[PATCH 0/4] btrfs: some more speedups for directory logging/fsync

[PATCH 0/4] btrfs: some more speedups for directory logging/fsync

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

This patchset brings some more performance improvements for directory
logging/fsync, by doing some changes to the logging algorithm to avoid
logging dentries created in past transactions, which helps reducing a
lot the amount of logged metadata, and therefore less IO as well for
large directories.

It specially benefits the case when some dentries were removed, either
due to file deletes or renames, where it can reduce the total time spent
in an fsync by an order of magnitude even if the number of dentries removed
is a small percentage of the total dentries in the directory (for e.g. as
little as 1%, like in the test results of the changelog of patch 3/4).

This builds on top of my previous patchset to make directory logging copy
only dir index keys and skip dir item keys, which has been on misc-next
since the previous merge window closed.

Patches 1/4 and 3/4 are the changes that accomplish this, while patch 2/4
is just preparation for patch 3/4, and patch 4/4 is more of a cleanup of
old, unnecessary and unreliable logic. Test case generic/335 was recently
updated in fstests, so that after applying patch 4/4 it passes.

Patch 3/4 contains in its changelog the test and results.

We are close to the 5.17 merge window, holiday season is approaching and
there's already a significant change for directory logging coming to 5.17
(log only dir index keys and skip dir item keys), so I think this patchset
is better suited for the 5.18 merge window.

Thanks.

Filipe Manana (4):
  btrfs: don't log unnecessary boundary keys when logging directory
  btrfs: put initial index value of a directory in a constant
  btrfs: stop copying old dir items when logging a directory
  btrfs: stop trying to log subdirectories created in past transactions

 fs/btrfs/btrfs_inode.h |  12 +++-
 fs/btrfs/inode.c       |  10 +---
 fs/btrfs/tree-log.c    | 123 +++++++++++++++++++++++++----------------
 3 files changed, 86 insertions(+), 59 deletions(-)

-- 
2.33.0

[PATCH 1/4] btrfs: don't log unnecessary boundary keys when logging directory

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Before we start to log dir index keys from a leaf, we check if there is a
previous index key, which normally is at the end of a leaf that was not
changed in the current transaction. Then we log that key and set the start
of logged range (item of type BTRFS_DIR_LOG_INDEX_KEY) to the offset of
that key. This is to ensure that if there were deleted index keys between
that key and the first key we are going to log, those deletions are
replayed in case we need to replay to the log after a power failure.
However we really don't need to log that previous key, we can just set the
start of the logged range to that key's offset plus 1. This achieves the
same and avoids logging one dir index key.

The same logic is performed when we finish logging the index keys of a
leaf and we find that the next leaf has index keys and was not changed in
the current transaction. We are logging the first key of that next leaf
and use its offset as the end of range we log. This is just to ensure that
if there were deleted index keys between the last index key we logged and
the first key of that next leaf, those index keys are deleted if we end
up replaying the log. However that is not necessary, we can avoid logging
that first index key of the next leaf and instead set the end of the
logged range to match the offset of that index key minus 1.

So avoid logging those index keys at the boundaries and adjust the start
and end offsets of the logged ranges as described above.

This patch is part of a patchset comprised of the following patches:

  1/4 btrfs: don't log unnecessary boundary keys when logging directory
  2/4 btrfs: put initial index value of a directory in a constant
  3/4 btrfs: stop copying old dir items when logging a directory
  4/4 btrfs: stop trying to log subdirectories created in past transactions

