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=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 2522AC4727E for ; Thu, 24 Sep 2020 20:08:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BC6FB23899 for ; Thu, 24 Sep 2020 20:08:35 +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="AjvHKaw4" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726588AbgIXUIe (ORCPT ); Thu, 24 Sep 2020 16:08:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53838 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726500AbgIXUId (ORCPT ); Thu, 24 Sep 2020 16:08:33 -0400 Received: from mail-qv1-xf43.google.com (mail-qv1-xf43.google.com [IPv6:2607:f8b0:4864:20::f43]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 35F8EC0613D6 for ; Thu, 24 Sep 2020 13:02:56 -0700 (PDT) Received: by mail-qv1-xf43.google.com with SMTP id cr8so152868qvb.10 for ; Thu, 24 Sep 2020 13:02:56 -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=V6UdUdkmkvok2W93q7i5z7Tnyyn+1qXAK4+CaS4d9dg=; b=AjvHKaw4mi9aIoc/WkGzvkf247jeN/YEE4XZED2eYU203MUOwuyVP6BIdt27dZ5Uv2 Vwm/AnZjhBsuoIlALObXtvkXbsMYtqOjVjHHBWGUkTSnq4Kn8OgbakWt82URN5SWYKsK xrKpxzu81GsDBCk6f2x4+K+IV289VvQLK3yqL0SuncB9riMXQyx5OjZ1KiAHsLjBnRRe wdSMAWV6k6pBoZfzW7tVgiXFgfVVtVJcn5KqjdSL8uCpDewUOfoEOw/8fwflhoHOx2QI 9aOJKQli2QY4QIymcHTe5Pqhu5OHN7xMlt4hgfGSThLVOMn2j+HlYLDAOR8ZS0o5I+B4 z9QA== 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=V6UdUdkmkvok2W93q7i5z7Tnyyn+1qXAK4+CaS4d9dg=; b=PWsmfnM145GsvlHteACTqP/tiMqqx4MA/VTPju3OK/Ngfq4bVoQv+2cnXYqcXYKH3c dM6xGLhVYqulSTUaPFNJ0HFup//GcsRVBxEdPJonCjxpvkfTzJ3i8Wyx45/u0S6u0Vi8 7Mhzhy/GqlD0XC++1558NW3wbYfreeO4yYBFVjHItzk33tkukQJKbqGT3XcLEaXKA8Uw dUhWo7xe9lsGO5wF/vl87ex2DW7u8l1fhogUXmjYqW/FQqZQsJ4whZPeCpBSDjjlVKN5 gJsFryP7Io3181i0Eq7OWM/5PNtRxWsaaM2vm0R2+z7yOZda5TZucDHVAw8+9fKwmJ7p UW3Q== X-Gm-Message-State: AOAM531LXmBHlZ3dUZOJDeTsopQH6AIx9R+XOPgFUhKZkoLEW3T3sn+V F+xNV+mq7cRF3at+L3YK9115SQ== X-Google-Smtp-Source: ABdhPJwQ58cQqwPYpnLzikAMkGBaC11Z6mEv8T+V8eydsQEavysyUGWO0mF3RmUMHBGQK43jVk1dMQ== X-Received: by 2002:a0c:8406:: with SMTP id l6mr834013qva.15.1600977775408; Thu, 24 Sep 2020 13:02:55 -0700 (PDT) Received: from localhost ([2620:10d:c091:480::1:4d7a]) by smtp.gmail.com with ESMTPSA id f8sm384545qtx.81.2020.09.24.13.02.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Sep 2020 13:02:54 -0700 (PDT) Date: Thu, 24 Sep 2020 16:01:22 -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: <20200924200122.GC329853@cmpxchg.org> References: <20200922203700.2879671-1-guro@fb.com> <20200922203700.2879671-4-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922203700.2879671-4-guro@fb.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? > @@ -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.