263813@devbig004.ftw2.facebook.com> <20190830164211.GD25069@quack2.suse.cz> <20190830170903.GB2263813@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190830170903.GB2263813@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 Hello Tejun, On Fri 30-08-19 10:09:03, Tejun Heo wrote: > On Fri, Aug 30, 2019 at 06:42:11PM +0200, Jan Kara wrote: > > 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. > > So, account_page_dirtied() is called from two places. > > __set_page_dirty() and __set_page_dirty_nobuffers(). The following is > from the latter. > > lock_page_memcg(page); > if (!TestSetPageDirty(page)) { > struct address_space *mapping = page_mapping(page); > ... > > if (!mapping) { > unlock_page_memcg(page); > return 1; > } > > xa_lock_irqsave(&mapping->i_pages, flags); > BUG_ON(page_mapping(page) != mapping); > WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page)); > account_page_dirtied(page, mapping); > ... > > If I'm reading it right, it's saying that at this point if mapping > exists after setting page dirty, it must not change while locking > i_pages. Correct __set_page_dirty_nobuffers() is supposed to be called serialized with truncation either through page lock or other means. At least the comment says so and the code looks like that. > > __set_page_dirty_nobuffers() is more brief but seems to be making the > same assumption. I suppose you mean __set_page_dirty() here. > xa_lock_irqsave(&mapping->i_pages, flags); > if (page->mapping) { /* Race with truncate? */ > WARN_ON_ONCE(warn && !PageUptodate(page)); > account_page_dirtied(page, mapping); > __xa_set_mark(&mapping->i_pages, page_index(page), > PAGECACHE_TAG_DIRTY); > } > xa_unlock_irqrestore(&mapping->i_pages, flags); > > Both are clearly assuming that once i_pages is locked, mapping can't > change. So, inside account_page_dirtied(), mapping clearly can't > change. The TP in question - track_foreign_dirty - is invoked from > mem_cgroup_track_foreign_dirty() which is only called from > account_page_dirty(), so I'm failing to see how mapping would change > there. I'm not sure where we depend here on page->mapping not getting cleared. The point is even if page->mapping is getting cleared while we work on the page, we have 'mapping' stored locally so we just account everything against the original mapping. I've researched this a bit more and commit 2d6d7f982846 "mm: protect set_page_dirty() from ongoing truncation" introduced the idea that __set_page_dirty_nobuffers() should be only called synchronized with truncation. Now I know for a fact that this is not always the case (e.g. various RDMA drivers calling set_page_dirty() without a lock or any other protection against truncate) but let's consider this a bug in the caller of set_page_dirty(). So in the end I agree that you're fine with relying on page_mapping() not changing under you. Honza -- Jan Kara SUSE Labs, CR