[RFC PATCH] mm: set memory section offline when all its pages are offline.

[lantianyu1986@gmail.com]

From: Tianyu Lan <Tianyu.Lan@microsoft.com>

If size of offline memory region passed to offline_pages() is
not aligned with PAGES_PER_SECTION, memory section will be set
to offline in the offline_mem_sections() with some pages of
memory section online. Fix it, Update memory section status after
marking offline pages as "reserved" in __offline_isolated_pages()
and check all pages in memory are reserved or not before setting
memory section offline.

Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
---
This patch is to prepare for hot remove memory function in Hyper-V
balloon driver. It requests to offline memory with random size.
---
 mm/page_alloc.c |  3 ++-
 mm/sparse.c     | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dbd0d5cbbcbb..cc02866924ae 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8540,7 +8540,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 	if (pfn == end_pfn)
 		return offlined_pages;
 
-	offline_mem_sections(pfn, end_pfn);
 	zone = page_zone(pfn_to_page(pfn));
 	spin_lock_irqsave(&zone->lock, flags);
 	pfn = start_pfn;
@@ -8576,6 +8575,8 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 	}
 	spin_unlock_irqrestore(&zone->lock, flags);
 
+	offline_mem_sections(pfn, end_pfn);
+
 	return offlined_pages;
 }
 #endif
diff --git a/mm/sparse.c b/mm/sparse.c
index fd13166949b5..eb5860487b84 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -571,6 +571,7 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
 void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
 {
 	unsigned long pfn;
+	int i;
 
 	for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
 		unsigned long section_nr = pfn_to_section_nr(pfn);
@@ -583,6 +584,15 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
 		if (WARN_ON(!valid_section_nr(section_nr)))
 			continue;
 
+		/*
+		 * Check whether all pages in the section are reserverd before
+		 * setting setction offline.
+		 */
+		for (i = 0; i < PAGES_PER_SECTION; i++)
+			if (!PageReserved(pfn_to_page(
+			    SECTION_ALIGN_DOWN(pfn + i))))
+				continue;
+
 		ms = __nr_to_section(section_nr);
 		ms->section_mem_map &= ~SECTION_IS_ONLINE;
 	}
-- 
2.14.5