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 <zab@zabbo.net>
CC: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

3 files changed, 19 insertions, 12 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 7c0841ecde6..0df5ee102b6 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1271,6 +1271,9 @@ struct ext4_sb_info {
         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;
 
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e8755c21f4b..2f082abf499 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3085,7 +3085,6 @@ out:
         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
@@ -3111,13 +3110,14 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
                                            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;
 
@@ -3125,6 +3125,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
                 "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)
@@ -3224,9 +3225,12 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
          */
         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;
@@ -3250,9 +3254,8 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
         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);
@@ -3264,9 +3267,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
                                 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;
@@ -3286,7 +3287,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
         }
 
         allocated = ext4_split_extent(handle, inode, path,
-                                       &split_map, split_flag, 0);
+                                      &split_map, split_flag, 0);
         if (allocated < 0)
                 err = allocated;
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5a97e590692..0423e2e7f61 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2541,6 +2541,7 @@ EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
 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[] = {
@@ -2556,6 +2557,7 @@ 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,
 };
@@ -3756,6 +3758,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 
         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