Performance test results are listed in the changelog of patch 3/4.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 39 +++++++++++++++++++++------------------
 1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index c1ddbe800897..a80f14df5b81 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3883,17 +3883,18 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 	ret = btrfs_previous_item(root, path, ino, BTRFS_DIR_INDEX_KEY);
 	if (ret == 0) {
 		struct btrfs_key tmp;
+
 		btrfs_item_key_to_cpu(path->nodes[0], &tmp, path->slots[0]);
-		if (tmp.type == BTRFS_DIR_INDEX_KEY) {
-			first_offset = tmp.offset;
-			ret = overwrite_item(trans, log, dst_path,
-					     path->nodes[0], path->slots[0],
-					     &tmp);
-			if (ret) {
-				err = ret;
-				goto done;
-			}
-		}
+		/*
+		 * The dir index key before the first one we found that needs to
+		 * be logged might be in a previous leaf, and there might be a
+		 * gap between these keys, meaning that we had deletions that
+		 * happened. So the key range item we log (key type
+		 * BTRFS_DIR_LOG_INDEX_KEY) must cover a range that starts at the
+		 * previous key's offset plus 1, so that those deletes are replayed.
+		 */
+		if (tmp.type == BTRFS_DIR_INDEX_KEY)
+			first_offset = tmp.offset + 1;
 	}
 	btrfs_release_path(path);
 
@@ -3941,14 +3942,16 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 			goto done;
 		}
 		if (btrfs_header_generation(path->nodes[0]) != trans->transid) {
-			ctx->last_dir_item_offset = min_key.offset;
-			ret = overwrite_item(trans, log, dst_path,
-					     path->nodes[0], path->slots[0],
-					     &min_key);
-			if (ret)
-				err = ret;
-			else
-				last_offset = min_key.offset;
+			/*
+			 * The next leaf was not changed in the current transaction
+			 * and has at least one dir index key.
+			 * We check for the next key because there might have been
+			 * one or more deletions between the last key we logged and
+			 * that next key. So the key range item we log (key type
+			 * BTRFS_DIR_LOG_INDEX_KEY) must end at the next key's
+			 * offset minus 1, so that those deletes are replayed.
+			 */
+			last_offset = min_key.offset - 1;
 			goto done;
 		}
 		if (need_resched()) {
-- 
2.33.0

[PATCH 2/4] btrfs: put initial index value of a directory in a constant

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

At btrfs_set_inode_index_count() we refer twice to the number 2 as the
initial index value for a directory (when it's empty), with a proper
comment explaining the reason for that value. In the next patch I'll
have to use that magic value in the directory logging code, so put
the value in a #define at btrfs_inode.h, to avoid hardcoding the
magic value again at tree-log.c.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/btrfs_inode.h | 12 ++++++++++--
 fs/btrfs/inode.c       | 10 ++--------
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index b3e46aabc3d8..6d4f42b0fdce 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -13,6 +13,13 @@
 #include "ordered-data.h"
 #include "delayed-inode.h"
 
+/*
+ * Since we search a directory based on f_pos (struct dir_context::pos) we have
+ * to start at 2 since '.' and '..' have f_pos of 0 and 1 respectively, so
+ * everybody else has to start at 2 (see btrfs_real_readdir() and dir_emit_dots()).
+ */
+#define BTRFS_DIR_START_INDEX 2
+
 /*
  * ordered_data_close is set by truncate when a file that used
  * to have good data has been truncated to zero.  When it is set
@@ -173,8 +180,9 @@ struct btrfs_inode {
 	u64 disk_i_size;
 
 	/*
-	 * if this is a directory then index_cnt is the counter for the index
-	 * number for new files that are created
+	 * If this is a directory then index_cnt is the counter for the index
+	 * number for new files that are created. For an empty directory, this
+	 * must be initialized to BTRFS_DIR_START_INDEX.
 	 */
 	u64 index_cnt;
 
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a88130c7782e..97fd33d599eb 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5971,14 +5971,8 @@ static int btrfs_set_inode_index_count(struct btrfs_inode *inode)
 		goto out;
 	ret = 0;
 
-	/*
-	 * MAGIC NUMBER EXPLANATION:
-	 * since we search a directory based on f_pos we have to start at 2
-	 * since '.' and '..' have f_pos of 0 and 1 respectively, so everybody
-	 * else has to start at 2
-	 */
 	if (path->slots[0] == 0) {
-		inode->index_cnt = 2;
+		inode->index_cnt = BTRFS_DIR_START_INDEX;
 		goto out;
 	}
 
