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From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     alastair@d-silva.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Wei Yang <richardw.yang@linux.intel.com>,
        David Hildenbrand <david@redhat.com>, Qian Cai <cai@lca.pw>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: [PATCH 2/2] mm: don't hide potentially null memmap pointer in sparse_remove_section
Date:   Tue, 27 Aug 2019 15:36:55 +1000
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From: Alastair D'Silva <alastair@d-silva.org>

By adding offset to memmap before passing it in to clear_hwpoisoned_pages,
we hide a theoretically null memmap from the null check inside
clear_hwpoisoned_pages.

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

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
---
 mm/sparse.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index e41917a7e844..3ff84e627e58 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -882,7 +882,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
 }
 
 #ifdef CONFIG_MEMORY_FAILURE
-static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
+static void clear_hwpoisoned_pages(struct page *memmap, int start, int count)
 {
 	int i;
 
@@ -898,7 +898,7 @@ 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 = start; i < start + count; i++) {
 		if (PageHWPoison(&memmap[i])) {
 			num_poisoned_pages_dec();
 			ClearPageHWPoison(&memmap[i]);
@@ -906,7 +906,8 @@ 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, int start,
+		int count)
 {
 }
 #endif
@@ -915,7 +916,7 @@ void sparse_remove_section(struct mem_section *ms, unsigned long pfn,
 		unsigned long nr_pages, unsigned long map_offset,
 		struct vmem_altmap *altmap)
 {
-	clear_hwpoisoned_pages(pfn_to_page(pfn) + map_offset,
+	clear_hwpoisoned_pages(pfn_to_page(pfn), map_offset,
 			nr_pages - map_offset);
 	section_deactivate(pfn, nr_pages, altmap);
 }
-- 
2.21.0