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

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On 8/20/20 6:23 PM, Jan Kara wrote:
On Thu 20-08-20 17:06:28, Ritesh Harjani wrote:
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@xxxxxxxxx>
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>

Thanks for returning to this and I'm glad to see how much this helped :)
BTW, I'd suspect there could be also significant contention and cache line
bouncing on j_state_lock and transaction's atomic counters...

ok, will try and profile to see if this happens.



---
  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;

So the patch looks correct but using ext4_overwrite_io() seems a bit
foolish since under the hood it does ext4_map_blocks() only to be able to
decide whether to call ext4_map_blocks() once again with exactly the same
arguments :). So I'd rather slightly refactor the code in
ext4_iomap_begin() to avoid this double calling of ext4_map_blocks() for
the fast path.

Yes, agreed. Looking at the numbers I was excited to post out the RFC
for discussion. Will make above changes and post. :)


With DIO, we need to detect overwrite case early in
ext4_dio_write_iter() to determine whether we need shared or excl.
locks - so probably for DIO case we still need overwrite check in
ext4_dio_write_iter()

Thanks for review!!
-ritesh



								Honza

  	/*
  	 * 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




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