[RFC 1/1] ext4: Optimize ext4 DAX overwrites

[Ritesh Harjani <riteshh@linux.ibm.com>]

Currently in case of DAX, we are starting a transaction
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 random writes.

Reported-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
---
 fs/ext4/ext4.h  | 1 +
 fs/ext4/file.c  | 2 +-
 fs/ext4/inode.c | 8 +++++++-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 42f5060f3cdf..9a2138afc751 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3232,6 +3232,7 @@ extern const struct dentry_operations ext4_dentry_ops;
 extern const struct inode_operations ext4_file_inode_operations;
 extern const struct file_operations ext4_file_operations;
 extern loff_t ext4_llseek(struct file *file, loff_t offset, int origin);
+extern bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len);
 
 /* inline.c */
 extern int ext4_get_max_inline_size(struct inode *inode);
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 2a01e31a032c..51cd92ac1758 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -188,7 +188,7 @@ ext4_extending_io(struct inode *inode, loff_t offset, size_t len)
 }
 
 /* Is IO overwriting allocated and initialized blocks? */
-static bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len)
+bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len)
 {
 	struct ext4_map_blocks map;
 	unsigned int blkbits = inode->i_blkbits;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 10dd470876b3..f0ac0ee9e991 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3423,6 +3423,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	int ret;
 	struct ext4_map_blocks map;
 	u8 blkbits = inode->i_blkbits;
+	bool overwrite = false;
 
 	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
 		return -EINVAL;
@@ -3430,6 +3431,9 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	if (WARN_ON_ONCE(ext4_has_inline_data(inode)))
 		return -ERANGE;
 
+	if (IS_DAX(inode) && (flags & IOMAP_WRITE) &&
+	    ext4_overwrite_io(inode, offset, length))
+		overwrite = true;
 	/*
 	 * Calculate the first and last logical blocks respectively.
 	 */
@@ -3437,13 +3441,15 @@ 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;
 
-	if (flags & IOMAP_WRITE)
+	if ((flags & IOMAP_WRITE) && !overwrite)
 		ret = ext4_iomap_alloc(inode, &map, flags);
 	else
 		ret = ext4_map_blocks(NULL, inode, &map, 0);
 
 	if (ret < 0)
 		return ret;
+	if (IS_DAX(inode) && overwrite)
+		WARN_ON(!(map.m_flags & EXT4_MAP_MAPPED));
 
 	ext4_set_iomap(inode, iomap, &map, offset, length);
 
-- 
2.25.4