Re: [PATCH 7/8] btrfs: fix may_commit_transaction to deal with no partial filling

[Nikolay Borisov <nborisov@suse.com>]

On 16.08.19 г. 17:19 ч., Josef Bacik wrote:
> Now that we aren't partially filling tickets we may have some slack
> space left in the space_info.  We need to account for this in
> may_commit_transaction, otherwise we may choose to not commit the
> transaction despite it actually having enough space to satisfy our
> ticket.
> 
> Calculate the free space we have in the space_info, if any.  Then check
> to see if its >= the amount of bytes_needed after we've accounted for
> the space being used by the delayed refs rsv.  If it's not subtract it
> from the bytes_needed before doing the final pinned check.  If we still
> don't have enough space then we are truly out of space.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/space-info.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index bd485be783b8..f79afdc04925 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -471,12 +471,19 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,
>  	struct btrfs_trans_handle *trans;
>  	u64 bytes_needed;
>  	u64 reclaim_bytes = 0;
> +	u64 cur_free_bytes = 0;
>  
>  	trans = (struct btrfs_trans_handle *)current->journal_info;
>  	if (trans)
>  		return -EAGAIN;
>  
>  	spin_lock(&space_info->lock);
> +	cur_free_bytes = btrfs_space_info_used(space_info, true);
> +	if (cur_free_bytes < space_info->total_bytes)
> +		cur_free_bytes = space_info->total_bytes - cur_free_bytes;
> +	else
> +		cur_free_bytes = 0;
> +

Why don't you subtract  cur_free_bytes from bytes_needed right here,
giving you more chance to hit the:

if (reclaim_bytes >= bytes_needed)
                goto commit;

>  	if (!list_empty(&space_info->priority_tickets))
>  		ticket = list_first_entry(&space_info->priority_tickets,
>  					  struct reserve_ticket, list);
> @@ -522,6 +529,18 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,
>  		goto commit;
>  	bytes_needed -= reclaim_bytes;
>  
> +	/*
> +	 * We don't partially fill tickets anymore, so we could have some free
> +	 * bytes in the space_info already, just not enough to satisfy the
> +	 * ticket.  If bytes_needed is already below cur_free_bytes after taking
> +	 * away the delayed refs and delayed rsv's then we can commit.
> +	 * Otherwise subtract our cur_free_bytes from bytes_needed before we
> +	 * check pinned.
> +	 */
> +	if (bytes_needed <= cur_free_bytes)
> +		goto commit;

If you do that you won't need that check because you have already
accounted for the current free space.

> +	bytes_needed -= cur_free_bytes;

And this line will be gone as well.

> +
>  	if (__percpu_counter_compare(&space_info->total_bytes_pinned,
>  				   bytes_needed,
>  				   BTRFS_TOTAL_BYTES_PINNED_BATCH) < 0)
>