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=-12.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 2F5CFC433E0 for ; Wed, 10 Feb 2021 01:39:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EB4FF64E05 for ; Wed, 10 Feb 2021 01:39:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234686AbhBJBiz (ORCPT ); Tue, 9 Feb 2021 20:38:55 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45168 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233463AbhBIX3X (ORCPT ); Tue, 9 Feb 2021 18:29:23 -0500 Received: from mail-ej1-x632.google.com (mail-ej1-x632.google.com [IPv6:2a00:1450:4864:20::632]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 223A6C061786; Tue, 9 Feb 2021 15:28:43 -0800 (PST) Received: by mail-ej1-x632.google.com with SMTP id p20so502221ejb.6; Tue, 09 Feb 2021 15:28:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=+LOfUnbs2rjrDJJHm6hAN6K5xB7iMF06eg0OTBY+XJI=; b=hKF40R8ujeaEK9QaHJwh3Q7/gcPxTTRo5CzSQA00Xy0IB4Rlp5wrTBlxulJ/nZNuWl ig5yRREg4tHynFKLjKxxMFXAld1Ak51jizmo9hBLZRKrv5alMEw66nsxcVKS9SY+DOd3 E3nquhWnhwlXdJI4ryoHT93MXr+M84K31KgQSaJCUwhZoXzgnnxmosqCyWkCx8QM5p1o pZqQKFYs2wtVqk+laCxEy+DaymqTmm4syXbjCvN2qtuzcLfNf8jTC9DMT8vJtWYs7D6q Zvyl4ETPdD+01p4h7VOkIHpkXs+9U4vl15aC/JcR+FGplyeTK3Q70Z5xsouHfreVcpj5 MX9Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=+LOfUnbs2rjrDJJHm6hAN6K5xB7iMF06eg0OTBY+XJI=; b=mPSTONrOgZ7Bos4AYJIj8OyeyioR6W/vA0OiwORWX8GjEM9skur/lkM96YAaRCH4MZ frPEZnFGSScicdErZBgvY5BwHXxYzPaP1JwGsR+Ko04cjLpa3JjuLa+56HEx+bxNpqQg +lsR955hlb+szFHAuNgktUjRpa4DAx42h0wRILJr2gzLo66aibWQepS3k7m0+R2z5Pj9 hErtcVgyQSE5GZBExwq27NWJrA95PIF5+EXIBRI4dtGmLCD3Gxh2I/fXmrPnkHGz8k0v 9af1pgxX845/epUCS+casLui4xe1lse6KF3wjChMDSWhABvou+1Cl7UldL2+PBw/9unz AmAA== X-Gm-Message-State: AOAM530ejf0d1k3AIqzGj3pna+NcWzinXIWYKYFn4AQi9xzaYRWWZOWa bHiXzo9XxMZ4nKx91WjPB9jqgqr7wLZdZ703xa8= X-Google-Smtp-Source: ABdhPJy1/Q8eKamyH3M828qG9CNSWbfUR6JpVHCh723gGoVDjpQDzXNQgifkrKdm9F4jXpJcVEXg8oUMrI1xW3q1/gQ= X-Received: by 2002:a17:906:2e85:: with SMTP id o5mr127772eji.238.1612913321899; Tue, 09 Feb 2021 15:28:41 -0800 (PST) MIME-Version: 1.0 References: <20210209174646.1310591-1-shy828301@gmail.com> <20210209174646.1310591-4-shy828301@gmail.com> <20210209203307.GF524633@carbon.DHCP.thefacebook.com> In-Reply-To: <20210209203307.GF524633@carbon.DHCP.thefacebook.com> From: Yang Shi Date: Tue, 9 Feb 2021 15:28:30 -0800 Message-ID: Subject: Re: [v7 PATCH 03/12] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation To: Roman Gushchin Cc: Kirill Tkhai , Vlastimil Babka , Shakeel Butt , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 9, 2021 at 12:33 PM Roman Gushchin wrote: > > On Tue, Feb 09, 2021 at 09:46:37AM -0800, Yang Shi wrote: > > Since memcg_shrinker_map_size just can be changed under holding shrinker_rwsem > > exclusively, the read side can be protected by holding read lock, so it sounds > > superfluous to have a dedicated mutex. > > > > Kirill Tkhai suggested use write lock since: > > > > * We want the assignment to shrinker_maps is visible for shrink_slab_memcg(). > > * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but > > in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing > > is not actually protected. > > * READ lock makes alloc_shrinker_info() racy against memory allocation fail. > > alloc_shrinker_info()->free_shrinker_info() may free memory right after > > shrink_slab_memcg() dereferenced it. You may say > > shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure, > > but this is not the thing we want to remember in the future, since this > > spreads modularity. > > > > And a test with heavy paging workload didn't show write lock makes things worse. > > > > Acked-by: Vlastimil Babka > > Acked-by: Kirill Tkhai > > Signed-off-by: Yang Shi > > Acked-by: Roman Gushchin > > with a small nit (below): > > > --- > > mm/vmscan.c | 16 ++++++---------- > > 1 file changed, 6 insertions(+), 10 deletions(-) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 96b08c79f18d..e4ddaaaeffe2 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem); > > #ifdef CONFIG_MEMCG > > > > static int memcg_shrinker_map_size; > > -static DEFINE_MUTEX(memcg_shrinker_map_mutex); > > > > static void free_shrinker_map_rcu(struct rcu_head *head) > > { > > @@ -200,8 +199,6 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg, > > struct memcg_shrinker_map *new, *old; > > int nid; > > > > - lockdep_assert_held(&memcg_shrinker_map_mutex); > > - > > Why not check that shrinker_rwsem is down here? No special reason, just because we know it was acquired before. We could add the check, but not here. I think it'd be better to have the assert in expand_shrinker_maps() since the rwsem was acquired before calling it. > > > for_each_node(nid) { > > old = rcu_dereference_protected( > > mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true); > > @@ -249,7 +246,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) > > if (mem_cgroup_is_root(memcg)) > > return 0; > > > > - mutex_lock(&memcg_shrinker_map_mutex); > > + down_write(&shrinker_rwsem); > > size = memcg_shrinker_map_size; > > for_each_node(nid) { > > map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid); > > @@ -260,7 +257,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) > > } > > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map); > > } > > - mutex_unlock(&memcg_shrinker_map_mutex); > > + up_write(&shrinker_rwsem); > > > > return ret; > > } > > @@ -275,9 +272,8 @@ static int expand_shrinker_maps(int new_id) > > if (size <= old_size) > > return 0; > > > > - mutex_lock(&memcg_shrinker_map_mutex); > > And here as well. It will make the locking model more obvious and will help > to avoid errors in the future. > > > if (!root_mem_cgroup) > > - goto unlock; > > + goto out; > > > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > > do { > > @@ -286,13 +282,13 @@ static int int new_id) > > ret = expand_one_shrinker_map(memcg, size, old_size); > > if (ret) { > > mem_cgroup_iter_break(NULL, memcg); > > - goto unlock; > > + goto out; > > } > > } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); > > -unlock: > > +out: > > if (!ret) > > memcg_shrinker_map_size = size; > > - mutex_unlock(&memcg_shrinker_map_mutex); > > + > > return ret; > > } > > > > -- > > 2.26.2 > >