[PATCH v2] ext4: change sequential discard handling on commit complete phase into parallel manner

[PATCH v2] ext4: change sequential discard handling on commit complete phase into parallel manner

[Daeho Jeong]

Now, when we mount ext4 filesystem with '-o discard' option, we have to
issue all the discard commands for the blocks to be deallocated and
wait for the completion of the commands on the commit complete phase.
Because this procedure might involve a lot of sequential combinations of
issuing discard commands and waiting for that, the delay of this
procedure might be too much long, even to half a minute in our test,
and it results in long commit delay and fsync() performance degradation.

When we converted this sequential discard handling on commit complete
phase into a parallel manner like XFS filesystem, we could enhance the
discard command handling performance. The result was such that 17.0s
delay of a single commit in the worst case has been enhanced to 4.8s.

Signed-off-by: Daeho Jeong <daeho.jeong@samsung.com>
Tested-by: Hobin Woo <hobin.woo@samsung.com>
Tested-by: Kitae Lee <kitae87.lee@samsung.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/ext4.h    |   3 ++
 fs/ext4/mballoc.c | 122 ++++++++++++++++++++++++++++++++++++++++--------------
 fs/ext4/mballoc.h |   9 ++--
 fs/ext4/super.c   |   3 ++
 4 files changed, 102 insertions(+), 35 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index fb69ee2..7dce2cf 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1444,6 +1444,8 @@ struct ext4_sb_info {
 	unsigned int *s_mb_maxs;
 	unsigned int s_group_info_size;
 	unsigned int s_mb_free_pending;
+	struct list_head s_freed_data_list;	/* List of blocks to be freed
+						   after commit completed */
 
 	/* tunables */
 	unsigned long s_stripe;
@@ -2434,6 +2436,7 @@ extern int ext4_mb_add_groupinfo(struct super_block *sb,
 extern int ext4_group_add_blocks(handle_t *handle, struct super_block *sb,
 				ext4_fsblk_t block, unsigned long count);
 extern int ext4_trim_fs(struct super_block *, struct fstrim_range *);
+extern void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid);
 
 /* inode.c */
 int ext4_inode_is_fast_symlink(struct inode *inode);
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 354dc1a..ff3d037 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -367,8 +367,6 @@ static void ext4_mb_generate_from_pa(struct super_block *sb, void *bitmap,
 					ext4_group_t group);
 static void ext4_mb_generate_from_freelist(struct super_block *sb, void *bitmap,
 						ext4_group_t group);
-static void ext4_free_data_callback(struct super_block *sb,
-				struct ext4_journal_cb_entry *jce, int rc);
 
 static inline void *mb_correct_addr_and_bit(int *bit, void *addr)
 {
@@ -2639,6 +2637,7 @@ int ext4_mb_init(struct super_block *sb)
 	spin_lock_init(&sbi->s_md_lock);
 	spin_lock_init(&sbi->s_bal_lock);
 	sbi->s_mb_free_pending = 0;
+	INIT_LIST_HEAD(&sbi->s_freed_data_list);
 
 	sbi->s_mb_max_to_scan = MB_DEFAULT_MAX_TO_SCAN;
 	sbi->s_mb_min_to_scan = MB_DEFAULT_MIN_TO_SCAN;
@@ -2782,7 +2781,8 @@ int ext4_mb_release(struct super_block *sb)
 }
 
 static inline int ext4_issue_discard(struct super_block *sb,
-		ext4_group_t block_group, ext4_grpblk_t cluster, int count)
+		ext4_group_t block_group, ext4_grpblk_t cluster, int count,
+		struct bio **biop)
 {
 	ext4_fsblk_t discard_block;
 
@@ -2791,18 +2791,18 @@ static inline int ext4_issue_discard(struct super_block *sb,
 	count = EXT4_C2B(EXT4_SB(sb), count);
 	trace_ext4_discard_blocks(sb,
 			(unsigned long long) discard_block, count);
-	return sb_issue_discard(sb, discard_block, count, GFP_NOFS, 0);
+	if (biop) {
+		return __blkdev_issue_discard(sb->s_bdev,
+				discard_block << (sb->s_blocksize_bits - 9),
+				count << (sb->s_blocksize_bits - 9),
+				GFP_NOFS, 0, biop);
+	} else
+		return sb_issue_discard(sb, discard_block, count, GFP_NOFS, 0);
 }
 
