On 2019/11/27 上午12:25, Josef Bacik wrote:
> inc_block_group_ro does a calculation to see if we have enough room left
> over if we mark this block group as read only in order to see if it's ok
> to mark the block group as read only.
> 
> The problem is this calculation _only_ works for data, where our used is
> always less than our total.  For metadata we will overcommit, so this
> will almost always fail for metadata.
> 
> Fix this by exporting btrfs_can_overcommit, and then see if we have
> enough space to remove the remaining free space in the block group we
> are trying to mark read only.  If we do then we can mark this block
> group as read only.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Just as mentioned in patch 3, it would be better to revert b12de52896c0
("btrfs: scrub: Don't check free space before marking a block group RO")
in this patch too.

As your new over-commit check also prevent btrfs_inc_block_group_ro() to
create empty system chunk, it solves that mentioned bug in a better way.

Thanks,
Qu

> ---
>  fs/btrfs/block-group.c | 35 ++++++++++++++++++++++++-----------
>  fs/btrfs/space-info.c  | 19 ++++++++++---------
>  fs/btrfs/space-info.h  |  3 +++
>  3 files changed, 37 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 5961411500ed..ca55eb6758d1 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1184,7 +1184,6 @@ static int inc_block_group_ro(struct btrfs_block_group *cache, int force)
>  {
>  	struct btrfs_space_info *sinfo = cache->space_info;
>  	u64 num_bytes;
> -	u64 sinfo_used;
>  	int ret = -ENOSPC;
>  
>  	spin_lock(&sinfo->lock);
> @@ -1205,19 +1204,36 @@ static int inc_block_group_ro(struct btrfs_block_group *cache, int force)
>  
>  	num_bytes = cache->length - cache->reserved - cache->pinned -
>  		    cache->bytes_super - cache->used;
> -	sinfo_used = btrfs_space_info_used(sinfo, true);
>  
>  	/*
> -	 * sinfo_used + num_bytes should always <= sinfo->total_bytes.
> -	 *
> -	 * Here we make sure if we mark this bg RO, we still have enough
> -	 * free space as buffer.
> +	 * Data never overcommits, even in mixed mode, so do just the straight
> +	 * check of left over space in how much we have allocated.
>  	 */
> -	if (sinfo_used + num_bytes <= sinfo->total_bytes) {
> +	if (sinfo->flags & BTRFS_BLOCK_GROUP_DATA) {
> +		u64 sinfo_used = btrfs_space_info_used(sinfo, true);
> +
> +		/*
> +		 * Here we make sure if we mark this bg RO, we still have enough
> +		 * free space as buffer.
> +		 */
> +		if (sinfo_used + num_bytes <= sinfo->total_bytes)
> +			ret = 0;
> +	} else {
> +		/*
> +		 * We overcommit metadata, so we need to do the
> +		 * btrfs_can_overcommit check here, and we need to pass in
> +		 * BTRFS_RESERVE_NO_FLUSH to give ourselves the most amount of
> +		 * leeway to allow us to mark this block group as read only.
> +		 */
> +		if (btrfs_can_overcommit(cache->fs_info, sinfo, num_bytes,
> +					 BTRFS_RESERVE_NO_FLUSH))
> +			ret = 0;
> +	}
> +
> +	if (!ret) {
>  		sinfo->bytes_readonly += num_bytes;
>  		cache->ro++;
>  		list_add_tail(&cache->ro_list, &sinfo->ro_bgs);
> -		ret = 0;
>  	}
>  out:
>  	spin_unlock(&cache->lock);
> @@ -1225,9 +1241,6 @@ static int inc_block_group_ro(struct btrfs_block_group *cache, int force)
>  	if (ret == -ENOSPC && btrfs_test_opt(cache->fs_info, ENOSPC_DEBUG)) {
