ext4: implement allocation of pre-zeroed blocks

DAX page fault path needs to get blocks that are pre-zeroed to avoid
races when two concurrent page faults happen in the same block of a
file. Implement support for this in ext4_map_blocks().
Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -557,6 +557,10 @@ enum {
 #define EXT4_GET_BLOCKS_KEEP_SIZE		0x0080
 	/* Convert written extents to unwritten */
 #define EXT4_GET_BLOCKS_CONVERT_UNWRITTEN	0x0100
+	/* Write zeros to newly created written extents */
+#define EXT4_GET_BLOCKS_ZERO			0x0200
+#define EXT4_GET_BLOCKS_CREATE_ZERO		(EXT4_GET_BLOCKS_CREATE |\
+					EXT4_GET_BLOCKS_ZERO)
 
 /*
  * The bit position of these flags must not overlap with any of the

fs/ext4/extents.c

@@ -4044,6 +4044,14 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 	}
 	/* IO end_io complete, convert the filled extent to written */
 	if (flags & EXT4_GET_BLOCKS_CONVERT) {
+		if (flags & EXT4_GET_BLOCKS_ZERO) {
+			if (allocated > map->m_len)
+				allocated = map->m_len;
+			err = ext4_issue_zeroout(inode, map->m_lblk, newblock,
+						 allocated);
+			if (err < 0)
+				goto out2;
+		}
 		ret = ext4_convert_unwritten_extents_endio(handle, inode, map,
 							   ppath);
 		if (ret >= 0) {

fs/ext4/inode.c

@@ -636,6 +636,22 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 			WARN_ON(1);
 		}
 
+		/*
+		 * We have to zeroout blocks before inserting them into extent
+		 * status tree. Otherwise someone could look them up there and
+		 * use them before they are really zeroed.
+		 */
+		if (flags & EXT4_GET_BLOCKS_ZERO &&
+		    map->m_flags & EXT4_MAP_MAPPED &&
+		    map->m_flags & EXT4_MAP_NEW) {
+			ret = ext4_issue_zeroout(inode, map->m_lblk,
+						 map->m_pblk, map->m_len);
+			if (ret) {
+				retval = ret;
+				goto out_sem;
+			}
+		}
+
 		/*
 		 * If the extent has been zeroed out, we don't need to update
 		 * extent status tree.
@@ -643,7 +659,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		if ((flags & EXT4_GET_BLOCKS_PRE_IO) &&
 		    ext4_es_lookup_extent(inode, map->m_lblk, &es)) {
 			if (ext4_es_is_written(&es))
-				goto has_zeroout;
+				goto out_sem;
 		}
 		status = map->m_flags & EXT4_MAP_UNWRITTEN ?
 				EXTENT_STATUS_UNWRITTEN : EXTENT_STATUS_WRITTEN;
@@ -654,11 +670,13 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 			status |= EXTENT_STATUS_DELAYED;
 		ret = ext4_es_insert_extent(inode, map->m_lblk, map->m_len,
 					    map->m_pblk, status);
-		if (ret < 0)
+		if (ret < 0) {
 			retval = ret;
+			goto out_sem;
+		}
 	}
 
-has_zeroout:
+out_sem:
 	up_write((&EXT4_I(inode)->i_data_sem));
 	if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED) {
 		ret = check_block_validity(inode, map);
@@ -3083,6 +3101,7 @@ int ext4_get_block_dax(struct inode *inode, sector_t iblock,
 		   struct buffer_head *bh_result, int create)
 {
 	int flags = EXT4_GET_BLOCKS_PRE_IO | EXT4_GET_BLOCKS_UNWRIT_EXT;
+
 	if (create)
 		flags |= EXT4_GET_BLOCKS_CREATE;
 	ext4_debug("ext4_get_block_dax: inode %lu, create flag %d\n",

include/trace/events/ext4.h

@@ -42,7 +42,8 @@ struct extent_status;
 	{ EXT4_GET_BLOCKS_CONVERT,		"CONVERT" },		\
 	{ EXT4_GET_BLOCKS_METADATA_NOFAIL,	"METADATA_NOFAIL" },	\
 	{ EXT4_GET_BLOCKS_NO_NORMALIZE,		"NO_NORMALIZE" },	\
-	{ EXT4_GET_BLOCKS_KEEP_SIZE,		"KEEP_SIZE" })
+	{ EXT4_GET_BLOCKS_KEEP_SIZE,		"KEEP_SIZE" },		\
+	{ EXT4_GET_BLOCKS_ZERO,			"ZERO" })
 
 #define show_mflags(flags) __print_flags(flags, "",	\
 	{ EXT4_MAP_NEW,		"N" },			\