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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 24DD6C433B4 for ; Fri, 9 Apr 2021 12:43:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F3BCB610CF for ; Fri, 9 Apr 2021 12:43:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232763AbhDIMnP (ORCPT ); Fri, 9 Apr 2021 08:43:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:33012 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231127AbhDIMnM (ORCPT ); Fri, 9 Apr 2021 08:43:12 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1617972178; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=YiL8GIn+kNksZGX+sHZ8dtDnwl03BF0KcDznW8aXSqA=; b=UfUcXlLJ8zbFuZ+4eapJA8Cvjn/dHDzZs21SonEP/japaikGIw41zEGfTlkPMNA18t7Rpk S1ofdPXsp7dzaIZgCh1dtjD4bUettRcKarIIuy7BS3r1/pa9fu2qPqPkkuD8X3mndq9OlJ 7HxjCjK09MIBf1l2Qklpp6ucGdUaSaE= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 2C172AF4C; Fri, 9 Apr 2021 12:42:58 +0000 (UTC) Date: Fri, 9 Apr 2021 14:42:57 +0200 From: Michal Hocko To: Mel Gorman Cc: Andrew Morton , Linux-MM , LKML , Oscar Salvador , "Michael S. Tsirkin" , David Hildenbrand , Vlastimil Babka , Alexander Duyck , Minchan Kim Subject: Re: [PATCH] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove Message-ID: References: <20210409120957.GM3697@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210409120957.GM3697@techsingularity.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 09-04-21 13:09:57, Mel Gorman wrote: > zone_pcp_reset allegedly protects against a race with drain_pages > using local_irq_save but this is bogus. local_irq_save only operates > on the local CPU. If memory hotplug is running on CPU A and drain_pages > is running on CPU B, disabling IRQs on CPU A does not affect CPU B and > offers no protection. Yes, the synchronization aspect is bogus indeed. > This patch reorders memory hotremove such that the PCP structures > relevant to the zone are no longer reachable by the time the structures > are freed. With this reordering, no protection is required to prevent > a use-after-free and the IRQs can be left enabled. zone_pcp_reset is > renamed to zone_pcp_destroy to make it clear that the per-cpu structures > are deleted when the function returns. Wouldn't it be much easier to simply not destroy/reset pcp of an empty zone at all? The whole point of this exercise seems to be described in 340175b7d14d5. setup_zone_pageset can check for an already allocated pcp and simply reinitialize it. -- Michal Hocko SUSE Labs