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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9770DC433EF for ; Wed, 25 May 2022 09:53:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239187AbiEYJxp (ORCPT ); Wed, 25 May 2022 05:53:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35676 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240162AbiEYJxi (ORCPT ); Wed, 25 May 2022 05:53:38 -0400 Received: from mail-pj1-x102d.google.com (mail-pj1-x102d.google.com [IPv6:2607:f8b0:4864:20::102d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 10AD7338AC for ; Wed, 25 May 2022 02:53:36 -0700 (PDT) Received: by mail-pj1-x102d.google.com with SMTP id cv10so2574325pjb.4 for ; Wed, 25 May 2022 02:53:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=9rlBY3PNi6qAmOpJr9mzzdY/bjGa01SJNfhk40so+9E=; b=wB/9UutLwYmlC+hqMcMe5v7VpbPZIPOASKIhNz9TI/rIZ1iZqhiPzsV3k6XUuyFwVl wW4IxGffPtaW4/prC4C2aAqCJtfPlalwcq8Wv6vEefOE0/fBUVRKpEkfooGCniuHL7Pz dSwf18AUMYPjciMSpPEeI6qjH0O1BNyMXPddZuPwMzFuNxhX1pEXGhJK3jLVr/FSBi9f BXe5+QYLLLBMeSOAY9p5fmkKulrpayS+1qpMnGy2lQUMgQMQLCzwa/34Ucu8uhyBHr6m 92ICCkyEyw4C1v419exrTlpxo7R6iBrLAV6Jxrg9bEPC+S/khe/tSeTeAkXmrlrsIqqM lUYA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=9rlBY3PNi6qAmOpJr9mzzdY/bjGa01SJNfhk40so+9E=; b=iu/GVywARGF3X4b0iO2C1gnEb0k0Z40nuNbulVs7/AYqDO/prSSgWQATpYU55c9brn uOtyoQUPl/yE24bJrJGStS6nWpt2JYKrVtMB5y6jmu5bdssfrRQrD0PTurObIbfXNVbU xmekNY7y419k3Po57wMK8+BUyibvoDiusylmFiSCTeLTOhLTpheBAWzy6bKiDmw9Tcfn 5Uy0+GtbVNmsamKYw6TFw5/0na/IHTwYX6TsTeKXB1HhpIYdtbrZZYclRTQ2RstpbpWY RwCQUfsJoVESK9wYhvOMskCP9CbANS9+1P58/fUQJAPmNdGKqjYZiohsz76/hXL/ydWA OG1Q== X-Gm-Message-State: AOAM532EwmUTxn1B4TErK7jA1ZcNjWx9h7DtGe2aZCnbzOcb+Mq6kbu9 YAm5Gxsoj0awcXDRaC86S0diIw== X-Google-Smtp-Source: ABdhPJzMyMkX24hof16RTJ9atm/eD3WpDblvZiEcBgbvezw96LMmNP9gRlSJkV4qLO1RDOIR/ebBmg== X-Received: by 2002:a17:90b:1d10:b0:1e0:4bbb:3393 with SMTP id on16-20020a17090b1d1000b001e04bbb3393mr9406657pjb.218.1653472415573; Wed, 25 May 2022 02:53:35 -0700 (PDT) Received: from localhost ([2408:8207:18da:2310:c40f:7b5:4fa8:df3f]) by smtp.gmail.com with ESMTPSA id a12-20020aa780cc000000b005184640c939sm1474506pfn.207.2022.05.25.02.53.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 May 2022 02:53:34 -0700 (PDT) Date: Wed, 25 May 2022 17:53:30 +0800 From: Muchun Song To: Johannes Weiner Cc: mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, duanxiongchun@bytedance.com, longman@redhat.com Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented Message-ID: References: <20220524060551.80037-1-songmuchun@bytedance.com> <20220524060551.80037-4-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 24, 2022 at 03:27:20PM -0400, Johannes Weiner wrote: > On Tue, May 24, 2022 at 02:05:43PM +0800, Muchun Song wrote: > > The diagram below shows how to make the folio lruvec lock safe when LRU > > pages are reparented. > > > > folio_lruvec_lock(folio) > > retry: > > lruvec = folio_lruvec(folio); > > > > // The folio is reparented at this time. > > spin_lock(&lruvec->lru_lock); > > > > if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) > > // Acquired the wrong lruvec lock and need to retry. > > // Because this folio is on the parent memcg lruvec list. > > goto retry; > > > > // If we reach here, it means that folio_memcg(folio) is stable. > > > > memcg_reparent_objcgs(memcg) > > // lruvec belongs to memcg and lruvec_parent belongs to parent memcg. > > spin_lock(&lruvec->lru_lock); > > spin_lock(&lruvec_parent->lru_lock); > > > > // Move all the pages from the lruvec list to the parent lruvec list. > > > > spin_unlock(&lruvec_parent->lru_lock); > > spin_unlock(&lruvec->lru_lock); > > > > After we acquire the lruvec lock, we need to check whether the folio is > > reparented. If so, we need to reacquire the new lruvec lock. On the > > routine of the LRU pages reparenting, we will also acquire the lruvec > > lock (will be implemented in the later patch). So folio_memcg() cannot > > be changed when we hold the lruvec lock. > > > > Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after > > we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So > > remove it. > > > > This is a preparation for reparenting the LRU pages. > > > > Signed-off-by: Muchun Song > > This looks good to me. Just one question: > > > @@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio) > > */ > > struct lruvec *folio_lruvec_lock(struct folio *folio) > > { > > - struct lruvec *lruvec = folio_lruvec(folio); > > + struct lruvec *lruvec; > > > > + rcu_read_lock(); > > +retry: > > + lruvec = folio_lruvec(folio); > > spin_lock(&lruvec->lru_lock); > > - lruvec_memcg_debug(lruvec, folio); > > + > > + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { > > + spin_unlock(&lruvec->lru_lock); > > + goto retry; > > + } > > + > > + /* > > + * Preemption is disabled in the internal of spin_lock, which can serve > > + * as RCU read-side critical sections. > > + */ > > + rcu_read_unlock(); > > The code looks right to me, but I don't understand the comment: why do > we care that the rcu read-side continues? With the lru_lock held, > reparenting is on hold and the lruvec cannot be rcu-freed anyway, no? > Right. We could hold rcu read lock until end of reparting. So you mean we do rcu_read_unlock in folio_lruvec_lock()? Thanks. From mboxrd@z Thu Jan 1 00:00:00 1970 From: Muchun Song Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented Date: Wed, 25 May 2022 17:53:30 +0800 Message-ID: References: <20220524060551.80037-1-songmuchun@bytedance.com> <20220524060551.80037-4-songmuchun@bytedance.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=9rlBY3PNi6qAmOpJr9mzzdY/bjGa01SJNfhk40so+9E=; b=wB/9UutLwYmlC+hqMcMe5v7VpbPZIPOASKIhNz9TI/rIZ1iZqhiPzsV3k6XUuyFwVl wW4IxGffPtaW4/prC4C2aAqCJtfPlalwcq8Wv6vEefOE0/fBUVRKpEkfooGCniuHL7Pz dSwf18AUMYPjciMSpPEeI6qjH0O1BNyMXPddZuPwMzFuNxhX1pEXGhJK3jLVr/FSBi9f BXe5+QYLLLBMeSOAY9p5fmkKulrpayS+1qpMnGy2lQUMgQMQLCzwa/34Ucu8uhyBHr6m 92ICCkyEyw4C1v419exrTlpxo7R6iBrLAV6Jxrg9bEPC+S/khe/tSeTeAkXmrlrsIqqM lUYA== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Johannes Weiner Cc: mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org, shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, duanxiongchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org, longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org On Tue, May 24, 2022 at 03:27:20PM -0400, Johannes Weiner wrote: > On Tue, May 24, 2022 at 02:05:43PM +0800, Muchun Song wrote: > > The diagram below shows how to make the folio lruvec lock safe when LRU > > pages are reparented. > > > > folio_lruvec_lock(folio) > > retry: > > lruvec = folio_lruvec(folio); > > > > // The folio is reparented at this time. > > spin_lock(&lruvec->lru_lock); > > > > if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) > > // Acquired the wrong lruvec lock and need to retry. > > // Because this folio is on the parent memcg lruvec list. > > goto retry; > > > > // If we reach here, it means that folio_memcg(folio) is stable. > > > > memcg_reparent_objcgs(memcg) > > // lruvec belongs to memcg and lruvec_parent belongs to parent memcg. > > spin_lock(&lruvec->lru_lock); > > spin_lock(&lruvec_parent->lru_lock); > > > > // Move all the pages from the lruvec list to the parent lruvec list. > > > > spin_unlock(&lruvec_parent->lru_lock); > > spin_unlock(&lruvec->lru_lock); > > > > After we acquire the lruvec lock, we need to check whether the folio is > > reparented. If so, we need to reacquire the new lruvec lock. On the > > routine of the LRU pages reparenting, we will also acquire the lruvec > > lock (will be implemented in the later patch). So folio_memcg() cannot > > be changed when we hold the lruvec lock. > > > > Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after > > we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So > > remove it. > > > > This is a preparation for reparenting the LRU pages. > > > > Signed-off-by: Muchun Song > > This looks good to me. Just one question: > > > @@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio) > > */ > > struct lruvec *folio_lruvec_lock(struct folio *folio) > > { > > - struct lruvec *lruvec = folio_lruvec(folio); > > + struct lruvec *lruvec; > > > > + rcu_read_lock(); > > +retry: > > + lruvec = folio_lruvec(folio); > > spin_lock(&lruvec->lru_lock); > > - lruvec_memcg_debug(lruvec, folio); > > + > > + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { > > + spin_unlock(&lruvec->lru_lock); > > + goto retry; > > + } > > + > > + /* > > + * Preemption is disabled in the internal of spin_lock, which can serve > > + * as RCU read-side critical sections. > > + */ > > + rcu_read_unlock(); > > The code looks right to me, but I don't understand the comment: why do > we care that the rcu read-side continues? With the lru_lock held, > reparenting is on hold and the lruvec cannot be rcu-freed anyway, no? > Right. We could hold rcu read lock until end of reparting. So you mean we do rcu_read_unlock in folio_lruvec_lock()? Thanks.