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 110E8C433EF for ; Wed, 25 May 2022 12:30:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232599AbiEYMaY (ORCPT ); Wed, 25 May 2022 08:30:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53308 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229590AbiEYMaU (ORCPT ); Wed, 25 May 2022 08:30:20 -0400 Received: from mail-qt1-x82b.google.com (mail-qt1-x82b.google.com [IPv6:2607:f8b0:4864:20::82b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 501C76D3B7 for ; Wed, 25 May 2022 05:30:18 -0700 (PDT) Received: by mail-qt1-x82b.google.com with SMTP id hh4so16757808qtb.10 for ; Wed, 25 May 2022 05:30:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=E4cl00r8fZFLtwJTErC/nGIuII++sdl50FiWVn7DYDg=; b=LQ44JAnhD9qNj4RiG4BM2QieawWgPoEVj/ws0ERKGtEhPFWMXrtNEm98uQ7TUyheVo rk/xoF8YnmB9J1kXL50vICactGTOfnUzWkgZ5Rjf6EfYzCl2W+IYJlJp33UWMk+WGB7T Dcl16HXheBKsBpvtLqQ+yx8Za8a3WR6Kn3Nju7kW8r9mqfz11Aniqi/lRVfU6DRwCwgQ 2yFPkQFNmwRVRHp7KaE3L9wsBmOW5Z2zeZWdhm6o1lQyde6g8wdx2SkgDj9sUgKdWe5A D/gdxNoQMWC3mFXaHsiFzQeLLgD9lgIkx1FigDCGECAU0PkhOej/l8N6FPGcdmbMDvWl ouVQ== 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=E4cl00r8fZFLtwJTErC/nGIuII++sdl50FiWVn7DYDg=; b=CVJD17ixaHVDhlrCFFprZ+oLotlXJ2/YAQIjgnzW9ZDw+i2so0mF0c0sOiPKg4WnIU VXuSmCK9ynK5hxm/LEqCUeHOyChY/BYWDAXTQKg1cKVDx5CSTkSl/GO72vqMtEu0/hXo +UpvnUsDQTQviaO2bhEu7WFfJY+EDxnNll1ir2XYuPdG2gpcAIYwhSHOAhVTjkma/Y4r 4eqcIKzhNs1vf2ZS7HIoUwsJWaGrfzqyrOzmlkXZ7jHc92weaCxMAQHq5mPYuJtlsSkT eAvJT3YaKYf0nuQEXAufi6h1sK9iWn/+v8nLgQSVqSIgPOnUBX6Wkckbn02lVdWEZ40W PeXg== X-Gm-Message-State: AOAM531Q+hLJXPP2xVPlMGQoFzQu0bo4cgbs5qkTeAwViSTDlA3TsRzM TLND4a0/gVrHdjwFu6AjSvL6Xg== X-Google-Smtp-Source: ABdhPJyytRziWVwawprnzpoB9zMapge3xdxp4ugjwWUqfzlZvOH+tTzED4y/XZJINd6rGGx4ow4q7w== X-Received: by 2002:ac8:5e4c:0:b0:2f3:c50e:53a6 with SMTP id i12-20020ac85e4c000000b002f3c50e53a6mr24090727qtx.499.1653481817698; Wed, 25 May 2022 05:30:17 -0700 (PDT) Received: from localhost ([2620:10d:c091:480::1:741f]) by smtp.gmail.com with ESMTPSA id o16-20020ac85550000000b002fb673aa50asm330344qtr.89.2022.05.25.05.30.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 May 2022 05:30:17 -0700 (PDT) Date: Wed, 25 May 2022 08:30:15 -0400 From: Johannes Weiner To: Muchun Song 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 Wed, May 25, 2022 at 05:53:30PM +0800, Muchun Song wrote: > 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()? The comment seems to suggest that disabling preemption is what keeps the lruvec alive. But it's the lru_lock that keeps it alive. The cgroup destruction path tries to take the lru_lock long before it even gets to synchronize_rcu(). Once you hold the lru_lock, having an implied read-side critical section as well doesn't seem to matter. Should the comment be deleted? From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented Date: Wed, 25 May 2022 08:30:15 -0400 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=cmpxchg-org.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=E4cl00r8fZFLtwJTErC/nGIuII++sdl50FiWVn7DYDg=; b=LQ44JAnhD9qNj4RiG4BM2QieawWgPoEVj/ws0ERKGtEhPFWMXrtNEm98uQ7TUyheVo rk/xoF8YnmB9J1kXL50vICactGTOfnUzWkgZ5Rjf6EfYzCl2W+IYJlJp33UWMk+WGB7T Dcl16HXheBKsBpvtLqQ+yx8Za8a3WR6Kn3Nju7kW8r9mqfz11Aniqi/lRVfU6DRwCwgQ 2yFPkQFNmwRVRHp7KaE3L9wsBmOW5Z2zeZWdhm6o1lQyde6g8wdx2SkgDj9sUgKdWe5A D/gdxNoQMWC3mFXaHsiFzQeLLgD9lgIkx1FigDCGECAU0PkhOej/l8N6FPGcdmbMDvWl ouVQ== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Muchun Song 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 Wed, May 25, 2022 at 05:53:30PM +0800, Muchun Song wrote: > 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()? The comment seems to suggest that disabling preemption is what keeps the lruvec alive. But it's the lru_lock that keeps it alive. The cgroup destruction path tries to take the lru_lock long before it even gets to synchronize_rcu(). Once you hold the lru_lock, having an implied read-side critical section as well doesn't seem to matter. Should the comment be deleted?