[PATCH] ext4: Fix call trace when remounting to read only in data=journal mode

[Gerald Yang <gerald.yang@canonical.com>]

When remounting the filesystem to read only in data=journal mode
it may dump the following call trace:

[   71.629350] CPU: 0 UID: 0 PID: 177 Comm: kworker/u96:5 Tainted: G            E       6.19.0-rc7 #1 PREEMPT(voluntary)
[   71.629352] Tainted: [E]=UNSIGNED_MODULE
[   71.629353] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS unknown 2/2/2022
[   71.629354] Workqueue: writeback wb_workfn (flush-7:4)
[   71.629359] RIP: 0010:ext4_journal_check_start+0x8b/0xd0
[   71.629360] Code: 31 ff 45 31 c0 45 31 c9 e9 42 ad c4 00 48 8b 5d f8 b8 fb ff ff ff c9 31 d2 31 c9 31 f6 31 ff 45 31 c0 45 31 c9 c3 cc cc cc cc <0f> 0b b8 e2 ff ff ff eb c2 0f 0b eb
 a9 44 8b 42 08 68 c7 53 ce b8
[   71.629361] RSP: 0018:ffffcf32c0fdf6a8 EFLAGS: 00010202
[   71.629364] RAX: ffff8f08c8505000 RBX: ffff8f08c67ee800 RCX: 0000000000000000
[   71.629366] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[   71.629367] RBP: ffffcf32c0fdf6b0 R08: 0000000000000001 R09: 0000000000000000
[   71.629368] R10: ffff8f08db18b3a8 R11: 0000000000000000 R12: 0000000000000000
[   71.629368] R13: 0000000000000002 R14: 0000000000000a48 R15: ffff8f08c67ee800
[   71.629369] FS:  0000000000000000(0000) GS:ffff8f0a7d273000(0000) knlGS:0000000000000000
[   71.629370] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   71.629371] CR2: 00007b66825905cc CR3: 000000011053d004 CR4: 0000000000772ef0
[   71.629374] PKRU: 55555554
[   71.629374] Call Trace:
[   71.629378]  <TASK>
[   71.629382]  __ext4_journal_start_sb+0x38/0x1c0
[   71.629383]  mpage_prepare_extent_to_map+0x4af/0x580
[   71.629389]  ? sbitmap_get+0x73/0x180
[   71.629399]  ext4_do_writepages+0x3cc/0x10a0
[   71.629400]  ? kvm_sched_clock_read+0x11/0x20
[   71.629409]  ext4_writepages+0xc8/0x1b0
[   71.629410]  ? ext4_writepages+0xc8/0x1b0
[   71.629411]  do_writepages+0xc4/0x180
[   71.629416]  __writeback_single_inode+0x45/0x350
[   71.629419]  ? _raw_spin_unlock+0xe/0x40
[   71.629423]  writeback_sb_inodes+0x260/0x5c0
[   71.629425]  ? __schedule+0x4d1/0x1870
[   71.629429]  __writeback_inodes_wb+0x54/0x100
[   71.629431]  ? queue_io+0x82/0x140
[   71.629433]  wb_writeback+0x1ab/0x330
[   71.629448]  wb_workfn+0x31d/0x410
[   71.629450]  process_one_work+0x191/0x3e0
[   71.629455]  worker_thread+0x2e3/0x420

This issue can be easily reproduced by:
mkdir -p mnt
dd if=/dev/zero of=ext4disk bs=1G count=2 oflag=direct
mkfs.ext4 ext4disk
tune2fs -o journal_data ext4disk
mount ext4disk mnt
fio --name=fiotest --rw=randwrite --bs=4k --runtime=3 --ioengine=libaio --iodepth=128 --numjobs=4 --filename=mnt/fiotest --filesize=1G --group_reporting
mount -o remount,ro ext4disk mnt
sync

In data=journal mode, metadata and data are both written to the journal
first, but for the second write, ext4 relies on the writeback thread to
flush the data to the real file location.

After the filesystem is remounted to read only, writeback thread still
writes data to it and causes the issue. Return early to avoid starting
a journal transaction on a read only filesystem, once the filesystem
becomes writable again, the write thread will continue writing data.

Signed-off-by: Gerald Yang <gerald.yang@canonical.com>
---
 fs/ext4/inode.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 15ba4d42982f..4e3bbf17995e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2787,6 +2787,17 @@ static int ext4_do_writepages(struct mpage_da_data *mpd)
 	if (unlikely(ret))
 		goto out_writepages;
 
+	/*
+	 * For data=journal, if the filesystem was remounted read-only,
+	 * the writeback thread may still write dirty pages to it.
+	 * Return early to avoid starting a journal transaction on a
+	 * read-only filesystem.
+	 */
+	if (ext4_should_journal_data(inode) && sb_rdonly(inode->i_sb)) {
+		ret = -EROFS;
+		goto out_writepages;
+	}
+
 	/*
 	 * If we have inline data and arrive here, it means that
 	 * we will soon create the block for the 1st page, so
-- 
2.43.0