+ mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch added to -mm tree

+ mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm/memory_hotplug: make unpopulated zones PCP structures unreachable during hot remove
has been added to the -mm tree.  Its filename is
     mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch

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------------------------------------------------------
From: Mel Gorman <mgorman@techsingularity.net>
Subject: 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.

Link: https://lkml.kernel.org/r/20210412090346.GQ3697@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/page_alloc.c~mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove
+++ a/mm/page_alloc.c
@@ -9020,12 +9020,9 @@ 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);
@@ -9034,7 +9031,6 @@ void zone_pcp_reset(struct zone *zone)
 		free_percpu(zone->pageset);
 		zone->pageset = &boot_pageset;
 	}
-	local_irq_restore(flags);
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
_

Patches currently in -mm which might be from mgorman@techsingularity.net are

mm-page_alloc-rename-alloced-to-allocated.patch
mm-page_alloc-add-a-bulk-page-allocator.patch
mm-page_alloc-add-a-bulk-page-allocator-fix.patch
mm-page_alloc-add-a-bulk-page-allocator-fix-fix.patch
mm-page_alloc-add-a-bulk-page-allocator-fix-fix-fix.patch
mm-page_alloc-add-an-array-based-interface-to-the-bulk-page-allocator.patch
mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove.patch