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From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     alastair@d-silva.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Baoquan He <bhe@redhat.com>, Qian Cai <cai@lca.pw>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in sparse_remove_one_section
Date:   Wed, 26 Jun 2019 16:11:22 +1000
In-Reply-To: <20190626061124.16013-1-alastair@au1.ibm.com>
References: <20190626061124.16013-1-alastair@au1.ibm.com>
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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 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 | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 57a1a3d9c1cf..1ec32aef5590 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -753,7 +753,8 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
 
 #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 start, unsigned long count)
 {
 	int i;
 
@@ -769,7 +770,7 @@ 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])) {
 			atomic_long_sub(1, &num_poisoned_pages);
 			ClearPageHWPoison(&memmap[i]);
@@ -777,7 +778,8 @@ 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 start, unsigned long count)
 {
 }
 #endif
@@ -824,7 +826,7 @@ void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
 		ms->pageblock_flags = NULL;
 	}
 
-	clear_hwpoisoned_pages(memmap + map_offset,
+	clear_hwpoisoned_pages(memmap, map_offset,
 			PAGES_PER_SECTION - map_offset);
 	free_section_usemap(memmap, usemap, altmap);
 }
-- 
2.21.0