-/*
- * This function is called by the jbd2 layer once the commit has finished,
- * so we know we can free the blocks that were released with that commit.
- */
-static void ext4_free_data_callback(struct super_block *sb,
-				    struct ext4_journal_cb_entry *jce,
-				    int rc)
+static void ext4_free_data_in_buddy(struct super_block *sb,
+				    struct ext4_free_data *entry)
 {
-	struct ext4_free_data *entry = (struct ext4_free_data *)jce;
 	struct ext4_buddy e4b;
 	struct ext4_group_info *db;
 	int err, count = 0, count2 = 0;
@@ -2810,18 +2810,6 @@ static void ext4_free_data_callback(struct super_block *sb,
 	mb_debug(1, "gonna free %u blocks in group %u (0x%p):",
 		 entry->efd_count, entry->efd_group, entry);
 
-	if (test_opt(sb, DISCARD)) {
-		err = ext4_issue_discard(sb, entry->efd_group,
-					 entry->efd_start_cluster,
-					 entry->efd_count);
-		if (err && err != -EOPNOTSUPP)
-			ext4_msg(sb, KERN_WARNING, "discard request in"
-				 " group:%d block:%d count:%d failed"
-				 " with %d", entry->efd_group,
-				 entry->efd_start_cluster,
-				 entry->efd_count, err);
-	}
-
 	err = ext4_mb_load_buddy(sb, entry->efd_group, &e4b);
 	/* we expect to find existing buddy because it's pinned */
 	BUG_ON(err != 0);
@@ -2862,6 +2850,62 @@ static void ext4_free_data_callback(struct super_block *sb,
 	mb_debug(1, "freed %u blocks in %u structures\n", count, count2);
 }
 
+/*
+ * This function is called by the jbd2 layer once the commit has finished,
+ * so we know we can free the blocks that were released with that commit.
+ */
+void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	struct ext4_free_data *entry;
+	struct bio *discard_bio = NULL;
+	struct list_head freed_data_list;
+	struct list_head *cur;
+	struct list_head *tmp;
+	int err;
+
+	INIT_LIST_HEAD(&freed_data_list);
+
+	spin_lock(&sbi->s_md_lock);
+	list_for_each_safe(cur, tmp, &sbi->s_freed_data_list) {
+		entry = list_entry(cur, struct ext4_free_data, efd_list);
+		if (entry->efd_tid != commit_tid)
+			break;
+		entry->efd_freed = true;
+		list_del(&entry->efd_list);
+		list_add_tail(&entry->efd_list, &freed_data_list);
+	}
+	spin_unlock(&sbi->s_md_lock);
+
+	if (test_opt(sb, DISCARD)) {
+		list_for_each(cur, &freed_data_list) {
+			entry = list_entry(cur, struct ext4_free_data,
+					   efd_list);
+
+			err = ext4_issue_discard(sb, entry->efd_group,
+						 entry->efd_start_cluster,
+						 entry->efd_count,
+						 &discard_bio);
+			if (err && err != -EOPNOTSUPP) {
+				ext4_msg(sb, KERN_WARNING, "discard request in"
+					 " group:%d block:%d count:%d failed"
+					 " with %d", entry->efd_group,
+					 entry->efd_start_cluster,
+					 entry->efd_count, err);
+			} else if (err == -EOPNOTSUPP)
+				break;
+		}
+
+		if (discard_bio)
+			submit_bio_wait(discard_bio);
+	}
+
+	list_for_each_safe(cur, tmp, &freed_data_list) {
+		entry = list_entry(cur, struct ext4_free_data, efd_list);
+		ext4_free_data_in_buddy(sb, entry);
+	}
+}
+
 int __init ext4_init_mballoc(void)
 {
 	ext4_pspace_cachep = KMEM_CACHE(ext4_prealloc_space,
@@ -4588,6 +4632,21 @@ static int can_merge(struct ext4_free_data *entry1,
 	return 0;
 }
 
