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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 2095FC3A5A6 for ; Fri, 30 Aug 2019 17:09:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E9A4823407 for ; Fri, 30 Aug 2019 17:09:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567184949; bh=gMd5xlYG8lJ3Gus8qIlNByXHRVxU/bMgXRnvywdLzM0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Eq8DcT+fgEXW5CGww3pEF4MLHCzOP0nVKjHZwtRWeo/APLZhqWqnArRoUEHIA58ub B0evO6+XxShl7T036aW3ECEMVf2QK2OA9SiTniIEk1IMi2FO911AiUgn4bMhbKlLpU DTuodWgQpracV/QQsj/oA5yFrOWKfOI7ZekET0Zw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727940AbfH3RJI (ORCPT ); Fri, 30 Aug 2019 13:09:08 -0400 Received: from mail-qt1-f196.google.com ([209.85.160.196]:33670 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727434AbfH3RJH (ORCPT ); Fri, 30 Aug 2019 13:09:07 -0400 Received: by mail-qt1-f196.google.com with SMTP id r5so3106293qtd.0; Fri, 30 Aug 2019 10:09:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=zag/cCurJuI8nINltP1hM69M5M7TCZ8LCVJ1Mr2Rw9E=; b=RNe+Qb5zoZXx6K4SAJ2I3Zg7N1BzEmvVWiTNb0YF2zfqjTcYHkhY9qX1bi4d8i1FGk r5oLZLXJoqeIx4V+/2k8Uv0HumpF+RyOTfj0Obk43jaimI4UPKRoRV6sd0+isOli0V6o X/madbTdHMGYh6q4hdTGhhNmB7eXSHU51hbAXjzsLDtOemDAqfHt0oziuvXhYvxstwjB XZmdVp1cjPl3cE06OpN8T/DhyL6ZzMy/O6kX3eiJBeAjDnjWeLGrOTe4i3Pa/GZJ3ehq X4lvG6Sn8TJrrFHAFfCPHo5waWEh9mJYemvG+wBOheerm120wHxeVbQYCwcpl2ctQLcl XFOQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=zag/cCurJuI8nINltP1hM69M5M7TCZ8LCVJ1Mr2Rw9E=; b=R3c7vYX+xMsRuU5gHFsDCFpVQhxIFYEflBWv32WGAf/oV+zRCYPXpehT6+H/DcWjzm yFtQahp+n9GilQsEygo/zRQlG58ithiiKpBnhwX6w5xc5uD27IZn57pGaVT0JZEN3ASg dwMmKFiISeBPa3FIxvaea8HpInfsOol7B09CAc/rRpZ4NymwcEwmNj0YQYTS8lNxAX3J C7s7JcgVocy9dEUWUa7Q7G0Yfe8X3f9+y/Cxp/t1zsjFKpJkWlaPPAvbCfPjn1XhzQcg NQNITuaMxttKg2fponWnFMkNO6bo8ttUu2IaAAQA0Qg20RRax7C1A74NHewwr+ckFMZp sIDQ== X-Gm-Message-State: APjAAAW2WFYWZtcQTbe73JEaFgHBLKFeVpeKOfCA/Za4gLkq2LGZIQ2y Dt18+3NQrOGKU2n/AHzFneM= X-Google-Smtp-Source: APXvYqwchRBbgZEE4wNHzNdKzi1iP+TxhWNP/ayEstj1LZU/qPNuW9VAzfDU9gQJLA47VKDIagx+Lg== X-Received: by 2002:a0c:a0e6:: with SMTP id c93mr6195303qva.109.1567184946122; Fri, 30 Aug 2019 10:09:06 -0700 (PDT) Received: from localhost ([2620:10d:c091:480::677e]) by smtp.gmail.com with ESMTPSA id c14sm1789122qta.80.2019.08.30.10.09.05 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 30 Aug 2019 10:09:05 -0700 (PDT) Date: Fri, 30 Aug 2019 10:09:03 -0700 From: Tejun Heo To: Jan Kara Cc: 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: <20190830170903.GB2263813@devbig004.ftw2.facebook.com> References: <20190829224701.GX2263813@devbig004.ftw2.facebook.com> <20190830154023.GC25069@quack2.suse.cz> <20190830154921.GZ2263813@devbig004.ftw2.facebook.com> <20190830164211.GD25069@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190830164211.GD25069@quack2.suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Jan. 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. __set_page_dirty_nobuffers() is more brief but seems to be making the same assumption. 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. Thanks. -- tejun