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=-9.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 DEFD3C4727E for ; Fri, 25 Sep 2020 18:09:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 86D8A235FD for ; Fri, 25 Sep 2020 18:09:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=cmpxchg-org.20150623.gappssmtp.com header.i=@cmpxchg-org.20150623.gappssmtp.com header.b="YwqbC9lP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729707AbgIYSJd (ORCPT ); Fri, 25 Sep 2020 14:09:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59836 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729354AbgIYSJd (ORCPT ); Fri, 25 Sep 2020 14:09:33 -0400 Received: from mail-qk1-x743.google.com (mail-qk1-x743.google.com [IPv6:2607:f8b0:4864:20::743]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A4D24C0613CE for ; Fri, 25 Sep 2020 11:09:32 -0700 (PDT) Received: by mail-qk1-x743.google.com with SMTP id q5so3726508qkc.2 for ; Fri, 25 Sep 2020 11:09:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=Wa9gWLx8z4A2Sdz4YX5ym9IbFeWjmAsR60a/YwhrF7g=; b=YwqbC9lPy36VZWkKl+K8aVGMJIBi1EI+wnp/laEjsdhzwamlwgXBtcYaBq207t/YkN O1kpjl5Umv897XlWSCzI8KDyqETPiyB3/woECIyysL2MYsrgihufs0yPtVaP34zwhzb9 hBHmLwXV3E2Q0EXbmz64ep+2P70dhdRVZdiHvc96Mr//knUp+EyHal2p0ov6Qng7hTBu 3+aGOtizgFi05EPrYm+CHzFyCWm/G1JPojWihtVPw1Rbbt3p34pVQy/DxxBVlHpV+jYJ ncI3ORDBdDt870ca85FaNOZsGZpSkITH0GlcjaGUdW+dD09pcJxqeVse6ddpBKut4QFc aXuA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Wa9gWLx8z4A2Sdz4YX5ym9IbFeWjmAsR60a/YwhrF7g=; b=oMZGnSFpXnK0bPp335Z8K7LYdpzHiz+2KbvDy7oozYqA0S/ojsrLmFvFbg6/18FOpj ADUhOnMA6zSoJr0VAG3+xZET1Nyg3AFSrYc9MlHIK5lsYfakhYdypZfMPFJee2MG3mQt lm7kGY3lNYCOCTiFvZGNC0XSGNCCSFnLuE+evu2AkfWfLhWC8idQ4AToZTET2p2C/wbH L49qMpNdBfVgX54oK9iqJFzd/7N6vw5OclWVZjW7iL11e9xTSl+iL2AmAO5yg9uJWo3o bApmBycQb0vVSASw3GXG3s1pA8pA/EZn0gdI3H+PqcgO0XZGSEXt07Q9E5hvWeGhYnvt ddBg== X-Gm-Message-State: AOAM531deatlYGefFoPbMwOg3JTrSm0iOZTC4c40i69SOWKXUgbf4xk/ hC7qMvNiyN8YmAcKDRQjiTOFiw== X-Google-Smtp-Source: ABdhPJwqUx1fk0fsqTKbsJfr9GkWo66OhRQUzkuYvej53ZH02a7apfdzcG2VGQfp/ZOKxMtMrY25ww== X-Received: by 2002:a37:e53:: with SMTP id 80mr1165768qko.243.1601057371751; Fri, 25 Sep 2020 11:09:31 -0700 (PDT) Received: from localhost ([2620:10d:c091:480::1:6423]) by smtp.gmail.com with ESMTPSA id g18sm2348430qtu.69.2020.09.25.11.09.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Sep 2020 11:09:31 -0700 (PDT) Date: Fri, 25 Sep 2020 14:07:57 -0400 From: Johannes Weiner To: Roman Gushchin Cc: Andrew Morton , Shakeel Butt , Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com Subject: Re: [PATCH v1 3/4] mm: introduce page memcg flags Message-ID: <20200925180757.GB362187@cmpxchg.org> References: <20200922203700.2879671-1-guro@fb.com> <20200922203700.2879671-4-guro@fb.com> <20200924200122.GC329853@cmpxchg.org> <20200924203905.GD1899519@carbon.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200924203905.GD1899519@carbon.dhcp.thefacebook.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 24, 2020 at 01:39:05PM -0700, Roman Gushchin wrote: > On Thu, Sep 24, 2020 at 04:01:22PM -0400, Johannes Weiner wrote: > > On Tue, Sep 22, 2020 at 01:36:59PM -0700, Roman Gushchin wrote: > > > The lowest bit in page->memcg_data is used to distinguish between > > > struct memory_cgroup pointer and a pointer to a objcgs array. > > > All checks and modifications of this bit are open-coded. > > > > > > Let's formalize it using page memcg flags, defined in page_memcg_flags > > > enum and replace all