[PATCH 2/5] mm: don't hide potentially null memmap pointer in sparse_remove_one_section

["Alastair D'Silva" <alastair@au1.ibm.com>]

From: Alastair D'Silva <alastair@d-silva.org>

By adding offset to memmap before passing it in to clear_hwpoisoned_pages,
is hides a potentially null memmap from the null check inside
clear_hwpoisoned_pages.

This patch passes the offset to clear_hwpoisoned_pages instead, allowing
memmap to successfully peform it's null check.

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
---
 mm/sparse.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 104a79fedd00..66a99da9b11b 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -746,12 +746,14 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
 		kfree(usemap);
 		__kfree_section_memmap(memmap, altmap);
 	}
+
 	return ret;
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
 #ifdef CONFIG_MEMORY_FAILURE
-static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
+static void clear_hwpoisoned_pages(struct page *memmap,
+		unsigned long map_offset, int nr_pages)
 {
 	int i;
 
@@ -767,7 +769,7 @@ static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
 	if (atomic_long_read(&num_poisoned_pages) == 0)
 		return;
 
-	for (i = 0; i < nr_pages; i++) {
+	for (i = map_offset; i < nr_pages; i++) {
 		if (PageHWPoison(&memmap[i])) {
 			atomic_long_sub(1, &num_poisoned_pages);
 			ClearPageHWPoison(&memmap[i]);
@@ -775,7 +777,8 @@ static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
 	}
 }
 #else
-static inline void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
+static inline void clear_hwpoisoned_pages(struct page *memmap,
+		unsigned long map_offset, int nr_pages)
 {
 }
 #endif
@@ -822,8 +825,7 @@ void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
 		ms->pageblock_flags = NULL;
 	}
 
-	clear_hwpoisoned_pages(memmap + map_offset,
-			PAGES_PER_SECTION - map_offset);
+	clear_hwpoisoned_pages(memmap, map_offset, PAGES_PER_SECTION);
 	free_section_usemap(memmap, usemap, altmap);
 }
 #endif /* CONFIG_MEMORY_HOTREMOVE */
-- 
2.21.0