Re: [PATCH 2/2] btrfs: relocation: review the call sites which can be interruped by signal

[Nikolay Borisov <nborisov@suse.com>]

On 8.07.20 г. 13:00 ч., Qu Wenruo wrote:
> Since most metadata reservation calls can return -EINTR when get
> interruped by fatal signal, we need to review the all the metadata
> reservation call sites.
> 
> In relocation code, the metadata reservation happens in the following
> sites:
> - btrfs_block_rsv_refill() in merge_reloc_root()
>   merge_reloc_root() is a pretty critial section, we don't want get
>   interrupted by signal, so change the flush status to
>   BTRFS_RESERVE_FLUSH_LIMIT, so it won't get interrupted by signal.
>   Since such change can be ENPSPC-prone, also shrink the amount of
>   metadata to reserve a little to avoid deadly ENOSPC there.

You need to either document why do you think this smaller reservation is
fine in the changelog or document it it with a comment above it.

> 
> - btrfs_block_rsv_refill() in reserve_metadata_space()
>   It calls with BTRFS_RESERVE_FLUSH_LIMIT, which won't get interrupred
>   by signal.
> 
> - btrfs_block_rsv_refill() in prepare_to_relocate()
> - btrfs_block_rsv_add() in prepare_to_relocate()
> - btrfs_block_rsv_refill() in relocate_block_group()
> - btrfs_delalloc_reserve_metadata() in relocate_file_extent_cluster()
> - btrfs_start_transaction() in relocate_block_group()
> - btrfs_start_transaction() in create_reloc_inode()
>   Can be interruped by fatal signal and we can handle it easily.
>   For these call sites, just catch the -EINTR value in btrfs_balance()
>   and count them as canceled.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/relocation.c | 4 ++--
>  fs/btrfs/volumes.c    | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 2b869fb2e62c..29bbead29be5 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -1686,12 +1686,12 @@ static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
>  		btrfs_unlock_up_safe(path, 0);
>  	}
>  
> -	min_reserved = fs_info->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
> +	min_reserved = fs_info->nodesize * level * 2;
>  	memset(&next_key, 0, sizeof(next_key));
>  
>  	while (1) {
>  		ret = btrfs_block_rsv_refill(root, rc->block_rsv, min_reserved,
> -					     BTRFS_RESERVE_FLUSH_ALL);
> +					     BTRFS_RESERVE_FLUSH_LIMIT);
>  		if (ret) {
>  			err = ret;
>  			goto out;
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index aabc6c922e04..d60df30bdc47 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4135,7 +4135,7 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>  	mutex_lock(&fs_info->balance_mutex);
>  	if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req))
>  		btrfs_info(fs_info, "balance: paused");
> -	else if (ret == -ECANCELED && atomic_read(&fs_info->balance_cancel_req))
> +	else if (ret == -ECANCELED  || ret == -EINTR)
>  		btrfs_info(fs_info, "balance: canceled");
>  	else
>  		btrfs_info(fs_info, "balance: ended with status: %d", ret);
>