[PATCH v2 1/2] mm/page_alloc.c: Don't set pages PageReserved() when offlining

[David Hildenbrand <david@redhat.com>]

We call __offline_isolated_pages() from __offline_pages() after all
pages were isolated and are either free (PageBuddy()) or PageHWPoison.
Nothing can stop us from offlining memory at this point.

In __offline_isolated_pages() we first set all affected memory sections
offline (offline_mem_sections(pfn, end_pfn)), to mark the memmap as
invalid (pfn_to_online_page() will no longer succeed), and then walk over
all pages to pull the free pages from the free lists (to the isolated
free lists, to be precise).

Note that re-onlining a memory block will result in the whole memmap
getting reinitialized, overwriting any old state. We already poision the
memmap when offlining is complete to find any access to
stale/uninitialized memmaps.

So, setting the pages PageReserved() is not helpful. The memap is marked
offline and all pageblocks are isolated. As soon as offline, the memmap
is stale either way.

This looks like a leftover from ancient times where we initialized the
memmap when adding memory and not when onlining it (the pages were set
PageReserved so re-onling would work as expected).

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memory_hotplug.c | 4 +---
 mm/page_alloc.c     | 5 +----
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 5e6b2a312362..3524e2e1a9df 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1346,9 +1346,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 	return ret;
 }
 
-/*
- * remove from free_area[] and mark all as Reserved.
- */
+/* Mark all sections offline and remove all free pages from the buddy. */
 static int
 offline_isolated_pages_cb(unsigned long start, unsigned long nr_pages,
 			void *data)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ed8884dc0c47..bf6b21f02154 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8667,7 +8667,7 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 {
 	struct page *page;
 	struct zone *zone;
-	unsigned int order, i;
+	unsigned int order;
 	unsigned long pfn;
 	unsigned long flags;
 	unsigned long offlined_pages = 0;
@@ -8695,7 +8695,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 		 */
 		if (unlikely(!PageBuddy(page) && PageHWPoison(page))) {
 			pfn++;
-			SetPageReserved(page);
 			offlined_pages++;
 			continue;
 		}
@@ -8709,8 +8708,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 			pfn, 1 << order, end_pfn);
 #endif
 		del_page_from_free_area(page, &zone->free_area[order]);
-		for (i = 0; i < (1 << order); i++)
-			SetPageReserved((page+i));
 		pfn += (1 << order);
 	}
 	spin_unlock_irqrestore(&zone->lock, flags);
-- 
2.21.0