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 957F3C38145 for ; Sat, 3 Sep 2022 13:53:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230521AbiICNxT (ORCPT ); Sat, 3 Sep 2022 09:53:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50198 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229506AbiICNxQ (ORCPT ); Sat, 3 Sep 2022 09:53:16 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 125475B7BF; Sat, 3 Sep 2022 06:53:14 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id B7D31B80113; Sat, 3 Sep 2022 13:53:12 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 35A48C433D6; Sat, 3 Sep 2022 13:53:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1662213191; bh=lMwzQ02SEWG/34khEpyuiktbPBlHfdcpRGPVn5PeCnc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=SqPZnqs+b2jBHPlRwoOtrbs9x/hUs9LB+pPc6Wh4Mq7GHZZaGSrAPiHXWnO6t4vne 7iSthZoYW4hbxbsouXL4Q+IP/u56AjgkMHQKDY8uetVvdD3lxMtNwBxEvVzOTAMGPG rJD+B8svihtBQVC+tsaaJTpMdUsF4vLedF06lRw0GGYQX/5x1h0ZdpYJQUKuW3jKyc kFIfJhZYoRcwR/9YZGYdgHoEiipJlXXP41LHpo4T0Ir2fL8XMuM7xVRbqlYw4jd+aU EZjlfQHSdvAdj5LBqg2zoFOwm1rZ8ScSei4PC2Cr/KbNaJEMWZAPjyHJeG5lC1hkOe TtSpUGkaqGkXQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C9A575C0531; Sat, 3 Sep 2022 06:53:10 -0700 (PDT) Date: Sat, 3 Sep 2022 06:53:10 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Vlastimil Babka , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org, rostedt@goodmis.org, vineeth@bitbyteword.org, boqun.feng@gmail.com, Hyeonggon Yoo <42.hyeyoo@gmail.com> Subject: Re: [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head Message-ID: <20220903135310.GS6159@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220901221720.1105021-1-joel@joelfernandes.org> <20220901221720.1105021-3-joel@joelfernandes.org> <75ad3ed3-7dcd-474c-472c-ca32c1734281@suse.cz> <013ca416-72de-8059-a5d5-cd63c92ef119@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <013ca416-72de-8059-a5d5-cd63c92ef119@joelfernandes.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 02, 2022 at 11:09:08AM -0400, Joel Fernandes wrote: > > > On 9/2/2022 5:30 AM, Vlastimil Babka wrote: > > On 9/2/22 11:26, Vlastimil Babka wrote: > >> On 9/2/22 00:17, Joel Fernandes (Google) wrote: > >>> From: Vlastimil Babka > >>> > >>> Joel reports [1] that increasing the rcu_head size for debugging > >>> purposes used to work before struct slab was split from struct page, but > >>> now runs into the various SLAB_MATCH() sanity checks of the layout. > >>> > >>> This is because the rcu_head in struct page is in union with large > >>> sub-structures and has space to grow without exceeding their size, while > >>> in struct slab (for SLAB and SLUB) it's in union only with a list_head. > >>> > >>> On closer inspection (and after the previous patch) we can put all > >>> fields except slab_cache to a union with rcu_head, as slab_cache is > >>> sufficient for the rcu freeing callbacks to work and the rest can be > >>> overwritten by rcu_head without causing issues. > >>> > >>> This is only somewhat complicated by the need to keep SLUB's > >>> freelist+counters aligned for cmpxchg_double. As a result the fields > >>> need to be reordered so that slab_cache is first (after page flags) and > >>> the union with rcu_head follows. For consistency, do that for SLAB as > >>> well, although not necessary there. > >>> > >>> As a result, the rcu_head field in struct page and struct slab is no > >>> longer at the same offset, but that doesn't matter as there is no > >>> casting that would rely on that in the slab freeing callbacks, so we can > >>> just drop the respective SLAB_MATCH() check. > >>> > >>> Also we need to update the SLAB_MATCH() for compound_head to reflect the > >>> new ordering. > >>> > >>> While at it, also add a static_assert to check the alignment needed for > >>> cmpxchg_double so mistakes are found sooner than a runtime GPF. > >>> > >>> [1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@joelfernandes.org/ > >>> > >>> Reported-by: Joel Fernandes > >>> Signed-off-by: Vlastimil Babka > >> > >> I've added patches 01 and 02 to slab tree for -next exposure before Joel's > >> full series posting, but it should be also ok if rcu tree carries them with > >> the whole patchset. I can then drop them from slab tree (there are no > >> dependencies with other stuff there) so we don't introduce duplicite commits > >> needlessly, just give me a heads up. > > > > Ah but in that case please apply the reviews from my posting [1] > > > > patch 1: > > Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com> > > > > patch 2 > > Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com> > > > > [1] https://lore.kernel.org/all/20220826090912.11292-1-vbabka@suse.cz/ > > > Sorry for injecting confusion - my main intent with including the mm patches in > this series is to make it easier for other reviewers/testers to backport the > series to their kernels in one shot. Some reviewers expressed interested in > trying out the series. > > I think it is best to let the -mm patches in the series go through the slab > tree, as you also have the Acks/Reviews there and will take sure those > dependencies are out of the way. > > My lesson here is to be more clear, I could have added some notes for context > below the "---" of those mm patches. > > Thanks again for your help, Hello, Vlastimil, and thank you for putting these together! I believe that your two patches should go in via the slab tree. I am queueing them in -rcu only temporarily and just for convenience in testing. I expect that I will rebase them so that I can let your versions cover things in -next. Thanx, Paul