[PATCHv2 3/3] ext4: Optimize ext4 DAX overwrites

[Ritesh Harjani]

Currently in case of DAX, we are starting a journal txn everytime for
IOMAP_WRITE case. This can be optimized away in case of an overwrite
(where the blocks were already allocated).

This could give a significant performance boost for multi-threaded writes
specially random writes.
On PPC64 VM with simulated pmem device, ~10x perf improvement could be
seen in random writes (overwrite). Also bcoz this optimizes away the
spinlock contention during jbd2 slab cache allocation (jbd2_journal_handle)
On x86 VM, ~2x perf improvement was observed.

Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
 fs/ext4/inode.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 10dd470876b3..c18009c91e68 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3437,6 +3437,14 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
 			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
 
+	/*
+	 * In case of DAX write, we check if this is overwrite request, to avoid
+	 * starting a journal txn in ext4_iomap_alloc()
+	 */
+	if ((flags & IOMAP_WRITE) && IS_DAX(inode) &&
+	    ext4_overwrite_io(inode, &map, true))
+		goto out_set;
+
 	if (flags & IOMAP_WRITE)
 		ret = ext4_iomap_alloc(inode, &map, flags);
 	else
@@ -3444,9 +3452,8 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 
 	if (ret < 0)
 		return ret;
-
+out_set:
 	ext4_set_iomap(inode, iomap, &map, offset, length);
-
 	return 0;
 }
 
-- 
2.25.4