btrfs: Ensure replaced device doesn't have pending chunk allocation
Recent FITRIM work, namelybbbf7243d6
("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. Reported-by: David Sterba <dsterba@suse.com> Fixes:391cd9df81
("Btrfs: fix unprotected alloc list insertion during the finishing procedure of replace") CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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@ -603,17 +603,33 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
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}
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btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);
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trans = btrfs_start_transaction(root, 0);
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if (IS_ERR(trans)) {
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mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
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return PTR_ERR(trans);
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}
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ret = btrfs_commit_transaction(trans);
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WARN_ON(ret);
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/*
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* We have to use this loop approach because at this point src_device
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* has to be available for transaction commit to complete, yet new
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* chunks shouldn't be allocated on the device.
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*/
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while (1) {
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trans = btrfs_start_transaction(root, 0);
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if (IS_ERR(trans)) {
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mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
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return PTR_ERR(trans);
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}
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ret = btrfs_commit_transaction(trans);
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WARN_ON(ret);
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/* Prevent write_all_supers() during the finishing procedure */
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mutex_lock(&fs_info->fs_devices->device_list_mutex);
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/* Prevent new chunks being allocated on the source device */
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mutex_lock(&fs_info->chunk_mutex);
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if (!list_empty(&src_device->post_commit_list)) {
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mutex_unlock(&fs_info->fs_devices->device_list_mutex);
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mutex_unlock(&fs_info->chunk_mutex);
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} else {
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break;
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}
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}
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/* keep away write_all_supers() during the finishing procedure */
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mutex_lock(&fs_info->fs_devices->device_list_mutex);
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mutex_lock(&fs_info->chunk_mutex);
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down_write(&dev_replace->rwsem);
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dev_replace->replace_state =
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scrub_ret ? BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED
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@ -662,7 +678,6 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
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btrfs_device_set_disk_total_bytes(tgt_device,
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src_device->disk_total_bytes);
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btrfs_device_set_bytes_used(tgt_device, src_device->bytes_used);
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ASSERT(list_empty(&src_device->post_commit_list));
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tgt_device->commit_total_bytes = src_device->commit_total_bytes;
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tgt_device->commit_bytes_used = src_device->bytes_used;
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