From b79f1c2caadc5c6251241977c7987fefdeadc2d9 Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Thu, 25 Jul 2024 10:46:01 +0100 Subject: btrfs: reschedule when updating chunk maps at the end of a device replace At the end of a device replace we must go over all the chunk maps and update their stripes to point to the target device instead of the source device. We iterate over the chunk maps while holding a write lock and we never reschedule, which can result in monopolizing a CPU for too long and blocking readers for too long (it's a rw lock, non-blocking). So improve on this by rescheduling if necessary. This is safe because at this point we are holding the chunk mutex, which means no new chunks can be allocated and therefore we don't risk missing a new chunk map that covers a range behind the last one we processed before rescheduling. Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/dev-replace.c | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'fs/btrfs/dev-replace.c') diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c index f638c458d285..20cf5e95f2bc 100644 --- a/fs/btrfs/dev-replace.c +++ b/fs/btrfs/dev-replace.c @@ -827,6 +827,14 @@ static void btrfs_dev_replace_update_device_in_mapping_tree( u64 start = 0; int i; + /* + * The chunk mutex must be held so that no new chunks can be created + * while we are updating existing chunks. This guarantees we don't miss + * any new chunk that gets created for a range that falls before the + * range of the last chunk we processed. + */ + lockdep_assert_held(&fs_info->chunk_mutex); + write_lock(&fs_info->mapping_tree_lock); do { struct btrfs_chunk_map *map; @@ -839,6 +847,7 @@ static void btrfs_dev_replace_update_device_in_mapping_tree( map->stripes[i].dev = tgtdev; start = map->start + map->chunk_len; btrfs_free_chunk_map(map); + cond_resched_rwlock_write(&fs_info->mapping_tree_lock); } while (start); write_unlock(&fs_info->mapping_tree_lock); } -- cgit v1.2.3 From 68a505bb87f948f72e5d230dfd7b40debdb195ed Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Thu, 25 Jul 2024 11:48:10 +0100 Subject: btrfs: more efficient chunk map iteration when device replace finishes When iterating the chunk maps when a device replace finishes we are doing a full rbtree search for each chunk map, which is not the most efficient thing to do, wasting CPU time. As we are holding a write lock on the tree during the whole iteration, we can simply start from the first node in the tree and then move to the next chunk map by doing a rb_next() call - the only exception is when we need to reschedule, in which case we have to do a full rbtree search since we dropped the write lock and the tree may have changed (chunk maps may have been removed and the tree got rebalanced). So just do that. Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/dev-replace.c | 36 +++++++++++++++++++++++++----------- 1 file changed, 25 insertions(+), 11 deletions(-) (limited to 'fs/btrfs/dev-replace.c') diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c index 20cf5e95f2bc..83d5cdd77f29 100644 --- a/fs/btrfs/dev-replace.c +++ b/fs/btrfs/dev-replace.c @@ -824,8 +824,7 @@ static void btrfs_dev_replace_update_device_in_mapping_tree( struct btrfs_device *srcdev, struct btrfs_device *tgtdev) { - u64 start = 0; - int i; + struct rb_node *node; /* * The chunk mutex must be held so that no new chunks can be created @@ -836,19 +835,34 @@ static void btrfs_dev_replace_update_device_in_mapping_tree( lockdep_assert_held(&fs_info->chunk_mutex); write_lock(&fs_info->mapping_tree_lock); - do { + node = rb_first_cached(&fs_info->mapping_tree); + while (node) { + struct rb_node *next = rb_next(node); struct btrfs_chunk_map *map; + u64 next_start; - map = btrfs_find_chunk_map_nolock(fs_info, start, U64_MAX); - if (!map) - break; - for (i = 0; i < map->num_stripes; i++) + map = rb_entry(node, struct btrfs_chunk_map, rb_node); + next_start = map->start + map->chunk_len; + + for (int i = 0; i < map->num_stripes; i++) if (srcdev == map->stripes[i].dev) map->stripes[i].dev = tgtdev; - start = map->start + map->chunk_len; - btrfs_free_chunk_map(map); - cond_resched_rwlock_write(&fs_info->mapping_tree_lock); - } while (start); + + if (cond_resched_rwlock_write(&fs_info->mapping_tree_lock)) { + map = btrfs_find_chunk_map_nolock(fs_info, next_start, U64_MAX); + if (!map) + break; + node = &map->rb_node; + /* + * Drop the lookup reference since we are holding the + * lock in write mode and no one can remove the chunk + * map from the tree and drop its tree reference. + */ + btrfs_free_chunk_map(map); + } else { + node = next; + } + } write_unlock(&fs_info->mapping_tree_lock); } -- cgit v1.2.3