+static inline bool ext4_freed_data_try_del(struct ext4_sb_info *sbi,
+					   struct ext4_free_data *entry)
+{
+	bool deleted = false;
+
+	spin_lock(&sbi->s_md_lock);
+	if (!entry->efd_freed) {
+		entry->efd_freed = true;
+		list_del(&entry->efd_list);
+		deleted = true;
+	}
+	spin_unlock(&sbi->s_md_lock);
+	return deleted;
+}
+
 static noinline_for_stack int
 ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,
 		      struct ext4_free_data *new_entry)
@@ -4642,7 +4701,7 @@ static int can_merge(struct ext4_free_data *entry1,
 	if (node) {
 		entry = rb_entry(node, struct ext4_free_data, efd_node);
 		if (can_merge(entry, new_entry) &&
-		    ext4_journal_callback_try_del(handle, &entry->efd_jce)) {
+		    ext4_freed_data_try_del(sbi, entry)) {
 			new_entry->efd_start_cluster = entry->efd_start_cluster;
 			new_entry->efd_count += entry->efd_count;
 			rb_erase(node, &(db->bb_free_root));
@@ -4654,16 +4713,15 @@ static int can_merge(struct ext4_free_data *entry1,
 	if (node) {
 		entry = rb_entry(node, struct ext4_free_data, efd_node);
 		if (can_merge(new_entry, entry) &&
-		    ext4_journal_callback_try_del(handle, &entry->efd_jce)) {
+		    ext4_freed_data_try_del(sbi, entry)) {
 			new_entry->efd_count += entry->efd_count;
 			rb_erase(node, &(db->bb_free_root));
 			kmem_cache_free(ext4_free_data_cachep, entry);
 		}
 	}
-	/* Add the extent to transaction's private list */
-	new_entry->efd_jce.jce_func = ext4_free_data_callback;
+
 	spin_lock(&sbi->s_md_lock);
-	_ext4_journal_callback_add(handle, &new_entry->efd_jce);
+	list_add_tail(&new_entry->efd_list, &sbi->s_freed_data_list);
 	sbi->s_mb_free_pending += clusters;
 	spin_unlock(&sbi->s_md_lock);
 	return 0;
@@ -4856,6 +4914,7 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
 		new_entry->efd_group = block_group;
 		new_entry->efd_count = count_clusters;
 		new_entry->efd_tid = handle->h_transaction->t_tid;
+		new_entry->efd_freed = false;
 
 		ext4_lock_group(sb, block_group);
 		mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
@@ -4866,7 +4925,8 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
 		 * them with group lock_held
 		 */
 		if (test_opt(sb, DISCARD)) {
-			err = ext4_issue_discard(sb, block_group, bit, count);
+			err = ext4_issue_discard(sb, block_group, bit, count,
+						 NULL);
 			if (err && err != -EOPNOTSUPP)
 				ext4_msg(sb, KERN_WARNING, "discard request in"
 					 " group:%d block:%d count:%lu failed"
@@ -5089,7 +5149,7 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
 	 */
 	mb_mark_used(e4b, &ex);
 	ext4_unlock_group(sb, group);
-	ret = ext4_issue_discard(sb, group, start, count);
+	ret = ext4_issue_discard(sb, group, start, count, NULL);
 	ext4_lock_group(sb, group);
 	mb_free_blocks(NULL, e4b, start, ex.fe_len);
 	return ret;
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 1aba469..f762509 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -78,10 +78,8 @@
 
 
 struct ext4_free_data {
-	/* MUST be the first member */
-	struct ext4_journal_cb_entry	efd_jce;
-
-	/* ext4_free_data private data starts from here */
+	/* this links the free block information from sb_info */
+	struct list_head		efd_list;
 
 	/* this links the free block information from group_info */
 	struct rb_node			efd_node;
@@ -95,6 +93,9 @@ struct ext4_free_data {
 
 	/* transaction which freed this extent */
 	tid_t				efd_tid;
+
+	/* indicate whether this is already handled */
+	bool				efd_freed;
 };
 
 struct ext4_prealloc_space {
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index a9448db..bf95387 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -371,6 +371,9 @@ static void ext4_journal_commit_callback(journal_t *journal, transaction_t *txn)
 	struct ext4_journal_cb_entry	*jce;
 
 	BUG_ON(txn->t_state == T_FINISHED);
+
+	ext4_process_freed_data(sb, txn->t_tid);
+
 	spin_lock(&sbi->s_md_lock);
 	while (!list_empty(&txn->t_private_list)) {
 		jce = list_entry(txn->t_private_list.next,
-- 
1.9.1

Re: [PATCH v2] ext4: change sequential discard handling on commit complete phase into parallel manner

[Jan Kara]

On Thu 08-06-17 10:44:48, Daeho Jeong wrote:
> Now, when we mount ext4 filesystem with '-o discard' option, we have to
> issue all the discard commands for the blocks to be deallocated and
> wait for the completion of the commands on the commit complete phase.
> Because this procedure might involve a lot of sequential combinations of
> issuing discard commands and waiting for that, the delay of this
> procedure might be too much long, even to half a minute in our test,
> and it results in long commit delay and fsync() performance degradation.
> 
> When we converted this sequential discard handling on commit complete
> phase into a parallel manner like XFS filesystem, we could enhance the
> discard command handling performance. The result was such that 17.0s
> delay of a single commit in the worst case has been enhanced to 4.8s.

Hmm, here you speak about 17s, in the above paragraph about half a minute
so what was it? Just curious...

> Signed-off-by: Daeho Jeong <daeho.jeong@samsung.com>
> Tested-by: Hobin Woo <hobin.woo@samsung.com>
> Tested-by: Kitae Lee <kitae87.lee@samsung.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

Actually, I didn't give my 'Reviewed-by' tag yet... But the patch looks
generally good, some smaller comments below.

> @@ -2862,6 +2850,62 @@ static void ext4_free_data_callback(struct super_block *sb,
>  	mb_debug(1, "freed %u blocks in %u structures\n", count, count2);
>  }
>  
> +/*
> + * This function is called by the jbd2 layer once the commit has finished,
> + * so we know we can free the blocks that were released with that commit.
> + */
> +void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
> +{
> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> +	struct ext4_free_data *entry;
> +	struct bio *discard_bio = NULL;
> +	struct list_head freed_data_list;
> +	struct list_head *cur;
> +	struct list_head *tmp;
> +	int err;
> +
> +	INIT_LIST_HEAD(&freed_data_list);
> +
> +	spin_lock(&sbi->s_md_lock);
> +	list_for_each_safe(cur, tmp, &sbi->s_freed_data_list) {
> +		entry = list_entry(cur, struct ext4_free_data, efd_list);

Use list_for_each_entry_safe() instead of explicit list_entry() call.

> +		if (entry->efd_tid != commit_tid)
> +			break;
> +		entry->efd_freed = true;
> +		list_del(&entry->efd_list);
> +		list_add_tail(&entry->efd_list, &freed_data_list);
> +	}
> +	spin_unlock(&sbi->s_md_lock);

Just find the right entry in the above loop and then use
list_cut_position() to split the list as needed. That saves us having to
remove and add each entry. Also you don't have to use _safe() list entry
iteration in that case.

> +
> +	if (test_opt(sb, DISCARD)) {
> +		list_for_each(cur, &freed_data_list) {
> +			entry = list_entry(cur, struct ext4_free_data,
> +					   efd_list);

Again use list_for_each_entry().

> +
> +			err = ext4_issue_discard(sb, entry->efd_group,
> +						 entry->efd_start_cluster,
> +						 entry->efd_count,
> +						 &discard_bio);
> +			if (err && err != -EOPNOTSUPP) {
> +				ext4_msg(sb, KERN_WARNING, "discard request in"
> +					 " group:%d block:%d count:%d failed"
> +					 " with %d", entry->efd_group,
> +					 entry->efd_start_cluster,
> +					 entry->efd_count, err);
> +			} else if (err == -EOPNOTSUPP)
> +				break;
> +		}
> +
> +		if (discard_bio)
> +			submit_bio_wait(discard_bio);
> +	}
> +
> +	list_for_each_safe(cur, tmp, &freed_data_list) {
> +		entry = list_entry(cur, struct ext4_free_data, efd_list);

And here use list_for_each_entry_safe().

> +		ext4_free_data_in_buddy(sb, entry);
> +	}
> +}
> +
>  int __init ext4_init_mballoc(void)
>  {
>  	ext4_pspace_cachep = KMEM_CACHE(ext4_prealloc_space,
> @@ -4588,6 +4632,21 @@ static int can_merge(struct ext4_free_data *entry1,
>  	return 0;
>  }
>  
> +static inline bool ext4_freed_data_try_del(struct ext4_sb_info *sbi,
> +					   struct ext4_free_data *entry)
> +{
> +	bool deleted = false;
> +
> +	spin_lock(&sbi->s_md_lock);
> +	if (!entry->efd_freed) {
> +		entry->efd_freed = true;
> +		list_del(&entry->efd_list);
> +		deleted = true;
> +	}
> +	spin_unlock(&sbi->s_md_lock);
> +	return deleted;
> +}

efd_freed can never be true here - see my explanation at the end. So you
can simplify the code by implementing something like:

void ext4_try_merge_freed_extent(struct ext4_sb_info *sbi,
				 struct ext4_free_data *entry,
				 struct ext4_free_data *new_entry)
{
	if ((entry->efd_tid != new_entry->efd_tid) ||
            (entry->efd_group != new_entry->efd_group))
		return;
	if (entry->efd_start_cluster + entry->efd_count ==
	    new_entry->efd_start_cluster) {
		new_entry->efd_start_cluster = entry->efd_start_cluster;
		new_entry->efd_count += entry->efd_count;
	} else if (new_entry->efd_start_cluster + new_entry->efd_count ==
		   entry->efd_start_cluster) {
		new_entry->efd_count += entry->efd_count;
	}
	spin_lock(&sbi->s_md_lock);
	list_del(&entry->efd_list);
	spin_unlock(&sbi->s_md_lock);
	rb_erase(entry->efd_node, &(db->bb_free_root));
	kmem_cache_free(ext4_free_data_cachep, entry);
}

which should handle all that is needed in one place.

> +
>  static noinline_for_stack int
>  ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,
>  		      struct ext4_free_data *new_entry)
> @@ -4642,7 +4701,7 @@ static int can_merge(struct ext4_free_data *entry1,
>  	if (node) {
>  		entry = rb_entry(node, struct ext4_free_data, efd_node);
>  		if (can_merge(entry, new_entry) &&
> -		    ext4_journal_callback_try_del(handle, &entry->efd_jce)) {
> +		    ext4_freed_data_try_del(sbi, entry)) {
>  			new_entry->efd_start_cluster = entry->efd_start_cluster;
>  			new_entry->efd_count += entry->efd_count;
>  			rb_erase(node, &(db->bb_free_root));
> @@ -4654,16 +4713,15 @@ static int can_merge(struct ext4_free_data *entry1,
>  	if (node) {
>  		entry = rb_entry(node, struct ext4_free_data, efd_node);
>  		if (can_merge(new_entry, entry) &&
> -		    ext4_journal_callback_try_del(handle, &entry->efd_jce)) {
> +		    ext4_freed_data_try_del(sbi, entry)) {
>  			new_entry->efd_count += entry->efd_count;
>  			rb_erase(node, &(db->bb_free_root));
>  			kmem_cache_free(ext4_free_data_cachep, entry);
>  		}
>  	}
> -	/* Add the extent to transaction's private list */
> -	new_entry->efd_jce.jce_func = ext4_free_data_callback;
> +
>  	spin_lock(&sbi->s_md_lock);
> -	_ext4_journal_callback_add(handle, &new_entry->efd_jce);
> +	list_add_tail(&new_entry->efd_list, &sbi->s_freed_data_list);
>  	sbi->s_mb_free_pending += clusters;
>  	spin_unlock(&sbi->s_md_lock);
>  	return 0;
> @@ -4856,6 +4914,7 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
>  		new_entry->efd_group = block_group;
>  		new_entry->efd_count = count_clusters;
>  		new_entry->efd_tid = handle->h_transaction->t_tid;
> +		new_entry->efd_freed = false;
>  
>  		ext4_lock_group(sb, block_group);
>  		mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
> @@ -4866,7 +4925,8 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
>  		 * them with group lock_held
>  		 */
>  		if (test_opt(sb, DISCARD)) {
> -			err = ext4_issue_discard(sb, block_group, bit, count);
> +			err = ext4_issue_discard(sb, block_group, bit, count,
> +						 NULL);
>  			if (err && err != -EOPNOTSUPP)
>  				ext4_msg(sb, KERN_WARNING, "discard request in"
>  					 " group:%d block:%d count:%lu failed"
> @@ -5089,7 +5149,7 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
>  	 */
>  	mb_mark_used(e4b, &ex);
>  	ext4_unlock_group(sb, group);
> -	ret = ext4_issue_discard(sb, group, start, count);
> +	ret = ext4_issue_discard(sb, group, start, count, NULL);
>  	ext4_lock_group(sb, group);
>  	mb_free_blocks(NULL, e4b, start, ex.fe_len);
>  	return ret;
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index 1aba469..f762509 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -78,10 +78,8 @@
>  
>  
>  struct ext4_free_data {
> -	/* MUST be the first member */
> -	struct ext4_journal_cb_entry	efd_jce;
> -
> -	/* ext4_free_data private data starts from here */
> +	/* this links the free block information from sb_info */
> +	struct list_head		efd_list;
>  
>  	/* this links the free block information from group_info */
>  	struct rb_node			efd_node;
> @@ -95,6 +93,9 @@ struct ext4_free_data {
>  
>  	/* transaction which freed this extent */
>  	tid_t				efd_tid;
> +
> +	/* indicate whether this is already handled */
> +	bool				efd_freed;
>  };

Two comments here:

a) 'bool' type in structures is discouraged in kernel as it can lead to
strange alignment issues etc. Just use int or bitfields.

b) efd_freed seems in fact unnecessary. You use it only in
ext4_freed_data_try_del() which is called only if merge succeeded which
means tids match and thus we currently have handle open against the
transaction with that pid and so that transaction cannot commit... So all
in all efd_freed can never be set in the places where you call
ext4_freed_data_try_del().

Thanks for working on this!

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

RE: Re: [PATCH v2] ext4: change sequential discard handling on commit complete phase into parallel manner

[Daeho Jeong]

> Hmm, here you speak about 17s, in the above paragraph about half a minute

> so what was it? Just curious...



Oh, you're are right. 17.0s is true.

 

> Actually, I didn't give my 'Reviewed-by' tag yet... But the patch looks

> generally good, some smaller comments below.



Sorry, I don't know how to use the tag exactly.



> Use list_for_each_entry_safe() instead of explicit list_entry() call.

 

> Just find the right entry in the above loop and then use

> list_cut_position() to split the list as needed. That saves us having to

> remove and add each entry. Also you don't have to use _safe() list entry

> iteration in that case.



> efd_freed can never be true here - see my explanation at the end. So you

> can simplify the code by implementing something like:

 

> void ext4_try_merge_freed_extent(struct ext4_sb_info *sbi,

>                                  struct ext4_free_data *entry,

>                                  struct ext4_free_data *new_entry)

> {

>         if ((entry->efd_tid != new_entry->efd_tid) ||

>             (entry->efd_group != new_entry->efd_group))

>                 return;

>         if (entry->efd_start_cluster + entry->efd_count ==

>             new_entry->efd_start_cluster) {

>                 new_entry->efd_start_cluster = entry->efd_start_cluster;

>                 new_entry->efd_count += entry->efd_count;

>         } else if (new_entry->efd_start_cluster + new_entry->efd_count ==

>                    entry->efd_start_cluster) {

>                 new_entry->efd_count += entry->efd_count;

>         }

>         spin_lock(&sbi->s_md_lock);

>         list_del(&entry->efd_list);

>         spin_unlock(&sbi->s_md_lock);

>         rb_erase(entry->efd_node, &(db->bb_free_root));

>         kmem_cache_free(ext4_free_data_cachep, entry);

> }

 

> which should handle all that is needed in one place.

 

> Two comments here:

 

> a) 'bool' type in structures is discouraged in kernel as it can lead to

> strange alignment issues etc. Just use int or bitfields.

 

> b) efd_freed seems in fact unnecessary. You use it only in

> ext4_freed_data_try_del() which is called only if merge succeeded which

> means tids match and thus we currently have handle open against the

> transaction with that pid and so that transaction cannot commit... So all

> in all efd_freed can never be set in the places where you call

> ext4_freed_data_try_del().

 

> Thanks for working on this!

 

I like the modified version. It looks neat.

Thank you for your valuable comments.