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=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,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 85BECC432C0 for ; Thu, 21 Nov 2019 22:06:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 54E8C206D8 for ; Thu, 21 Nov 2019 22:06:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=cmpxchg-org.20150623.gappssmtp.com header.i=@cmpxchg-org.20150623.gappssmtp.com header.b="crXpS16b" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726952AbfKUWGR (ORCPT ); Thu, 21 Nov 2019 17:06:17 -0500 Received: from mail-qt1-f196.google.com ([209.85.160.196]:44601 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726920AbfKUWGR (ORCPT ); Thu, 21 Nov 2019 17:06:17 -0500 Received: by mail-qt1-f196.google.com with SMTP id o11so5458162qtr.11 for ; Thu, 21 Nov 2019 14:06:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to :user-agent; bh=Bf3zXVc8JgyndN70CrFb4zsWDVt1DI2cOBZZ54QutII=; b=crXpS16brbj5wcwSGg8ZUNW+WnFbky+6ndEsBKBBwqewLUyZC2KtPR06bmxf6kVV0x +XADaSG7XVEs8kl8hGMFWn6z4G/S2iNrNg+CMACl4gcVvkcOBydeqNL2Ns+YzhGMxGgS PoyNNpLZLqeT5y44r7ouxVEZyHJ+9xp1ZwgERCu3+/9v7foYL77J2/4FdM7VxPoiZyJa MQOkloOGTfuv5odbW2dAfcsnjD9UseKW3xuJTuBllUg9O4YX5BV24NY2VWje8XU4rsVe mSOlipeQetbcpiGdNL2OdQpEQoycht9RyfCrvg+Blyffk8wGm/bGCfNnrb2BBpuB75nf +Ufg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to:user-agent; bh=Bf3zXVc8JgyndN70CrFb4zsWDVt1DI2cOBZZ54QutII=; b=DB0sH3SPNO6BB7Ay4pwLl0OE30PqxXDC4MxNQzKcoqZxrWp99DuuLL/Br/AVrmM1o+ RwdtE5oY2qZsaWVViL+OtAl2QT07YizO8vAnQwfR3rck2whhukaDburJQxSQQ0sVDyU6 eHDQpTYq699aOwcUQkVjCmXOovuJuKMwEqdh+4uUdVc07lfRZvJoGdRo4oLyemnkaCnN BGKKi/kA1VB9TgTPsQmq62Ug1wsVx9niEHIUETqktKnY722h0v4vODnte+crGeQXMJ6A lrJuN/XhjF/XJfmr2HUnjArnbsX6qZ+BCBvHW0xV4hTxdnp0vxtDFfUPY/lE2hagU51b D+uQ== X-Gm-Message-State: APjAAAXm4/Ylb322tt2s9npKJDTIGwx2QZlok1bKbY1uMFj46/ic8SNx oxV2YJb9zVQse5PEdoDabzJP+g== X-Google-Smtp-Source: APXvYqxMpQc7gkZFWT+4X1cGXGSebkY7gn/oAh6KeV78J6HsT1HgUuAZ7/0lsXcViU0bjQOBOqSvYQ== X-Received: by 2002:ac8:c4e:: with SMTP id l14mr1732376qti.87.1574373975657; Thu, 21 Nov 2019 14:06:15 -0800 (PST) Received: from localhost ([2620:10d:c091:500::1:dbed]) by smtp.gmail.com with ESMTPSA id j89sm2282706qte.72.2019.11.21.14.06.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 Nov 2019 14:06:14 -0800 (PST) Date: Thu, 21 Nov 2019 17:06:13 -0500 From: Johannes Weiner To: Alex Shi Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, mgorman@techsingularity.net, tj@kernel.org, hughd@google.com, khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com, yang.shi@linux.alibaba.com, willy@infradead.org, shakeelb@google.com, Michal Hocko , Vladimir Davydov , Roman Gushchin , Chris Down , Thomas Gleixner , Vlastimil Babka , Qian Cai , Andrey Ryabinin , "Kirill A. Shutemov" , =?iso-8859-1?B?Suly9G1l?= Glisse , Andrea Arcangeli , David Rientjes , "Aneesh Kumar K.V" , swkhack , "Potyra, Stefan" , Mike Rapoport , Stephen Rothwell , Colin Ian King , Jason Gunthorpe , Mauro Carvalho Chehab , Peng Fan , Nikolay Borisov , Ira Weiny , Kirill Tkhai , Yafang Shao Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock Message-ID: <20191121220613.GB487872@cmpxchg.org> References: <1574166203-151975-1-git-send-email-alex.shi@linux.alibaba.com> <1574166203-151975-4-git-send-email-alex.shi@linux.alibaba.com> <20191119160456.GD382712@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 20, 2019 at 07:41:44PM +0800, Alex Shi wrote: > 在 2019/11/20 上午12:04, Johannes Weiner 写道: > >> @@ -1246,6 +1245,46 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd > >> return lruvec; > >> } > >> > >> +struct lruvec *lock_page_lruvec_irq(struct page *page, > >> + struct pglist_data *pgdat) > >> +{ > >> + struct lruvec *lruvec; > >> + > >> +again: > >> + rcu_read_lock(); > >> + lruvec = mem_cgroup_page_lruvec(page, pgdat); > >> + spin_lock_irq(&lruvec->lru_lock); > >> + rcu_read_unlock(); > > The spinlock doesn't prevent the lruvec from being freed > > > > You deleted the rules from the mem_cgroup_page_lruvec() documentation, > > but they still apply: if the page is already !PageLRU() by the time > > you get here, it could get reclaimed or migrated to another cgroup, > > and that can free the memcg/lruvec. Merely having the lru_lock held > > does not prevent this. > > > Forgive my idiot, I still don't know the details of unsafe lruvec here. > From my shortsight, the spin_lock_irq(embedded a preempt_disable) could block all rcu syncing thus, keep all memcg alive until the preempt_enabled in unspinlock, is this right? > If so even the page->mem_cgroup is migrated to others cgroups, the new and old cgroup should still be alive here. You are right about the freeing part, I missed this. And I should have read this email here before sending out my "fix" to the current code; thankfully Hugh re-iterated my mistake on that thread. My apologies. But I still don't understand how the moving part is safe. You look up the lruvec optimistically, lock it, then verify the lookup. What keeps page->mem_cgroup from changing after you verified it? lock_page_lruvec(): mem_cgroup_move_account(): again: rcu_read_lock() lruvec = page->mem_cgroup->lruvec isolate_lru_page() spin_lock_irq(&lruvec->lru_lock) rcu_read_unlock() if page->mem_cgroup->lruvec != lruvec: spin_unlock_irq(&lruvec->lru_lock) goto again; page->mem_cgroup = new cgroup putback_lru_page() // new lruvec SetPageLRU() return lruvec; // old lruvec The caller assumes page belongs to the returned lruvec and will then change the page's lru state with a mismatched page and lruvec. If we could restrict lock_page_lruvec() to working only on PageLRU pages, we could fix the problem with memory barriers. But this won't work for split_huge_page(), which is AFAICT the only user that needs to freeze the lru state of a page that could be isolated elsewhere. So AFAICS the only option is to lock out mem_cgroup_move_account() entirely when the lru_lock is held. Which I guess should be fine.