[PATCH] btrfs: zoned: reset meta_write_pointer on zone reset

btrfs_reset_unused_block_groups() resets a block group's zone and sets
alloc_offset back to 0 so the space can be reused, but it leaves
meta_write_pointer pointing at the previous end of the zone.

Once the block group is reactivated and reused for metadata, newly
allocated tree blocks live before that stale write pointer.
btrfs_check_meta_write_pointer() then sees them behind the write pointer,
so they can never be written out in sequential order: the dirty extent
buffers are stranded and pin their btree_inode folios until unmount.

Reset meta_write_pointer back to the start of the block group for
metadata and system block groups.

Fixes: 453a73c3069a ("btrfs: zoned: reclaim unused zone by zone resetting")
Cc: [email protected]
Signed-off-by: Johannes Thumshirn <[email protected]>
---
 fs/btrfs/zoned.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
index ab7c3cc52599..765655473263 100644
--- a/fs/btrfs/zoned.c
+++ b/fs/btrfs/zoned.c
@@ -3218,6 +3218,17 @@ int btrfs_reset_unused_block_groups(struct btrfs_space_info *space_info, u64 num
 		reclaimed = bg->alloc_offset;
 		bg->zone_unusable = bg->length - bg->zone_capacity;
 		bg->alloc_offset = 0;
+		/*
+		 * The zone was just reset to empty, so alloc_offset went back to
+		 * the start of the zone. For metadata/system block groups the
+		 * write pointer must follow it back to the start of the zone;
+		 * otherwise it stays stale at the previous (finished) zone end,
+		 * and metadata written into the reused zone would sit behind the
+		 * write pointer, could never be written out in sequential order,
+		 * and would be stranded (pinning its folio) until unmount.
+		 */
+		if (bg->flags & (BTRFS_BLOCK_GROUP_METADATA | BTRFS_BLOCK_GROUP_SYSTEM))
+			bg->meta_write_pointer = bg->start;
 		/*
 		 * This holds because we currently reset fully used then freed
 		 * block group.
-- 
2.54.0