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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 95B3FC433E0 for ; Thu, 11 Mar 2021 16:26:59 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 162E164F9A for ; Thu, 11 Mar 2021 16:26:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 162E164F9A Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 738378D02CB; Thu, 11 Mar 2021 11:26:58 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6E7588D02B2; Thu, 11 Mar 2021 11:26:58 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5893A8D02CB; Thu, 11 Mar 2021 11:26:58 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0030.hostedemail.com [216.40.44.30]) by kanga.kvack.org (Postfix) with ESMTP id 3E9BC8D02B2 for ; Thu, 11 Mar 2021 11:26:58 -0500 (EST) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id F1EB92495 for ; Thu, 11 Mar 2021 16:26:57 +0000 (UTC) X-FDA: 77908122474.08.F5C86A6 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf10.hostedemail.com (Postfix) with ESMTP id 7A50E4080F54 for ; Thu, 11 Mar 2021 16:26:52 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1615480014; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hZ0PtrodtSFeTEipYW7mvjHwoefyVlEAYBMe9XX6QM8=; b=aekJaOlAAG6CNJAyUaRWxlaKLeGTHlmTXhd6YKWemSgqKFo9loFGMoNnyRJ2sXwW2CsB0H Rhwu+OHtuIDJ8ND1vv5lvSZVjz3CRXC9LeibGRWLKlgNGdilqgfNjs6cL7ritguwRhI+7E 8d2GnruXAWGp0zswaFW9yi/ZmBCF86I= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id BCB63AE1F; Thu, 11 Mar 2021 16:26:54 +0000 (UTC) Date: Thu, 11 Mar 2021 17:26:53 +0100 From: Michal Hocko To: Johannes Weiner Cc: Matthew Wilcox , Zhou Guanghui , linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, hughd@google.com, kirill.shutemov@linux.intel.com, npiggin@gmail.com, ziy@nvidia.com, wangkefeng.wang@huawei.com, guohanjun@huawei.com, dingtianhong@huawei.com, chenweilong@huawei.com, rui.xiang@huawei.com Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page Message-ID: References: <20210304074053.65527-1-zhouguanghui1@huawei.com> <20210304074053.65527-3-zhouguanghui1@huawei.com> <20210308210225.GF3479805@casper.infradead.org> <20210309123255.GI3479805@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: igjuu9zgksetncpxjbcgkf9prjaoru9o X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 7A50E4080F54 Received-SPF: none (suse.com>: No applicable sender policy available) receiver=imf10; identity=mailfrom; envelope-from=""; helo=mx2.suse.de; client-ip=195.135.220.15 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1615480012-594887 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: On Thu 11-03-21 10:21:39, Johannes Weiner wrote: > On Thu, Mar 11, 2021 at 09:37:02AM +0100, Michal Hocko wrote: > > Johannes, Hugh, > > > > what do you think about this approach? If we want to stick with > > split_page approach then we need to update the missing place Matthew has > > pointed out. > > I find the __free_pages() code quite tricky as well. But for that > reason I would actually prefer to initiate the splitting in there, > since that's the place where we actually split the page, rather than > spread the handling of this situation further out. > > The race condition shouldn't be hot, so I don't think we need to be as > efficient about setting page->memcg_data only on the higher-order > buddies as in Willy's scratch patch. We can call split_page_memcg(), > which IMO should actually help document what's happening to the page. > > I think that function could also benefit a bit more from step-by-step > documentation about what's going on. The kerneldoc is helpful, but I > don't think it does justice to how tricky this race condition is. > > Something like this? > > void __free_pages(struct page *page, unsigned int order) > { > /* > * Drop the base reference from __alloc_pages and free. In > * case there is an outstanding speculative reference, from > * e.g. the page cache, it will put and free the page later. > */ > if (likely(put_page_testzero(page))) { > free_the_page(page, order); > return; > } > > /* > * The speculative reference will put and free the page. > * > * However, if the speculation was into a higher-order page > * that isn't marked compound, the other side will know > * nothing about our buddy pages and only free the order-0 > * page at the start of our chunk! We must split off and free > * the buddy pages here. > * > * The buddy pages aren't individually refcounted, so they > * can't have any pending speculative references themselves. > */ > if (!PageHead(page) && order > 0) { > split_page_memcg(page, 1 << order); > while (order-- > 0) > free_the_page(page + (1 << order), order); > } > } Fine with me. Mathew was concerned about more places that do something similar but I would say that if we find out more places we might reconsider and currently stay with a reasonably clear model that it is only head patch that carries the memcg information and split_page_memcg is necessary to break such page into smaller pieces. -- Michal Hocko SUSE Labs