btrfs: teach __process_pages_contig about PAGE_LOCK operation

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ changes to the helper separated from the following patch ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -1554,6 +1554,11 @@ out:
 	return found;
 }
 
+static int __process_pages_contig(struct address_space *mapping,
+				  struct page *locked_page,
+				  pgoff_t start_index, pgoff_t end_index,
+				  unsigned long page_ops, pgoff_t *index_ret);
+
 static noinline void __unlock_for_delalloc(struct inode *inode,
 					   struct page *locked_page,
 					   u64 start, u64 end)
@@ -1731,17 +1736,24 @@ out_failed:
 	return found;
 }
 
-static void __process_pages_contig(struct address_space *mapping,
-				   struct page *locked_page,
-				   pgoff_t start_index, pgoff_t end_index,
-				   unsigned long page_ops)
+static int __process_pages_contig(struct address_space *mapping,
+				  struct page *locked_page,
+				  pgoff_t start_index, pgoff_t end_index,
+				  unsigned long page_ops, pgoff_t *index_ret)
 {
 	unsigned long nr_pages = end_index - start_index + 1;
+	unsigned long pages_locked = 0;
 	pgoff_t index = start_index;
 	struct page *pages[16];
 	unsigned ret;
+	int err = 0;
 	int i;
 
+	if (page_ops & PAGE_LOCK) {
+		ASSERT(page_ops == PAGE_LOCK);
+		ASSERT(index_ret && *index_ret == start_index);
+	}
+
 	if ((page_ops & PAGE_SET_ERROR) && nr_pages > 0)
 		mapping_set_error(mapping, -EIO);
 
@@ -1749,13 +1761,22 @@ static void __process_pages_contig(struct address_space *mapping,
 		ret = find_get_pages_contig(mapping, index,
 				     min_t(unsigned long,
 				     nr_pages, ARRAY_SIZE(pages)), pages);
-		for (i = 0; i < ret; i++) {
+		if (ret == 0) {
+			/*
+			 * Only if we're going to lock these pages,
+			 * can we find nothing at @index.
+			 */
+			ASSERT(page_ops & PAGE_LOCK);
+			return ret;
+		}
 
+		for (i = 0; i < ret; i++) {
 			if (page_ops & PAGE_SET_PRIVATE2)
 				SetPagePrivate2(pages[i]);
 
 			if (pages[i] == locked_page) {
 				put_page(pages[i]);
+				pages_locked++;
 				continue;
 			}
 			if (page_ops & PAGE_CLEAR_DIRTY)
@@ -1768,12 +1789,27 @@ static void __process_pages_contig(struct address_space *mapping,
 				end_page_writeback(pages[i]);
 			if (page_ops & PAGE_UNLOCK)
 				unlock_page(pages[i]);
+			if (page_ops & PAGE_LOCK) {
+				lock_page(pages[i]);
+				if (!PageDirty(pages[i]) ||
+				    pages[i]->mapping != mapping) {
+					unlock_page(pages[i]);
+					put_page(pages[i]);
+					err = -EAGAIN;
+					goto out;
+				}
+			}
 			put_page(pages[i]);
+			pages_locked++;
 		}
 		nr_pages -= ret;
 		index += ret;
 		cond_resched();
 	}
+out:
+	if (err && index_ret)
+		*index_ret = start_index + pages_locked - 1;
+	return err;
 }
 
 void extent_clear_unlock_delalloc(struct inode *inode, u64 start, u64 end,
@@ -1786,7 +1822,7 @@ void extent_clear_unlock_delalloc(struct inode *inode, u64 start, u64 end,
 
 	__process_pages_contig(inode->i_mapping, locked_page,
 			       start >> PAGE_SHIFT, end >> PAGE_SHIFT,
-			       page_ops);
+			       page_ops, NULL);
 }
 
 /*

fs/btrfs/extent_io.h

@@ -45,13 +45,14 @@
 #define EXTENT_BUFFER_IN_TREE 10
 #define EXTENT_BUFFER_WRITE_ERR 11    /* write IO error */
 
-/* these are flags for extent_clear_unlock_delalloc */
+/* these are flags for __process_pages_contig */
 #define PAGE_UNLOCK		(1 << 0)
 #define PAGE_CLEAR_DIRTY	(1 << 1)
 #define PAGE_SET_WRITEBACK	(1 << 2)
 #define PAGE_END_WRITEBACK	(1 << 3)
 #define PAGE_SET_PRIVATE2	(1 << 4)
 #define PAGE_SET_ERROR		(1 << 5)
+#define PAGE_LOCK		(1 << 6)
 
 /*
  * page->private values.  Every page that is controlled by the extent