[PATCH 6.18 247/752] ext4: mark group add fast-commit ineligible

Sasha Levin
2026-02-28T17:51:36Z

From: Li Chen <me@linux.beauty>

[ Upstream commit 89b4336fd5ec78f51f9d3a1d100f3ffa3228e604 ]

Fast commits only log operations that have dedicated replay support.
Online resize via EXT4_IOC_GROUP_ADD updates the superblock and group
descriptor metadata 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 when
ext4_ioctl_group_add() adds new block groups.
This forces those transactions to fall back to a full commit,
ensuring that the filesystem geometry updates are captured by the
normal journal rather than partially encoded in fast commit TLVs.
This change should not affect common workloads but makes online
resize via GROUP_ADD safer and easier to reason about under fast
commit.

Testing:
1. prepare:
dd if=/dev/zero of=/root/fc_resize.img bs=1M count=0 seek=256
mkfs.ext4 -O fast_commit -F /root/fc_resize.img
mkdir -p /mnt/fc_resize && mount -t ext4 -o loop /root/fc_resize.img /mnt/fc_resize
2. Ran a helper that issues EXT4_IOC_GROUP_ADD on the mounted
filesystem and checked the resize ineligible reason:
./group_add_helper /mnt/fc_resize
cat /proc/fs/ext4/loop0/fc_info
shows "Resize": > 0.
3. Fsynced a file on the resized filesystem and verified that the fast
commit stats report at least one ineligible commit:
touch /mnt/fc_resize/file
/root/fsync_file /mnt/fc_resize/file
sync
cat /proc/fs/ext4/loop0/fc_info
shows fc stats ineligible > 0.

Signed-off-by: Li Chen <me@linux.beauty>
Link: https://patch.msgid.link/20251211115146.897420-5-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/ioctl.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 3dec26c939fde..d26914fa0cb46 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -966,6 +966,7 @@ static long ext4_ioctl_group_add(struct file *file,

err = ext4_group_add(sb, input);
if (EXT4_SB(sb)->s_journal) {
+ ext4_fc_mark_ineligible(sb, EXT4_FC_REASON_RESIZE, NULL);
jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
err2 = jbd2_journal_flush(EXT4_SB(sb)->s_journal, 0);
jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
--
2.51.0