[PATCH v6 19/51] mm: Zero the head page, not the tail page

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>

Pass the head page to zero_user_segment(), not the tail page, and adjust
the byte offsets appropriately.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/shmem.c    | 7 +++++++
 mm/truncate.c | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index a05d129a45e9..55405d811cfd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -898,11 +898,18 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 		struct page *page = NULL;
 		shmem_getpage(inode, start - 1, &page, SGP_READ);
 		if (page) {
+			struct page *head = thp_head(page);
 			unsigned int top = PAGE_SIZE;
 			if (start > end) {
 				top = partial_end;
 				partial_end = 0;
 			}
+			if (head != page) {
+				unsigned int diff = start - 1 - head->index;
+				partial_start += diff << PAGE_SHIFT;
+				top += diff << PAGE_SHIFT;
+				page = head;
+			}
 			zero_user_segment(page, partial_start, top);
 			set_page_dirty(page);
 			unlock_page(page);
diff --git a/mm/truncate.c b/mm/truncate.c
index dd9ebc1da356..152974888124 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -374,12 +374,19 @@ void truncate_inode_pages_range(struct address_space *mapping,
 	if (partial_start) {
 		struct page *page = find_lock_page(mapping, start - 1);
 		if (page) {
+			struct page *head = thp_head(page);
 			unsigned int top = PAGE_SIZE;
 			if (start > end) {
 				/* Truncation within a single page */
 				top = partial_end;
 				partial_end = 0;
 			}
+			if (head != page) {
+				unsigned int diff = start - 1 - head->index;
+				partial_start += diff << PAGE_SHIFT;
+				top += diff << PAGE_SHIFT;
+				page = head;
+			}
 			wait_on_page_writeback(page);
 			zero_user_segment(page, partial_start, top);
 			cleancache_invalidate_page(mapping, page);
-- 
2.26.2