f2fs: zone: fix to avoid bio split on sequential zone

From: Chao Yu <[email protected]>

In the block layer, bio_split_to_limits() splits a bio if it exceeds any underlying queue limits:
1. queue_max_sectors: Splits bios larger than the transfer limit. This limit is set by the device driver/DMA controller (max_hw_sectors) and device-mapper targets. When flushing dirty pages (governed by VFS writeback_chunk_size() and f2fs nr_pages_to_skip()), f2fs merges contiguous pages into bios up to 512KB (1024 sectors) or more without checking queue_max_sectors, which will be split if the device or DM target enforces a smaller limit.
2. queue_max_segments: Splits bios exceeding scatter-gather segment limits. f2fs allocates bios with up to 256 pages, exceeding the 128-segment limit of DM targets (dm-default-key/dm-crypt) if pages are discontiguous.
3. chunk_sectors: Splits bios crossing chunk/zone boundaries. In f2fs, section_size strictly matches zone_size, and block allocation never spans across section boundaries, so bios never cross chunk_sectors.

Crucially, when bio_split_to_limits() splits a bio, bio_submit_split() submits the tail half via submit_bio_noacct() before returning the head half to the caller. On a sequential zoned device, this causes the tail bio to arrive at the zone write plug before the head:

dm-3: zone 7: prepare_bio wp mismatch: bio sector 30720 offset 2048 != wp_offset 248, len 248

The write plug rejects the out-of-order tail with BLK_STS_IOERR, causing f2fs_stop_checkpoint() and remounting the filesystem read-only:

2026-09-30T22:09:23.725678+08:00 localhost kernel: CPU: 9 UID: 0 PID: 243 Comm: kworker/9:1H Not tainted 6.12.81-gb3ef518fb26d #5
2026-09-30T22:09:23.725750+08:00 localhost kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
2026-09-30T22:09:23.725753+08:00 localhost kernel: Workqueue: dm-3_zwplugs blk_zone_wplug_bio_work
2026-09-30T22:09:23.725756+08:00 localhost kernel: Call Trace:
2026-09-30T22:09:23.725758+08:00 localhost kernel:  <TASK>
2026-09-30T22:09:23.725758+08:00 localhost kernel:  dump_stack_lvl+0x4d/0x70
2026-09-30T22:09:23.725759+08:00 localhost kernel:  f2fs_stop_checkpoint+0x1df/0x5d0
2026-09-30T22:09:23.725759+08:00 localhost kernel:  ? blk_zone_wplug_bio_work+0x4eb/0x740
2026-09-30T22:09:23.725760+08:00 localhost kernel:  f2fs_cache_write_end_io+0x545/0x6e0
2026-09-30T22:09:23.725761+08:00 localhost kernel:  blk_zone_wplug_bio_work+0x4eb/0x740
2026-09-30T22:09:23.725762+08:00 localhost kernel:  process_one_work+0x5c6/0xde0
2026-09-30T22:09:23.725764+08:00 localhost kernel:  ? assign_work+0x124/0x4e0
2026-09-30T22:09:23.725765+08:00 localhost kernel:  worker_thread+0x403/0xb70
2026-09-30T22:09:23.725765+08:00 localhost kernel:  ? __kthread_parkme+0x82/0x140
2026-09-30T22:09:23.725766+08:00 localhost kernel:  ? __pfx_worker_thread+0x10/0x10
2026-09-30T22:09:23.725766+08:00 localhost kernel:  kthread+0x235/0x2e0
2026-09-30T22:09:23.725767+08:00 localhost kernel:  ? recalc_sigpending+0x128/0x1d0
2026-09-30T22:09:23.725767+08:00 localhost kernel:  ? __pfx_kthread+0x10/0x10
2026-09-30T22:09:23.725769+08:00 localhost kernel:  ret_from_fork+0x2f/0x70
2026-09-30T22:09:23.725769+08:00 localhost kernel:  ? __pfx_kthread+0x10/0x10
2026-09-30T22:09:23.725770+08:00 localhost kernel:  ret_from_fork_asm+0x1a/0x30
2026-09-30T22:09:23.725771+08:00 localhost kernel:  </TASK>
2026-09-30T22:09:23.725771+08:00 localhost kernel: F2FS-fs (dm-0): Stopped filesystem due to reason: 3

To prevent write bios destined for sequential zones from being split:
1. In __bio_alloc(), bound npages to bdev_max_segments(bdev) for writes to sequential zone areas.
2. In page_is_mergeable(), check queue_max_segments() and queue_max_sectors() for sequential zone areas, so f2fs will not merge beyond the queue limits.

---
 fs/f2fs/data.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

index 4bfa301a5df6..15c6af8515f3 100644
@@ -536,6 +536,10 @@ static struct bio *__bio_alloc(struct f2fs_io_info *fio, int npages)
 	struct bio *bio;
 
 	bdev = f2fs_target_device(sbi, fio->new_blkaddr, &sector);
+	/* avoid reversed IO if bio is split due to exceeding max_segments */
+	if (npages > 1 && !is_read_io(fio->op) &&
+	    f2fs_is_sequential_zone_area(sbi, fio->new_blkaddr))
+		npages = min_t(int, npages, bdev_max_segments(bdev));
 	bio = bio_alloc_bioset(bdev, npages,
 				fio->op | fio->op_flags | f2fs_io_flags(fio),
 				GFP_NOIO, &f2fs_bioset);

@@ -892,7 +896,19 @@ static bool page_is_mergeable(struct f2fs_sb_info *sbi, struct bio *bio,
 		return false;
 	if (last_blkaddr + 1 != cur_blkaddr)
 		return false;
-	return bio->bi_bdev == f2fs_target_device(sbi, cur_blkaddr, NULL);
+	if (bio->bi_bdev != f2fs_target_device(sbi, cur_blkaddr, NULL))
+		return false;
+	/* avoid reversed IO if bio is split due to exceeding max_segments/sectors */
+	if (f2fs_is_sequential_zone_area(sbi, cur_blkaddr)) {
+		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
+
+		if (bio->bi_vcnt >= queue_max_segments(q))
+			return false;
+		if (bio_sectors(bio) + (F2FS_BLKSIZE(sbi) >> SECTOR_SHIFT) >
+				queue_max_sectors(q))
+			return false;
+	}
+	return true;
 }
 
 static bool io_type_is_mergeable(struct f2fs_bio_info *io,

2.49.0