@@ -5989,7 +5983,7 @@ static int btrfs_set_inode_index_count(struct btrfs_inode *inode)
 
 	if (found_key.objectid != btrfs_ino(inode) ||
 	    found_key.type != BTRFS_DIR_INDEX_KEY) {
-		inode->index_cnt = 2;
+		inode->index_cnt = BTRFS_DIR_START_INDEX;
 		goto out;
 	}
 
-- 
2.33.0

[PATCH 3/4] btrfs: stop copying old dir items when logging a directory

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

When logging a directory, we go over every leaf of the subvolume tree that
was changed in the current transaction and copy all its dir index keys to
the log tree.

That includes copying dir index keys created in past transactions. This is
done mostly for simplicity, as after logging the keys we log an item that
specifies the start and end ranges of the keys we logged. That item is
then used during log replay to figure out which keys need to be deleted -
every key in that range that we find in the subvolume tree and is not in
the log tree, needs to be deleted.

Now that we log only dir index keys, and not dir item keys anymore, when
we remove dentries from a directory (due to unlink and rename operations),
we can get entire leaves that we changed only for deleting old dir index
keys, or that have few dir index keys that are new - this is due to the
fact that the offset for new index keys comes from a monotonically
increasing counter.

We can avoid logging dir index keys from past transactions, and in order
to track the deletions, only log range items (BTRFS_DIR_LOG_INDEX_KEY key
type) when we find gaps between consecutive index keys. This massively
reduces the amount of logged metadata when we have deleted directory
entries, even if it's a small percentage of the total number of entries.
The reduction comes from both less items that are logged and instead of
logging many dir index items (struct btrfs_dir_item), which have a size
of 30 bytes plus a file name, we typically log just a few range items
(struct btrfs_dir_log_item), which take only 8 bytes each.

Even if no entries were deleted from a directory and only new entries
were added, we typically still get a reduction on the amount of logged
metadata, because it's very likely the first leaf that got the new
dir index entries also has several old dir index entries.

So change the logging logic to not log dir index keys created in past
transactions and log a range item for every gap it finds between each
pair of consecutive index keys, to ensure deletions are tracked and
replayed on log replay.

This patch is part of a patchset comprised of the following patches:

 1/4 btrfs: don't log unnecessary boundary keys when logging directory
 2/4 btrfs: put initial index value of a directory in a constant
 3/4 btrfs: stop copying old dir items when logging a directory
 4/4 btrfs: stop trying to log subdirectories created in past transactions

The following test was run on a branch without this patchset and on a
branch with the first three patches applied:

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/nvme0n1
  MNT=/mnt/nvme0n1

  NUM_FILES=1000000
  NUM_FILE_DELETES=10000

  MKFS_OPTIONS="-O no-holes -R free-space-tree"
  MOUNT_OPTIONS="-o ssd"

  mkfs.btrfs -f $MKFS_OPTIONS $DEV
  mount $MOUNT_OPTIONS $DEV $MNT

  mkdir $MNT/testdir
  for ((i = 1; i <= $NUM_FILES; i++)); do
      echo -n > $MNT/testdir/file_$i
  done

  sync

  del_inc=$(( $NUM_FILES / $NUM_FILE_DELETES ))
  for ((i = 1; i <= $NUM_FILES; i += $del_inc)); do
      rm -f $MNT/testdir/file_$i
  done

  start=$(date +%s%N)
  xfs_io -c "fsync" $MNT/testdir
  end=$(date +%s%N)

  dur=$(( (end - start) / 1000000 ))
  echo "dir fsync took $dur ms after deleting $NUM_FILE_DELETES files"
  echo

  umount $MNT

The test was run on a non-debug kernel (Debian's default kernel config),
and the results were the following for various values of NUM_FILES and
NUM_FILE_DELETES:

** before, NUM_FILES = 1 000 000, NUM_FILE_DELETES = 10 000 **

dir fsync took 585 ms after deleting 10000 files

** after, NUM_FILES = 1 000 000, NUM_FILE_DELETES = 10 000 **

dir fsync took 34 ms after deleting 10000 files   (-94.2%)

