1 files changed, 43 insertions, 55 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index e23fea40767e..8d0aba1cc155 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -789,6 +789,32 @@ void shmem_unlock_mapping(struct address_space *mapping)
 }
 
 /*
+ * Check whether a hole-punch or truncation needs to split a huge page,
+ * returning true if no split was required, or the split has been successful.
+ *
+ * Eviction (or truncation to 0 size) should never need to split a huge page;
+ * but in rare cases might do so, if shmem_undo_range() failed to trylock on
+ * head, and then succeeded to trylock on tail.
+ *
+ * A split can only succeed when there are no additional references on the
+ * huge page: so the split below relies upon find_get_entries() having stopped
+ * when it found a subpage of the huge page, without getting further references.
+ */
+static bool shmem_punch_compound(struct page *page, pgoff_t start, pgoff_t end)
+{
+	if (!PageTransCompound(page))
+		return true;
+
+	/* Just proceed to delete a huge page wholly within the range punched */
+	if (PageHead(page) &&
+	    page->index >= start && page->index + HPAGE_PMD_NR <= end)
+		return true;
+
+	/* Try to split huge page, so we can truly punch the hole or truncate */
+	return split_huge_page(page) >= 0;
+}
+
+/*
  * Remove range of pages and swap entries from page cache, and free them.
  * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
  */
@@ -838,31 +864,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 			if (!trylock_page(page))
 				continue;
 
-			if (PageTransTail(page)) {
-				/* Middle of THP: zero out the page */
-				clear_highpage(page);
-				unlock_page(page);
-				continue;
-			} else if (PageTransHuge(page)) {
-				if (index == round_down(end, HPAGE_PMD_NR)) {
-					/*
-					 * Range ends in the middle of THP:
-					 * zero out the page
-					 */
-					clear_highpage(page);
-					unlock_page(page);
-					continue;
-				}
-				index += HPAGE_PMD_NR - 1;
-				i += HPAGE_PMD_NR - 1;
-			}
-
-			if (!unfalloc || !PageUptodate(page)) {
-				VM_BUG_ON_PAGE(PageTail(page), page);
-				if (page_mapping(page) == mapping) {
-					VM_BUG_ON_PAGE(PageWriteback(page), page);
+			if ((!unfalloc || !PageUptodate(page)) &&
+			    page_mapping(page) == mapping) {
+				VM_BUG_ON_PAGE(PageWriteback(page), page);
+				if (shmem_punch_compound(page, start, end))
 					truncate_inode_page(mapping, page);
-				}
 			}
 			unlock_page(page);
 		}
@@ -936,43 +942,25 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 
 			lock_page(page);
 
-			if (PageTransTail(page)) {
-				/* Middle of THP: zero out the page */
-				clear_highpage(page);
-				unlock_page(page);
-				/*
-				 * Partial thp truncate due 'start' in middle
-				 * of THP: don't need to look on these pages
-				 * again on !pvec.nr restart.
-				 */
-				if (index != round_down(end, HPAGE_PMD_NR))
-					start++;
-				continue;
-			} else if (PageTransHuge(page)) {
-				if (index == round_down(end, HPAGE_PMD_NR)) {
-					/*
-					 * Range ends in the middle of THP:
-					 * zero out the page
-					 */
-					clear_highpage(page);
-					unlock_page(page);
-					continue;
-				}
-				index += HPAGE_PMD_NR - 1;
-				i += HPAGE_PMD_NR - 1;
-			}
-
 			if (!unfalloc || !PageUptodate(page)) {
-				VM_BUG_ON_PAGE(PageTail(page), page);
-				if (page_mapping(page) == mapping) {
-					VM_BUG_ON_PAGE(PageWriteback(page), page);
-					truncate_inode_page(mapping, page);
-				} else {
+				if (page_mapping(page) != mapping) {
 					/* Page was replaced by swap: retry */
 					unlock_page(page);
 					index--;
 					break;
 				}
+				VM_BUG_ON_PAGE(PageWriteback(page), page);
+				if (shmem_punch_compound(page, start, end))
+					truncate_inode_page(mapping, page);
+				else {
+					/* Wipe the page and don't get stuck */
+					clear_highpage(page);
+					flush_dcache_page(page);
+					set_page_dirty(page);
+					if (index <
+					    round_up(start, HPAGE_PMD_NR))
+						start = index + 1;
+				}
 			}
 			unlock_page(page);
 		}