[PATCH v2 2/2] mm: fix contiguous memmap assumptions about alloc/free pages

[Chen Wandun <chenwandun@huawei.com>]

It isn't true for only SPARSEMEM configs to assume that a compound page
has virtually contiguous page structs, so use nth_page to iterate each
page.

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
---
 mm/page_alloc.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 855211dea13e..758d8f069b32 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -721,7 +721,7 @@ static void prep_compound_head(struct page *page, unsigned int order)
 
 static void prep_compound_tail(struct page *head, int tail_idx)
 {
-	struct page *p = head + tail_idx;
+	struct page *p = nth_page(head, tail_idx);
 
 	p->mapping = TAIL_MAPPING;
 	set_compound_head(p, head);
@@ -1199,10 +1199,10 @@ static inline int check_free_page(struct page *page)
 	return 1;
 }
 
-static int free_tail_pages_check(struct page *head_page, struct page *page)
+static int free_tail_pages_check(struct page *head_page, int index)
 {
+	struct page *page = nth_page(head_page, index);
 	int ret = 1;
-
 	/*
 	 * We rely page->lru.next never has bit 0 set, unless the page
 	 * is PageTail(). Let's make sure that's true even for poisoned ->lru.
@@ -1213,7 +1213,7 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
 		ret = 0;
 		goto out;
 	}
-	switch (page - head_page) {
+	switch (index) {
 	case 1:
 		/* the first tail page: ->mapping may be compound_mapcount() */
 		if (unlikely(compound_mapcount(page))) {
@@ -1322,6 +1322,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	if (unlikely(order)) {
 		bool compound = PageCompound(page);
 		int i;
+		struct page *tail_page;
 
 		VM_BUG_ON_PAGE(compound && compound_order(page) != order, page);
 
@@ -1330,13 +1331,14 @@ static __always_inline bool free_pages_prepare(struct page *page,
 			ClearPageHasHWPoisoned(page);
 		}
 		for (i = 1; i < (1 << order); i++) {
+			tail_page = nth_page(page, i);
 			if (compound)
-				bad += free_tail_pages_check(page, page + i);
-			if (unlikely(check_free_page(page + i))) {
+				bad += free_tail_pages_check(page, i);
+			if (unlikely(check_free_page(tail_page))) {
 				bad++;
 				continue;
 			}
-			(page + i)->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
+			tail_page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
 		}
 	}
 	if (PageMappingFlags(page))
-- 
2.18.0.huawei.25