** before, NUM_FILES = 100 000, NUM_FILE_DELETES = 1 000 **

dir fsync took 50 ms after deleting 1000 files

** after, NUM_FILES = 100 000, NUM_FILE_DELETES = 1 000 **

dir fsync took 7 ms after deleting 1000 files    (-86.0%)

** before, NUM_FILES = 10 000, NUM_FILE_DELETES = 100 **

dir fsync took 9 ms after deleting 100 files

** after, NUM_FILES = 10 000, NUM_FILE_DELETES = 100 **

dir fsync took 5 ms after deleting 100 files     (-44.4%)

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 83 +++++++++++++++++++++++++++++----------------
 1 file changed, 53 insertions(+), 30 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index a80f14df5b81..7c239fedf192 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3702,7 +3702,8 @@ static int process_dir_items_leaf(struct btrfs_trans_handle *trans,
 				  struct btrfs_inode *inode,
 				  struct btrfs_path *path,
 				  struct btrfs_path *dst_path,
-				  struct btrfs_log_ctx *ctx)
+				  struct btrfs_log_ctx *ctx,
+				  u64 *last_old_dentry_offset)
 {
 	struct btrfs_root *log = inode->root->log_root;
 	struct extent_buffer *src = path->nodes[0];
@@ -3715,6 +3716,7 @@ static int process_dir_items_leaf(struct btrfs_trans_handle *trans,
 	int i;
 
 	for (i = path->slots[0]; i < nritems; i++) {
+		struct btrfs_dir_item *di;
 		struct btrfs_key key;
 		int ret;
 
@@ -3725,7 +3727,34 @@ static int process_dir_items_leaf(struct btrfs_trans_handle *trans,
 			break;
 		}
 
+		di = btrfs_item_ptr(src, i, struct btrfs_dir_item);
 		ctx->last_dir_item_offset = key.offset;
+
+		/*
+		 * Skip ranges of items that consist only of dir item keys created
+		 * in past transactions. However if we find a gap, we must log a
+		 * dir index range item for that gap, so that index keys in that
+		 * gap are deleted during log replay.
+		 */
+		if (btrfs_dir_transid(src, di) < trans->transid) {
+			if (key.offset > *last_old_dentry_offset + 1) {
+				ret = insert_dir_log_key(trans, log, dst_path,
+						 ino, *last_old_dentry_offset + 1,
+						 key.offset - 1);
+				/*
+				 * -EEXIST should never happen because when we
+				 * log a directory in full mode (LOG_INODE_ALL)
+				 * we drop all BTRFS_DIR_LOG_INDEX_KEY keys from
+				 * the log tree.
+				 */
+				ASSERT(ret != -EEXIST);
+				if (ret < 0)
+					return ret;
+			}
+
+			*last_old_dentry_offset = key.offset;
+			continue;
+		}
 		/*
 		 * We must make sure that when we log a directory entry, the
 		 * corresponding inode, after log replay, has a matching link
@@ -3749,14 +3778,10 @@ static int process_dir_items_leaf(struct btrfs_trans_handle *trans,
 		 * resulting in -ENOTEMPTY errors.
 		 */
 		if (!ctx->log_new_dentries) {
-			struct btrfs_dir_item *di;
 			struct btrfs_key di_key;
 
-			di = btrfs_item_ptr(src, i, struct btrfs_dir_item);
 			btrfs_dir_item_key_to_cpu(src, di, &di_key);
-			if ((btrfs_dir_transid(src, di) == trans->transid ||
-			     btrfs_dir_type(src, di) == BTRFS_FT_DIR) &&
-			    di_key.type != BTRFS_ROOT_ITEM_KEY)
+			if (di_key.type != BTRFS_ROOT_ITEM_KEY)
 				ctx->log_new_dentries = true;
 		}
 
@@ -3837,7 +3862,7 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 	struct btrfs_root *log = root->log_root;
 	int err = 0;
 	int ret;
-	u64 first_offset = min_offset;
+	u64 last_old_dentry_offset = min_offset - 1;
 	u64 last_offset = (u64)-1;
 	u64 ino = btrfs_ino(inode);
 
