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

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Message-ID: <20210412120842.GY3697@techsingularity.net>
Date:   Mon, 12 Apr 2021 13:08:42 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Oscar Salvador <osalvador@...e.de>,
        "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: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP
 structures unreachable during hot remove

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>
---
Resending for email address correction and adding lists

Changelog since v1
o Minimal fix

 mm/page_alloc.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5e8aedb64b57..9bf0db982f14 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8952,12 +8952,9 @@ void zone_pcp_enable(struct zone *zone)
 
 void zone_pcp_reset(struct zone *zone)
 {
-	unsigned long flags;
 	int cpu;
 	struct per_cpu_pageset *pset;
 
-	/* avoid races with drain_pages()  */
-	local_irq_save(flags);
 	if (zone->pageset != &boot_pageset) {
 		for_each_online_cpu(cpu) {
 			pset = per_cpu_ptr(zone->pageset, cpu);
@@ -8966,7 +8963,6 @@ void zone_pcp_reset(struct zone *zone)
 		free_percpu(zone->pageset);
 		zone->pageset = &boot_pageset;
 	}
-	local_irq_restore(flags);
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE