[RFC 3/5] ext4: mark move extents fast-commit ineligible

[Li Chen <me@linux.beauty>]

Fast commits only log operations that have dedicated replay support.
EXT4_IOC_MOVE_EXT swaps extents between regular files and may copy
data, rewriting the affected inodes' block mapping layout without
going through the fast commit tracking paths.
In practice these operations are rare and usually followed by further
updates, but mixing them into a fast commit makes the overall
semantics harder to reason about and risks replay gaps if new call
sites appear.

Teach ext4 to mark the filesystem fast-commit ineligible for the
journal transactions used by move_extent_per_page() when
EXT4_IOC_MOVE_EXT runs.
This forces those transactions to fall back to a full commit,
ensuring that these multi-inode extent swaps are captured by the
normal journal rather than partially encoded in fast commit TLVs.
This change should not affect common workloads but makes online
defragmentation safer and easier to reason about under fast commit.

Testing:
1. prepare:
        dd if=/dev/zero of=/root/fc_move.img bs=1M count=0 seek=256
        mkfs.ext4 -O fast_commit -F /root/fc_move.img
        mkdir -p /mnt/fc_move && mount -t ext4 -o loop \
/root/fc_move.img /mnt/fc_move
2. Created two files, ran EXT4_IOC_MOVE_EXT via e4defrag, and checked
   the ineligible reason statistics:
        fallocate -l 64M /mnt/fc_move/file1
        cp /mnt/fc_move/file1 /mnt/fc_move/file2
        e4defrag /mnt/fc_move/file1
        cat /proc/fs/ext4/loop0/fc_info
   shows "Move extents": > 0 and fc stats ineligible > 0.

Signed-off-by: Li Chen <me@linux.beauty>
---
 fs/ext4/fast_commit.c       | 1 +
 fs/ext4/fast_commit.h       | 1 +
 fs/ext4/move_extent.c       | 1 +
 include/trace/events/ext4.h | 4 +++-
 4 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 242b69e5fe13..0ef2154a2b1f 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -2304,6 +2304,7 @@ static const char * const fc_ineligible_reasons[] = {
 	[EXT4_FC_REASON_ENCRYPTED_FILENAME] = "Encrypted filename",
 	[EXT4_FC_REASON_MIGRATE] = "Inode format migration",
 	[EXT4_FC_REASON_VERITY] = "fs-verity enable",
+	[EXT4_FC_REASON_MOVE_EXT] = "Move extents",
 };
 
 int ext4_fc_info_show(struct seq_file *seq, void *v)
diff --git a/fs/ext4/fast_commit.h b/fs/ext4/fast_commit.h
index 20f65135208f..2f77a37fb101 100644
--- a/fs/ext4/fast_commit.h
+++ b/fs/ext4/fast_commit.h
@@ -99,6 +99,7 @@ enum {
 	EXT4_FC_REASON_ENCRYPTED_FILENAME,
 	EXT4_FC_REASON_MIGRATE,
 	EXT4_FC_REASON_VERITY,
+	EXT4_FC_REASON_MOVE_EXT,
 	EXT4_FC_REASON_MAX
 };
 
diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 4b091c21908f..5a5e91078528 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -287,6 +287,7 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
 		*err = PTR_ERR(handle);
 		return 0;
 	}
+	ext4_fc_mark_ineligible(sb, EXT4_FC_REASON_MOVE_EXT, handle);
 
 	orig_blk_offset = orig_page_offset * blocks_per_page +
 		data_offset_in_page;
diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
index 224ab12ee83f..56e60080e759 100644
--- a/include/trace/events/ext4.h
+++ b/include/trace/events/ext4.h
@@ -104,6 +104,7 @@ TRACE_DEFINE_ENUM(EXT4_FC_REASON_INODE_JOURNAL_DATA);
 TRACE_DEFINE_ENUM(EXT4_FC_REASON_ENCRYPTED_FILENAME);
 TRACE_DEFINE_ENUM(EXT4_FC_REASON_MIGRATE);
 TRACE_DEFINE_ENUM(EXT4_FC_REASON_VERITY);
+TRACE_DEFINE_ENUM(EXT4_FC_REASON_MOVE_EXT);
 TRACE_DEFINE_ENUM(EXT4_FC_REASON_MAX);
 
 #define show_fc_reason(reason)						\
@@ -119,7 +120,8 @@ TRACE_DEFINE_ENUM(EXT4_FC_REASON_MAX);
 		{ EXT4_FC_REASON_INODE_JOURNAL_DATA,	"INODE_JOURNAL_DATA"}, \
 		{ EXT4_FC_REASON_ENCRYPTED_FILENAME,	"ENCRYPTED_FILENAME"}, \
 		{ EXT4_FC_REASON_MIGRATE,		"MIGRATE"},	\
-		{ EXT4_FC_REASON_VERITY,		"VERITY"})
+		{ EXT4_FC_REASON_VERITY,		"VERITY"},	\
+		{ EXT4_FC_REASON_MOVE_EXT,		"MOVE_EXT"})
 
 TRACE_DEFINE_ENUM(CR_POWER2_ALIGNED);
 TRACE_DEFINE_ENUM(CR_GOAL_LEN_FAST);
-- 
2.51.0