Re: [PATCH 3/5] btrfs: use add_old_bytes when updating global reserve

[Nikolay Borisov <nborisov@suse.com>]

On 16.08.19 г. 18:20 ч., Josef Bacik wrote:
> We have some annoying xfstests tests that will create a very small fs,
> fill it up, delete it, and repeat to make sure everything works right.
> This trips btrfs up sometimes because we may commit a transaction to
> free space, but most of the free metadata space was being reserved by
> the global reserve.  So we commit and update the global reserve, but the
> space is simply added to bytes_may_use directly, instead of trying to
> add it to existing tickets.  This results in ENOSPC when we really did
> have space.  Fix this by returning the space via
> btrfs_space_info_add_old_bytes.  The global reserve _can_ be using
> overcommitted space, but the add_old_bytes checks this and won't add the
> reservation if we're still overcommitted, so we are safe in this regard.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com> but see below for one
suggestion.

> ---
>  fs/btrfs/block-rsv.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index 18a0af20ee5a..394b8fff3a4b 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -258,6 +258,7 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
>  	struct btrfs_space_info *sinfo = block_rsv->space_info;
>  	u64 num_bytes;
> +	u64 to_free = 0;
>  	unsigned min_items;
>  
>  	/*
> @@ -300,9 +301,7 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  		btrfs_space_info_update_bytes_may_use(fs_info, sinfo,
>  						      num_bytes);
>  	} else if (block_rsv->reserved > block_rsv->size) {
> -		num_bytes = block_rsv->reserved - block_rsv->size;
> -		btrfs_space_info_update_bytes_may_use(fs_info, sinfo,
> -						      -num_bytes);
> +		to_free = block_rsv->reserved - block_rsv->size;

nit: Since you hold  sinfo->lock here you could just call the try to
wakeup function, you already have the bytes_may_use update call, that
will also make the to_free variable private to this 'else if' branch.
The only reason to suggest is because further down the sinfo->lock is
released only to be acquired milliseconds later if to_free != 0

>  		block_rsv->reserved = block_rsv->size;
>  	}
>  
> @@ -313,6 +312,9 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  
>  	spin_unlock(&block_rsv->lock);
>  	spin_unlock(&sinfo->lock);
> +
> +	if (to_free)
> +		btrfs_space_info_add_old_bytes(fs_info, sinfo, to_free);
>  }
>  
>  void btrfs_init_global_block_rsv(struct btrfs_fs_info *fs_info)
>