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=-12.8 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 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 DD483C43461 for ; Wed, 31 Mar 2021 23:15:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B0AB66108C for ; Wed, 31 Mar 2021 23:15:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232406AbhCaXPL (ORCPT ); Wed, 31 Mar 2021 19:15:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46834 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232292AbhCaXOt (ORCPT ); Wed, 31 Mar 2021 19:14:49 -0400 Received: from mail-ed1-x533.google.com (mail-ed1-x533.google.com [IPv6:2a00:1450:4864:20::533]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 88379C061574 for ; Wed, 31 Mar 2021 16:14:48 -0700 (PDT) Received: by mail-ed1-x533.google.com with SMTP id bx7so24166529edb.12 for ; Wed, 31 Mar 2021 16:14:48 -0700 (PDT) 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=+h9iiR2ETy9snk4Rjw3/0FtbakmqcM90iNL/G5npVZ4=; b=oDsltAsVhMfX34nC0ya4Ses+z/aSYpIGarCGUrQBWp8KhNJg4VrgfO+34voug6kVkf 3CLBuPVFSwQMsklZgHWBzsmpGOZQJx4gZYwWEvBfuXSy4I9Yu8R56Bp59/GcBKkPmiR+ C+sGFkX59EQ3tDxb2Al8NdvNlqnpuwt7ZjSPsrJ7lVEUuyJEZXnCbPCZzXfOZhYMdfbt /q6h2r3OZhTZCjQW9S4/yW0dfNW0RO+Vjg8+FttpqsCA9Or0z0chfBbM6be1eNN4H+uW QzBVRrfpzVD/ECfIC9Cn+LBTHcIM9xiXcQyoUje+OOx+LrTjvfT71wjfn+cfwVcwdEm2 Uu6g== 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=+h9iiR2ETy9snk4Rjw3/0FtbakmqcM90iNL/G5npVZ4=; b=gKx+Zvj+VmxignM8T8npMex2uBNx/IPmw9qVZBvez/AsRnPRYaFLEQZxjjB3arDxwx jTEDSi40eGQKgXa9aQAN+aIoNtBefQs9Im8uJYdrLhrqurXCBO6P5O27bwyZsFuH4lT8 vGYoQ9bST30F+4QGvq48taXxfnTXH1hATvWm3Q6YVh5ihcdC2sekmJQBzighjNP8vVrY jSRGPypLg94n0g4Jh2zAq5XoVbT6rjonPzTFOHTu3QPEeIfsLSRLa9MkGwAeHr41Jm9f auOceq5ZE4IVGwM8UM4pNBTov1K+ye0yhwImx7cmfFk+Z1r9f7pY5FpyQbeBkvRFak08 8XFw== X-Gm-Message-State: AOAM533JNV/DG+0XI63JkiJB0fHq5o8MNFbaRndvugxnehQe3vWAgfK4 hv1eHCM9La7BOZJTvwgYqelOlHwJQWk4qNgVO9KHZq/g3vg= X-Google-Smtp-Source: ABdhPJyNAFcwBS9A1HYj+d5VqPFbHKIdG+b+86D+FZP03DVlhRYynUKUHoosRaSTO2mzV+WYKx7tQ344jJfVdJ6s/dc= X-Received: by 2002:a50:ef11:: with SMTP id m17mr6663973eds.151.1617232487296; Wed, 31 Mar 2021 16:14:47 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Yang Shi Date: Wed, 31 Mar 2021 16:14:35 -0700 Message-ID: Subject: Re: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info() To: Hugh Dickins Cc: Shakeel Butt , Andrew Morton , kernel test robot , Stephen Rothwell , Roman Gushchin , Kirill Tkhai , Vlastimil Babka , Dave Chinner , Johannes Weiner , Michal Hocko , LKML , Linux Memory Management List , lkp@lists.01.org, kernel test robot Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 31, 2021 at 2:13 PM Hugh Dickins wrote: > > On Wed, 31 Mar 2021, Yang Shi wrote: > > On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt wrote: > > > On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins wrote: > > > > > > > > Lockdep warns mm/vmscan.c: suspicious rcu_dereference_protected() usage! > > > > when free_shrinker_info() is called from mem_cgroup_css_free(): there it > > > > is called with no locking, whereas alloc_shrinker_info() calls it with > > > > down_write of shrinker_rwsem - which seems appropriate. Rearrange that > > > > so free_shrinker_info() can manage the shrinker_rwsem for itself. > > > > > > > > Link: https://lkml.kernel.org/r/20210317140615.GB28839@xsang-OptiPlex-9020 > > > > Reported-by: kernel test robot > > > > Signed-off-by: Hugh Dickins > > > > Cc: Yang Shi > > > > --- > > > > Sorry, I've made no attempt to work out precisely where in the series > > > > the locking went missing, nor tried to fit this in as a fix on top of > > > > mm-vmscan-add-shrinker_info_protected-helper.patch > > > > which Oliver reported (and which you notated in mmotm's "series" file). > > > > This patch just adds the fix to the end of the series, after > > > > mm-vmscan-shrink-deferred-objects-proportional-to-priority.patch > > > > > > The patch "mm: vmscan: add shrinker_info_protected() helper" replaces > > > rcu_dereference_protected(shrinker_info, true) with > > > rcu_dereference_protected(shrinker_info, > > > lockdep_is_held(&shrinker_rwsem)). > > > > > > I think we don't really need shrinker_rwsem in free_shrinker_info() > > > which is called from css_free(). The bits of the map have already been > > > 'reparented' in css_offline. I think we can remove > > > lockdep_is_held(&shrinker_rwsem) for free_shrinker_info(). > > > > Thanks, Hugh and Shakeel. I missed the report. > > > > I think Shakeel is correct, shrinker_rwsem is not required in css_free > > path so Shakeel's proposal should be able to fix it. > > Yes, looking at it again, I am sure that Shakeel is right, and > that my patch was overkill - no need for shrinker_rwsem there. > > Whether it's RCU-safe to free the info there, I have not reviewed at > all: but shrinker_rwsem would not help even if there were an issue. > > > I prepared a patch: > > Unsigned, white-space damaged, so does not apply. > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 64bf07cc20f2..7348c26d4cac 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -251,7 +251,12 @@ void free_shrinker_info(struct mem_cgroup *memcg) > > for_each_node(nid) { > > pn = memcg->nodeinfo[nid]; > > - info = shrinker_info_protected(memcg, nid); > > + /* > > + * Don't use shrinker_info_protected() helper since > > + * free_shrinker_info() could be called by css_free() > > + * without holding shrinker_rwsem. > > + */ > > Just because I mis-inferred from the use of shrinker_info_protected() > that shrinker_rwsem was needed here, is no reason to add that comment: > imagine how unhelpfully bigger the kernel source would be if we added > a comment everywhere I had misunderstood something! Yes, I agree the comment may incur more confusion. Better remove it. > > > + info = rcu_dereference_protected(pn->shrinker_info, true); > > kvfree(info); > > rcu_assign_pointer(pn->shrinker_info, NULL); > > } > > That does it, but I bikeshedded with myself in the encyclopaedic > rcupdate.h, and decided rcu_replace_pointer(pn->shrinker_info, NULL, true) > would be best. But now see that patch won't fit so well into your series, > and I can't spend more time writing up a justification for it. > > I think Andrew should simply delete my fix patch from his queue, > and edit out the > @@ -232,7 +239,7 @@ void free_shrinker_info(struct mem_cgrou > > for_each_node(nid) { > pn = memcg->nodeinfo[nid]; > - info = rcu_dereference_protected(pn->shrinker_info, true); > + info = shrinker_info_protected(memcg, nid); > kvfree(info); > rcu_assign_pointer(pn->shrinker_info, NULL); > } > hunk from your mm-vmscan-add-shrinker_info_protected-helper.patch > which will then restore free_shrinker_info() to what you propose above. Yes. I saw Andrew already had this fix in -mm tree. > > Thanks, > Hugh