On Thu, Jul 09, 2020 at 04:33:33PM +0800, 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.
>
> - btrfs_block_rsv_refill() in reserve_metadata_space()
> It calls with BTRFS_RESERVE_FLUSH_LIMIT, which won't get interrupred
> by signal.
This semantics of BTRFS_RESERVE_FLUSH_LIMIT regarding signals should be
documented, right now there's a comment but says something about avoidig
deadlocks.
> - 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@xxxxxxxx>
> ---
> fs/btrfs/relocation.c | 13 +++++++++++--
> fs/btrfs/volumes.c | 2 +-
> 2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 2b869fb2e62c..23914edd4710 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -1686,12 +1686,21 @@ 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;
> + /*
> + * In merge_reloc_root(), we modify the upper level pointer to swap
> + * the tree blocks between reloc tree and subvolume tree.
> + * Thus for tree block COW, we COW at most from level 1 to root level
> + * for each tree.
> + *
> + * Thus the needed metadata space is at most root_level * nodesize,
> + * and * 2 since we have two trees to COW.
> + */
> + min_reserved = fs_info->nodesize * btrfs_root_level(root_item) * 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)
Why do you remove atomic_read(&fs_info->balance_cancel_req) ?
> btrfs_info(fs_info, "balance: canceled");
I'm not sure if it would be useful to print the reason, like
- 'canceled: user request'
- 'canceled: interrupted'
> else
> btrfs_info(fs_info, "balance: ended with status: %d", ret);
> --
> 2.27.0
