[PATCH v2] mm/hotplug: fix offline undo_isolate_page_range()

[Qian Cai]

The commit f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded
memory to zones until online") introduced move_pfn_range_to_zone() which
calls memmap_init_zone() during onlining a memory block.
memmap_init_zone() will reset pagetype flags and makes migrate type to
be MOVABLE.

However, in __offline_pages(), it also call undo_isolate_page_range()
after offline_isolated_pages() to do the same thing. Due to
the commit 2ce13640b3f4 ("mm: __first_valid_page skip over offline
pages") changed __first_valid_page() to skip offline pages,
undo_isolate_page_range() here just waste CPU cycles looping around the
offlining PFN range while doing nothing, because __first_valid_page()
will return NULL as offline_isolated_pages() has already marked all
memory sections within the pfn range as offline via
offline_mem_sections().

Also, after calling the "useless" undo_isolate_page_range() here, it
reaches the point of no returning by notifying MEM_OFFLINE. Those pages
will be marked as MIGRATE_MOVABLE again once onlining. The only thing
left to do is to decrease the number of isolated pageblocks zone
counter which would make some paths of the page allocation slower that
the above commit introduced. A memory block is usually at most 1GiB in
size, so an "int" should be enough to represent the number of pageblocks
in a block. Fix an incorrect comment along the way.

Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
Signed-off-by: Qian Cai <cai@xxxxxx>
---

v2: return the nubmer of isolated pageblocks in start_isolate_page_range() per
    Oscar; take the zone lock when undoing zone->nr_isolate_pageblock per
    Michal.

 mm/memory_hotplug.c | 17 +++++++++++++----
 mm/page_alloc.c     |  2 +-
 mm/page_isolation.c | 16 ++++++++++------
 mm/sparse.c         |  2 +-
 4 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index cd23c081924d..8ffe844766da 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1580,7 +1580,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
 {
 	unsigned long pfn, nr_pages;
 	long offlined_pages;
-	int ret, node;
+	int ret, node, count;
 	unsigned long flags;
 	unsigned long valid_start, valid_end;
 	struct zone *zone;
@@ -1606,10 +1606,11 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	ret = start_isolate_page_range(start_pfn, end_pfn,
 				       MIGRATE_MOVABLE,
 				       SKIP_HWPOISON | REPORT_FAILURE);
-	if (ret) {
+	if (ret < 0) {
 		reason = "failure to isolate range";
 		goto failed_removal;
 	}
+	count = ret;
 
 	arg.start_pfn = start_pfn;
 	arg.nr_pages = nr_pages;
@@ -1661,8 +1662,16 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	/* Ok, all of our target is isolated.
 	   We cannot do rollback at this point. */
 	offline_isolated_pages(start_pfn, end_pfn);
-	/* reset pagetype flags and makes migrate type to be MOVABLE */
-	undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
+
+	/*
+	 * Onlining will reset pagetype flags and makes migrate type
+	 * MOVABLE, so just need to decrease the number of isolated
+	 * pageblocks zone counter here.
+	 */
+	spin_lock_irqsave(&zone->lock, flags);
+	zone->nr_isolate_pageblock -= count;
+	spin_unlock_irqrestore(&zone->lock, flags);
+
 	/* removal success */
 	adjust_managed_page_count(pfn_to_page(start_pfn), -offlined_pages);
 	zone->present_pages -= offlined_pages;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 03fcf73d47da..d96ca5bc555b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8233,7 +8233,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 
 	ret = start_isolate_page_range(pfn_max_align_down(start),
 				       pfn_max_align_up(end), migratetype, 0);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
 	/*
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index ce323e56b34d..bf67b63227ca 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -172,7 +172,8 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
  * future will not be allocated again.
  *
  * start_pfn/end_pfn must be aligned to pageblock_order.
- * Return 0 on success and -EBUSY if any part of range cannot be isolated.
+ * Return the number of isolated pageblocks on success and -EBUSY if any part of
+ * range cannot be isolated.
  *
  * There is no high level synchronization mechanism that prevents two threads
  * from trying to isolate overlapping ranges.  If this happens, one thread
@@ -188,6 +189,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
 	unsigned long pfn;
 	unsigned long undo_pfn;
 	struct page *page;
+	int count = 0;
 
 	BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
 	BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
@@ -196,13 +198,15 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
 	     pfn < end_pfn;
 	     pfn += pageblock_nr_pages) {
 		page = __first_valid_page(pfn, pageblock_nr_pages);
-		if (page &&
-		    set_migratetype_isolate(page, migratetype, flags)) {
-			undo_pfn = pfn;
-			goto undo;
+		if (page) {
+			if (set_migratetype_isolate(page, migratetype, flags)) {
+				undo_pfn = pfn;
+				goto undo;
+			}
+			count++;
 		}
 	}
-	return 0;
+	return count;
 undo:
 	for (pfn = start_pfn;
 	     pfn < undo_pfn;
diff --git a/mm/sparse.c b/mm/sparse.c
index 69904aa6165b..56e057c432f9 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -567,7 +567,7 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
-/* Mark all memory sections within the pfn range as online */
+/* Mark all memory sections within the pfn range as offline */
 void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
 {
 	unsigned long pfn;
-- 
2.17.2 (Apple Git-113)