open-coded accesses with test_bit()/__set_bit(). > > > > > > Few additional flags might be added later. Flags are intended to be > > > mutually exclusive. > > > > > > Signed-off-by: Roman Gushchin > > > --- > > > include/linux/memcontrol.h | 29 +++++++++++++++++++---------- > > > 1 file changed, 19 insertions(+), 10 deletions(-) > > > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > > index ab3ea3e90583..9a49f1e1c0c7 100644 > > > --- a/include/linux/memcontrol.h > > > +++ b/include/linux/memcontrol.h > > > @@ -343,6 +343,11 @@ struct mem_cgroup { > > > > > > extern struct mem_cgroup *root_mem_cgroup; > > > > > > +enum page_memcg_flags { > > > + /* page->memcg_data is a pointer to an objcgs vector */ > > > + PG_MEMCG_OBJ_CGROUPS, > > > > How about enum memcg_data_flags and PGMEMCG_OBJCG? > > Honestly I prefer the original names. I'm ok with enum memcg_data_flags, > if you prefer it. PGMEMCG_OBJCG looks bulky with too many letters > without a separator, also we use object cgroups (plural) everywhere, > like OBJCGS vs OBJCG. PG_MEMCG_OBJCGS works for me. Fair enough, it's a bit dense. MEMCG_DATA_OBJCGS could work too. It wouldn't introduce a new prefix and would relate to the field those flags belong to. > > > @@ -371,13 +376,7 @@ static inline struct mem_cgroup *page_mem_cgroup_check(struct page *page) > > > { > > > unsigned long memcg_data = page->memcg_data; > > > > > > - /* > > > - * The lowest bit set means that memcg isn't a valid > > > - * memcg pointer, but a obj_cgroups pointer. > > > - * In this case the page is shared and doesn't belong > > > - * to any specific memory cgroup. > > > - */ > > > - if (memcg_data & 0x1UL) > > > + if (test_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data)) > > > return NULL; > > > > > > return (struct mem_cgroup *)memcg_data; > > > @@ -422,7 +421,13 @@ static inline void clear_page_mem_cgroup(struct page *page) > > > */ > > > static inline struct obj_cgroup **page_obj_cgroups(struct page *page) > > > { > > > - return (struct obj_cgroup **)(page->memcg_data & ~0x1UL); > > > + unsigned long memcg_data = page->memcg_data; > > > + > > > + VM_BUG_ON_PAGE(memcg_data && !test_bit(PG_MEMCG_OBJ_CGROUPS, > > > + &memcg_data), page); > > > + __clear_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data); > > > > The flag names make sense to me, but this shouldn't be using test_bit, > > __clear_bit, __set_bit etc. on local variables. It suggests that it's > > modifying some shared/global state, when it's just masking out a bit > > during a read. We usually just open-code the bitwise ops for that. > > It will be way more bulky otherwise, all those memcg_data & (1UL << PG_MEMCG_OBJ_CGROUPS) etc. Does anybody need the bit numbers? You can make them masks directly: enum memcg_data_flags { MEMCG_DATA_OBJCGS = (1 << 0), ... } and do memcg_data | MEMCG_DATA_OBJCGS. cgroup-defs.h alone has 3 examples of this. It's very common. > I don't see why these bitops helpers can't be used on local variables. > Is the preference to not use them this way documented anywhere? The bitops are for shared state, that's why set_bit(), clear_bit(), test_bit() provide atomicity, and the __ versions of them usually indicate that outside locking is provided. Grep for __clear_bit() and most of the time it's on a shared data structure and surrounded by some sort of lock or atomic context. Why would you want to replace a single | expression with an RMW transaction involving three statements and a function call to __set_bit()?