Recent FITRIM work, namely bbbf7243d62d ("btrfs: combine device update operations during transaction commit") combined the way certain operations are recoded in a transaction. As a result an ASSERT was added in dev_replace_finish to ensure the new code works correctly. Unfortunately I got reports that it's possible to trigger the assert, meaning that during a device replace it's possible to have an unfinished chunk allocation on the source device.

This is supposed to be prevented by the fact that a transaction is committed before finishing the replace oepration and alter acquiring the chunk mutex. This is not sufficient since by the time the transaction is committed and the chunk mutex acquired it's possible to allocate a chunk depending on the workload being executed on the replaced device. This bug has been present ever since device replace was introduced but there was never code which checks for it.

The correct way to fix is to ensure that there is no pending device modification operation when the chunk mutex is acquire and if there is repeat transaction commit. Unfortunately it's not possible to just exclude the source device from btrfs_fs_devices::dev_alloc_list since this causes ENOSPC to be hit in transaction commit.

Fixing that in another way would need to add special cases to handle the last writes and forbid new ones. The looped transaction fix is more obvious, and can be easily backported. The runtime of dev-replace is long so there's no noticeable delay caused by that.

Signed-off-by: Nikolay Borisov nborisov@suse.com ---

Hello Greg,

Please merge the following backport of upstream commit debd1c065d2037919a7da67baf55cc683fee09f0 to 4.14.y stable branch. fs/btrfs/dev-replace.c | 29 +++++++++++++++++++---------- fs/btrfs/volumes.c | 2 ++ fs/btrfs/volumes.h | 5 +++++ 3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c index f86457713e60..f1e9dd246ab0 100644 --- a/fs/btrfs/dev-replace.c +++ b/fs/btrfs/dev-replace.c @@ -512,18 +512,27 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info, } btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);

- trans = btrfs_start_transaction(root, 0); - if (IS_ERR(trans)) { - mutex_unlock(&dev_replace->lock_finishing_cancel_unmount); - return PTR_ERR(trans); + while (1) { + trans = btrfs_start_transaction(root, 0); + if (IS_ERR(trans)) { + mutex_unlock(&dev_replace->lock_finishing_cancel_unmount); + return PTR_ERR(trans); + } + ret = btrfs_commit_transaction(trans); + WARN_ON(ret); + mutex_lock(&uuid_mutex); + /* keep away write_all_supers() during the finishing procedure */ + mutex_lock(&fs_info->fs_devices->device_list_mutex); + mutex_lock(&fs_info->chunk_mutex); + if (src_device->has_pending_chunks) { + mutex_unlock(&root->fs_info->chunk_mutex); + mutex_unlock(&root->fs_info->fs_devices->device_list_mutex); + mutex_unlock(&uuid_mutex); + } else { + break; + } } - ret = btrfs_commit_transaction(trans); - WARN_ON(ret);

- mutex_lock(&uuid_mutex); - /* keep away write_all_supers() during the finishing procedure */ - mutex_lock(&fs_info->fs_devices->device_list_mutex); - mutex_lock(&fs_info->chunk_mutex); btrfs_dev_replace_lock(dev_replace, 1); dev_replace->replace_state = scrub_ret ? BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 38ed8e259e00..85294fef1051 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -4851,6 +4851,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans, for (i = 0; i < map->num_stripes; i++) { num_bytes = map->stripes[i].dev->bytes_used + stripe_size; btrfs_device_set_bytes_used(map->stripes[i].dev, num_bytes); + map->stripes[i].dev->has_pending_chunks = true; }

atomic64_sub(stripe_size * map->num_stripes, &info->free_chunk_space); @@ -7310,6 +7311,7 @@ void btrfs_update_commit_device_bytes_used(struct btrfs_fs_info *fs_info, for (i = 0; i < map->num_stripes; i++) { dev = map->stripes[i].dev; dev->commit_bytes_used = dev->bytes_used; + dev->has_pending_chunks = false; } } mutex_unlock(&fs_info->chunk_mutex); diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h index 76fb6e84f201..f6ae6cdf233d 100644 --- a/fs/btrfs/volumes.h +++ b/fs/btrfs/volumes.h @@ -61,6 +61,11 @@ struct btrfs_device {

spinlock_t io_lock ____cacheline_aligned; int running_pending; + /* When true means this device has pending chunk alloc in + * current transaction. Protected by chunk_mutex. + */ + bool has_pending_chunks; + /* regular prio bios */ struct btrfs_pending_bios pending_bios; /* sync bios */