linux-kernel - Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove

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Message-ID: <20210414071756.GA20401@linux>
Date:   Wed, 14 Apr 2021 09:18:02 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexander Duyck <alexanderduyck@...com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP
 structures unreachable during hot remove

On Mon, Apr 12, 2021 at 01:08:42PM +0100, 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.
> 
> This patch deletes IRQ disable/enable on the grounds that IRQs protect
> nothing and assumes the existing hotplug paths guarantees the PCP cannot be
> used after zone_pcp_enable(). That should be the case already because all
> the pages have been freed and there is no page to put on the PCP lists.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

-- 
Oscar Salvador
SUSE L3