[PATCH v6 16/16] btrfs: populate fully_remapped_bgs_list on mount

[Mark Harmstone <mark@harmstone.com>]

Add a function btrfs_populate_fully_remapped_bgs_list() which gets
called on mount, which looks for fully remapped block groups
(i.e. identity_remap_count == 0) which haven't yet had their chunk
stripes and device extents removed.

This happens when a filesystem is unmounted while async discard has not
yet finished, as otherwise the data range occupied by the chunk stripes
would be permanently unusable.

Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/block-group.c      | 79 +++++++++++++++++++++++++++++++++++++
 fs/btrfs/block-group.h      |  2 +
 fs/btrfs/disk-io.c          |  9 +++++
 fs/btrfs/free-space-cache.c |  7 ++++
 fs/btrfs/relocation.c       |  4 ++
 5 files changed, 101 insertions(+)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 965ae904ec2e..de10c02d1852 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -4824,6 +4824,11 @@ void btrfs_mark_bg_fully_remapped(struct btrfs_block_group *bg,
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 
 	if (btrfs_test_opt(fs_info, DISCARD_ASYNC)) {
+		spin_lock(&bg->lock);
+		set_bit(BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING,
+			&bg->runtime_flags);
+		spin_unlock(&bg->lock);
+
 		btrfs_discard_queue_work(&fs_info->discard_ctl, bg);
 	} else {
 		spin_lock(&fs_info->unused_bgs_lock);
@@ -4843,3 +4848,77 @@ void btrfs_mark_bg_fully_remapped(struct btrfs_block_group *bg,
 		spin_unlock(&fs_info->unused_bgs_lock);
 	}
 }
+
+/*
+ * Compare the block group and chunk trees, and find any fully-remapped block
+ * groups which haven't yet had their chunk stripes and device extents removed,
+ * and put them on the fully_remapped_bgs list so this gets done.
+ *
+ * This happens when a block group becomes fully remapped, i.e. its last
+ * identity mapping is removed, and the volume is unmounted before async
+ * discard has finished. It's important this gets done as until it is the
+ * chunk's stripes are dead space.
+ */
+int btrfs_populate_fully_remapped_bgs_list(struct btrfs_fs_info *fs_info)
+{
+	struct rb_node *node_bg, *node_chunk;
+
+	node_bg = rb_first_cached(&fs_info->block_group_cache_tree);
+	node_chunk = rb_first_cached(&fs_info->mapping_tree);
+
+	while (node_bg && node_chunk) {
+		struct btrfs_block_group *bg;
+		struct btrfs_chunk_map *map;
+
+		bg = rb_entry(node_bg, struct btrfs_block_group, cache_node);
+		map = rb_entry(node_chunk, struct btrfs_chunk_map, rb_node);
+
+		ASSERT(bg->start == map->start);
+
+		if (!(bg->flags & BTRFS_BLOCK_GROUP_REMAPPED))
+			goto next;
+
+		if (bg->identity_remap_count != 0)
+			goto next;
+
+		if (map->num_stripes == 0)
+			goto next;
+
+		spin_lock(&fs_info->unused_bgs_lock);
+
+		if (list_empty(&bg->bg_list)) {
+			btrfs_get_block_group(bg);
+			list_add_tail(&bg->bg_list,
+				      &fs_info->fully_remapped_bgs);
+		} else {
+			list_move_tail(&bg->bg_list,
+				       &fs_info->fully_remapped_bgs);
+		}
+
+		spin_unlock(&fs_info->unused_bgs_lock);
+
+		/*
+		 * Ideally we'd want to call btrfs_discard_queue_work() here,
+		 * but it'd do nothing as the discard worker hasn't been
+		 * started yet.
+		 *
+		 * The block group will get added to the discard list when
+		 * btrfs_handle_fully_remapped_bgs() gets called, when we
+		 * commit the first transaction.
+		 */
+		if (btrfs_test_opt(fs_info, DISCARD_ASYNC)) {
+			spin_lock(&bg->lock);
+			set_bit(BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING,
+				&bg->runtime_flags);
+			spin_unlock(&bg->lock);
+		}
+
+next:
+		node_bg = rb_next(node_bg);
+		node_chunk = rb_next(node_chunk);
+	}
+
+	ASSERT(!node_bg && !node_chunk);
+
+	return 0;
+}
diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
index b0b16efea19a..03e8ad8a2ec7 100644
--- a/fs/btrfs/block-group.h
+++ b/fs/btrfs/block-group.h
@@ -93,6 +93,7 @@ enum btrfs_block_group_flags {
 	 * transaction.
 	 */
 	BLOCK_GROUP_FLAG_NEW,
+	BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING,
 };
 
 enum btrfs_caching_type {
@@ -416,5 +417,6 @@ int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
 bool btrfs_block_group_should_use_size_class(const struct btrfs_block_group *bg);
 void btrfs_mark_bg_fully_remapped(struct btrfs_block_group *bg,
 				  struct btrfs_trans_handle *trans);
+int btrfs_populate_fully_remapped_bgs_list(struct btrfs_fs_info *fs_info);
 
 #endif /* BTRFS_BLOCK_GROUP_H */
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 53221a0131fb..177a33cd9815 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3656,6 +3656,15 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		goto fail_sysfs;
 	}
 
+	if (btrfs_fs_incompat(fs_info, REMAP_TREE)) {
+		ret = btrfs_populate_fully_remapped_bgs_list(fs_info);
+		if (ret) {
+			btrfs_err(fs_info,
+			"failed to populate fully_remapped_bgs list: %d", ret);
+			goto fail_sysfs;
+		}
+	}
+
 	btrfs_zoned_reserve_data_reloc_bg(fs_info);
 	btrfs_free_zone_cache(fs_info);
 
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 1b8716b17031..ce853a9c0a4c 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -3068,6 +3068,7 @@ bool btrfs_is_free_space_trimmed(struct btrfs_block_group *block_group)
 	bool ret = true;
 
 	if (block_group->flags & BTRFS_BLOCK_GROUP_REMAPPED &&
+	    !test_bit(BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING, &block_group->runtime_flags) &&
 	    block_group->identity_remap_count == 0) {
 		return true;
 	}
@@ -3850,6 +3851,11 @@ void btrfs_trim_fully_remapped_block_group(struct btrfs_block_group *bg)
 	u64 end = btrfs_block_group_end(bg);
 	struct btrfs_chunk_map *map;
 
+	if (!test_bit(BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING, &bg->runtime_flags)) {
+		bg->discard_cursor = end;
+		goto skip_discard;
+	}
+
 	bytes = end - bg->discard_cursor;
 
 	if (max_discard_size &&
@@ -3888,6 +3894,7 @@ void btrfs_trim_fully_remapped_block_group(struct btrfs_block_group *bg)
 
 	btrfs_free_chunk_map(map);
 
+skip_discard:
 	if (bg->used == 0) {
 		spin_lock(&fs_info->unused_bgs_lock);
 		if (!list_empty(&bg->bg_list)) {
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 15c4a7c6b1ef..276201fe8f2d 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4788,6 +4788,10 @@ int btrfs_last_identity_remap_gone(struct btrfs_chunk_map *chunk,
 
 	btrfs_remove_bg_from_sinfo(bg);
 
+	spin_lock(&bg->lock);
+	clear_bit(BLOCK_GROUP_FLAG_STRIPE_REMOVAL_PENDING, &bg->runtime_flags);
+	spin_unlock(&bg->lock);
+
 	ret = remove_chunk_stripes(trans, chunk, path);
 	if (ret) {
 		btrfs_abort_transaction(trans, ret);
-- 
2.51.0