>  		btrfs_info(cache->fs_info,
>  			"unable to make block group %llu ro", cache->start);
> -		btrfs_info(cache->fs_info,
> -			"sinfo_used=%llu bg_num_bytes=%llu",
> -			sinfo_used, num_bytes);
>  		btrfs_dump_space_info(cache->fs_info, cache->space_info, 0, 0);
>  	}
>  	return ret;
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index df5fb68df798..01297c5b2666 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -159,9 +159,9 @@ static inline u64 calc_global_rsv_need_space(struct btrfs_block_rsv *global)
>  	return (global->size << 1);
>  }
>  
> -static int can_overcommit(struct btrfs_fs_info *fs_info,
> -			  struct btrfs_space_info *space_info, u64 bytes,
> -			  enum btrfs_reserve_flush_enum flush)
> +int btrfs_can_overcommit(struct btrfs_fs_info *fs_info,
> +			 struct btrfs_space_info *space_info, u64 bytes,
> +			 enum btrfs_reserve_flush_enum flush)
>  {
>  	u64 profile;
>  	u64 avail;
> @@ -226,7 +226,8 @@ void btrfs_try_granting_tickets(struct btrfs_fs_info *fs_info,
>  
>  		/* Check and see if our ticket can be satisified now. */
>  		if ((used + ticket->bytes <= space_info->total_bytes) ||
> -		    can_overcommit(fs_info, space_info, ticket->bytes, flush)) {
> +		    btrfs_can_overcommit(fs_info, space_info, ticket->bytes,
> +					 flush)) {
>  			btrfs_space_info_update_bytes_may_use(fs_info,
>  							      space_info,
>  							      ticket->bytes);
> @@ -639,14 +640,14 @@ btrfs_calc_reclaim_metadata_size(struct btrfs_fs_info *fs_info,
>  		return to_reclaim;
>  
>  	to_reclaim = min_t(u64, num_online_cpus() * SZ_1M, SZ_16M);
> -	if (can_overcommit(fs_info, space_info, to_reclaim,
> -			   BTRFS_RESERVE_FLUSH_ALL))
> +	if (btrfs_can_overcommit(fs_info, space_info, to_reclaim,
> +				 BTRFS_RESERVE_FLUSH_ALL))
>  		return 0;
>  
>  	used = btrfs_space_info_used(space_info, true);
>  
> -	if (can_overcommit(fs_info, space_info, SZ_1M,
> -			   BTRFS_RESERVE_FLUSH_ALL))
> +	if (btrfs_can_overcommit(fs_info, space_info, SZ_1M,
> +				 BTRFS_RESERVE_FLUSH_ALL))
>  		expected = div_factor_fine(space_info->total_bytes, 95);
>  	else
>  		expected = div_factor_fine(space_info->total_bytes, 90);
> @@ -1005,7 +1006,7 @@ static int __reserve_metadata_bytes(struct btrfs_fs_info *fs_info,
>  	 */
>  	if (!pending_tickets &&
>  	    ((used + orig_bytes <= space_info->total_bytes) ||
> -	     can_overcommit(fs_info, space_info, orig_bytes, flush))) {
> +	     btrfs_can_overcommit(fs_info, space_info, orig_bytes, flush))) {
>  		btrfs_space_info_update_bytes_may_use(fs_info, space_info,
>  						      orig_bytes);
>  		ret = 0;
> diff --git a/fs/btrfs/space-info.h b/fs/btrfs/space-info.h
> index 1a349e3f9cc1..24514cd2c6c1 100644
> --- a/fs/btrfs/space-info.h
> +++ b/fs/btrfs/space-info.h
> @@ -127,6 +127,9 @@ int btrfs_reserve_metadata_bytes(struct btrfs_root *root,
>  				 enum btrfs_reserve_flush_enum flush);
>  void btrfs_try_granting_tickets(struct btrfs_fs_info *fs_info,
>  				struct btrfs_space_info *space_info);
> +int btrfs_can_overcommit(struct btrfs_fs_info *fs_info,
> +			 struct btrfs_space_info *space_info, u64 bytes,
> +			 enum btrfs_reserve_flush_enum flush);
>  
>  static inline void btrfs_space_info_free_bytes_may_use(
>  				struct btrfs_fs_info *fs_info,
>