@@ -3871,10 +3896,11 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 		 */
 		if (ret == 0) {
 			struct btrfs_key tmp;
+
 			btrfs_item_key_to_cpu(path->nodes[0], &tmp,
 					      path->slots[0]);
 			if (tmp.type == BTRFS_DIR_INDEX_KEY)
-				first_offset = max(min_offset, tmp.offset) + 1;
+				last_old_dentry_offset = tmp.offset;
 		}
 		goto done;
 	}
@@ -3894,7 +3920,7 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 		 * previous key's offset plus 1, so that those deletes are replayed.
 		 */
 		if (tmp.type == BTRFS_DIR_INDEX_KEY)
-			first_offset = tmp.offset + 1;
+			last_old_dentry_offset = tmp.offset;
 	}
 	btrfs_release_path(path);
 
@@ -3916,7 +3942,8 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 	 * from our directory
 	 */
 	while (1) {
-		ret = process_dir_items_leaf(trans, inode, path, dst_path, ctx);
+		ret = process_dir_items_leaf(trans, inode, path, dst_path, ctx,
+					     &last_old_dentry_offset);
 		if (ret != 0) {
 			if (ret < 0)
 				err = ret;
@@ -3967,13 +3994,21 @@ static noinline int log_dir_items(struct btrfs_trans_handle *trans,
 	if (err == 0) {
 		*last_offset_ret = last_offset;
 		/*
-		 * insert the log range keys to indicate where the log
-		 * is valid
+		 * In case the leaf was changed in the current transaction but
+		 * all its dir items are from a past transaction, the last item
+		 * in the leaf is a dir item and there's no gap between that last
+		 * dir item and the first one on the next leaf (which did not
+		 * change in the current transaction), then we don't need to log
+		 * a range, last_old_dentry_offset is == to last_offset.
 		 */
-		ret = insert_dir_log_key(trans, log, path, ino, first_offset,
-					 last_offset);
-		if (ret)
-			err = ret;
+		ASSERT(last_old_dentry_offset <= last_offset);
+		if (last_old_dentry_offset < last_offset) {
+			ret = insert_dir_log_key(trans, log, path, ino,
+						 last_old_dentry_offset + 1,
+						 last_offset);
+			if (ret)
+				err = ret;
+		}
 	}
 	return err;
 }
@@ -4015,7 +4050,7 @@ static noinline int log_directory_changes(struct btrfs_trans_handle *trans,
 	if (inode->logged_trans != trans->transid)
 		inode->last_dir_index_offset = (u64)-1;
 
-	min_key = 0;
+	min_key = BTRFS_DIR_START_INDEX;
 	max_key = 0;
 	ctx->last_dir_item_offset = inode->last_dir_index_offset;
 
@@ -5869,7 +5904,6 @@ static int log_new_dir_dentries(struct btrfs_trans_handle *trans,
 				struct btrfs_log_ctx *ctx)
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
-	struct btrfs_root *log = root->log_root;
 	struct btrfs_path *path;
 	LIST_HEAD(dir_list);
 	struct btrfs_dir_list *dir_elem;
@@ -5911,7 +5945,7 @@ static int log_new_dir_dentries(struct btrfs_trans_handle *trans,
 		min_key.offset = 0;
 again:
 		btrfs_release_path(path);
-		ret = btrfs_search_forward(log, &min_key, path, trans->transid);
+		ret = btrfs_search_forward(root, &min_key, path, trans->transid);
 		if (ret < 0) {
 			goto next_dir_inode;
 		} else if (ret > 0) {
@@ -5919,7 +5953,6 @@ static int log_new_dir_dentries(struct btrfs_trans_handle *trans,
 			goto next_dir_inode;
 		}
 
