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Date:   Fri, 9 Apr 2021 14:42:57 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH] mm/memory_hotplug: Make unpopulated zones PCP structures
 unreachable during hot remove
Message-ID: <YHBL0e8s+EesIyDl@dhcp22.suse.cz>
References: <20210409120957.GM3697@techsingularity.net>
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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