ext4: make the zero-out chunk size tunable

Currently in ext4 the length of zero-out chunk is set to 7 file system
blocks. But if an inode has uninitailized extents from using
fallocate to preallocate space, and the workload issues many random
writes, this can cause a fragmented extent tree that will
unnecessarily grow the extent tree.

So create a new sysfs tunable, extent_max_zeroout_kb, which controls
the maximum size where blocks will be zeroed out instead of creating a
new uninitialized extent. The default of this has been sent to 32kb.

CC: Zach Brown <[e-mail]>
CC: Andreas Dilger <[e-mail]>
Signed-off-by: Zheng Liu <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

Documentation/ABI/testing/sysfs-fs-ext4

···
 Description:
 		The maximum number of megabytes the writeback code will
 		try to write out before move on to another inode.
+
+What:		/sys/fs/ext4/<disk>/extent_max_zeroout_kb
+Date:		August 2012
+Contact:	"Theodore Ts'o" <[e-mail]>
+Description:
+		The maximum number of kilobytes which will be zeroed
+		out in preference to creating a new uninitialized
+		extent when manipulating an inode's extent tree.  Note
+		that using a larger value will increase the
+		variability of time necessary to complete a random
+		write operation (since a 4k random write might turn
+		into a much larger write due to the zeroout
+		operation).

fs/ext4/ext4.h

···
 	unsigned long s_sectors_written_start;
 	u64 s_kbytes_written;
 
+	/* the size of zero-out chunk */
+	unsigned int s_extent_max_zeroout_kb;
+
 	unsigned int s_log_groups_per_flex;
 	struct flex_groups *s_flex_groups;
 

fs/ext4/extents.c

···
 	return err ? err : map->m_len;
 }
 
-#define EXT4_EXT_ZERO_LEN 7
 /*
  * This function is called by ext4_ext_map_blocks() if someone tries to write
  * to an uninitialized extent. It may result in splitting the uninitialized
···
 					   struct ext4_map_blocks *map,
 					   struct ext4_ext_path *path)
 {
+	struct ext4_sb_info *sbi;
 	struct ext4_extent_header *eh;
 	struct ext4_map_blocks split_map;
 	struct ext4_extent zero_ex;
 	struct ext4_extent *ex;
 	ext4_lblk_t ee_block, eof_block;
 	unsigned int ee_len, depth;
-	int allocated;
+	int allocated, max_zeroout = 0;
 	int err = 0;
 	int split_flag = 0;
 
···
 		"block %llu, max_blocks %u\n", inode->i_ino,
 		(unsigned long long)map->m_lblk, map->m_len);
 
+	sbi = EXT4_SB(inode->i_sb);
 	eof_block = (inode->i_size + inode->i_sb->s_blocksize - 1) >>
 		inode->i_sb->s_blocksize_bits;
 	if (eof_block < map->m_lblk + map->m_len)
···
 	 */
 	split_flag |= ee_block + ee_len <= eof_block ? EXT4_EXT_MAY_ZEROOUT : 0;
 
-	/* If extent has less than 2*EXT4_EXT_ZERO_LEN zerout directly */
-	if (ee_len <= 2*EXT4_EXT_ZERO_LEN &&
-	    (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
+	if (EXT4_EXT_MAY_ZEROOUT & split_flag)
+		max_zeroout = sbi->s_extent_max_zeroout_kb >>
+			inode->i_sb->s_blocksize_bits;
+
+	/* If extent is less than s_max_zeroout_kb, zeroout directly */
+	if (max_zeroout && (ee_len <= max_zeroout)) {
 		err = ext4_ext_zeroout(inode, ex);
 		if (err)
 			goto out;
···
 	split_map.m_lblk = map->m_lblk;
 	split_map.m_len = map->m_len;
 
-	if (allocated > map->m_len) {
-		if (allocated <= EXT4_EXT_ZERO_LEN &&
-		    (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
+	if (max_zeroout && (allocated > map->m_len)) {
+		if (allocated <= max_zeroout) {
 			/* case 3 */
 			zero_ex.ee_block =
 					 cpu_to_le32(map->m_lblk);
···
 				goto out;
 			split_map.m_lblk = map->m_lblk;
 			split_map.m_len = allocated;
-		} else if ((map->m_lblk - ee_block + map->m_len <
-			   EXT4_EXT_ZERO_LEN) &&
-			   (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
+		} else if (map->m_lblk - ee_block + map->m_len < max_zeroout) {
 			/* case 2 */
 			if (map->m_lblk != ee_block) {
 				zero_ex.ee_block = ex->ee_block;
···
 	}
 
 	allocated = ext4_split_extent(handle, inode, path,
-				       &split_map, split_flag, 0);
+				      &split_map, split_flag, 0);
 	if (allocated < 0)
 		err = allocated;
 

fs/ext4/super.c

···
 EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
 EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
 EXT4_RW_ATTR_SBI_UI(max_writeback_mb_bump, s_max_writeback_mb_bump);
+EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
 EXT4_ATTR(trigger_fs_error, 0200, NULL, trigger_test_error);
 
 static struct attribute *ext4_attrs[] = {
···
 	ATTR_LIST(mb_stream_req),
 	ATTR_LIST(mb_group_prealloc),
 	ATTR_LIST(max_writeback_mb_bump),
+	ATTR_LIST(extent_max_zeroout_kb),
 	ATTR_LIST(trigger_fs_error),
 	NULL,
 };
···
 
 	sbi->s_stripe = ext4_get_stripe_size(sbi);
 	sbi->s_max_writeback_mb_bump = 128;
+	sbi->s_extent_max_zeroout_kb = 32;
 
 	/*
 	 * set up enough so that it can read an inode