-process_leaf:
 		leaf = path->nodes[0];
 		nritems = btrfs_header_nritems(leaf);
 		for (i = path->slots[0]; i < nritems; i++) {
@@ -5976,16 +6009,6 @@ static int log_new_dir_dentries(struct btrfs_trans_handle *trans,
 			}
 			break;
 		}
-		if (i == nritems) {
-			ret = btrfs_next_leaf(log, path);
-			if (ret < 0) {
-				goto next_dir_inode;
-			} else if (ret > 0) {
-				ret = 0;
-				goto next_dir_inode;
-			}
-			goto process_leaf;
-		}
 		if (min_key.offset < (u64)-1) {
 			min_key.offset++;
 			goto again;
-- 
2.33.0

[PATCH 4/4] btrfs: stop trying to log subdirectories created in past transactions

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

When logging a directory we are trying to log subdirectories that were
changed in the current transaction and created in a past transaction.
This type of behaviour was introduced by commit 2f2ff0ee5e4303 ("Btrfs:
fix metadata inconsistencies after directory fsync"), to fix some metadata
inconsistencies that in the meanwhile no longer need this behaviour due to
numerous other changes that happened throughout the years.

This behaviour, besides not needed anymore, it's also undesirable because:

1) It's not reliable because it's only triggered for the directories
   of dentries (dir items) that happen to be present on a leaf that
   was changed in the current transaction. If a dentry that points to
   a directory resides on a leaf that was not changed in the current
   transaction, then it's not logged, as at log_dir_items() and
   log_new_dir_dentries() we use btrfs_search_forward();

2) It's not required by posix or any standard, it's undefined territory.
   The only way to guarantee a subdirectory is logged, it to explicitly
   fsync it;

Making the behaviour guaranteed would require scanning all directory
items, check which point to a directory, and then fsync each subdirectory
which was modified in the current transaction. This could be very
expensive for large directories with many subdirectories and/or large
subdirectories.

So remove that obsolete logic.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 7c239fedf192..4b89ac769347 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -5970,8 +5970,7 @@ static int log_new_dir_dentries(struct btrfs_trans_handle *trans,
 
 			di = btrfs_item_ptr(leaf, i, struct btrfs_dir_item);
 			type = btrfs_dir_type(leaf, di);
-			if (btrfs_dir_transid(leaf, di) < trans->transid &&
-			    type != BTRFS_FT_DIR)
+			if (btrfs_dir_transid(leaf, di) < trans->transid)
 				continue;
 			btrfs_dir_item_key_to_cpu(leaf, di, &di_key);
 			if (di_key.type == BTRFS_ROOT_ITEM_KEY)
-- 
2.33.0

Re: [PATCH 0/4] btrfs: some more speedups for directory logging/fsync

[David Sterba]

On Wed, Dec 15, 2021 at 12:19:57PM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> This patchset brings some more performance improvements for directory
> logging/fsync, by doing some changes to the logging algorithm to avoid
> logging dentries created in past transactions, which helps reducing a
> lot the amount of logged metadata, and therefore less IO as well for
> large directories.
> 
> It specially benefits the case when some dentries were removed, either
> due to file deletes or renames, where it can reduce the total time spent
> in an fsync by an order of magnitude even if the number of dentries removed
> is a small percentage of the total dentries in the directory (for e.g. as
> little as 1%, like in the test results of the changelog of patch 3/4).
> 
> This builds on top of my previous patchset to make directory logging copy
> only dir index keys and skip dir item keys, which has been on misc-next
> since the previous merge window closed.
> 
> Patches 1/4 and 3/4 are the changes that accomplish this, while patch 2/4
> is just preparation for patch 3/4, and patch 4/4 is more of a cleanup of
> old, unnecessary and unreliable logic. Test case generic/335 was recently
> updated in fstests, so that after applying patch 4/4 it passes.
> 
> Patch 3/4 contains in its changelog the test and results.
> 
> We are close to the 5.17 merge window, holiday season is approaching and
> there's already a significant change for directory logging coming to 5.17
> (log only dir index keys and skip dir item keys), so I think this patchset
> is better suited for the 5.18 merge window.

The for-5.17 has been branched, so this patchset is now in misc-next,
thanks.