Re: [PATCH 1/8] btrfs: do not allow reservations if we have pending tickets

[Nikolay Borisov <nborisov@suse.com>]

On 16.08.19 г. 17:19 ч., Josef Bacik wrote:
> If we already have tickets on the list we don't want to steal their
> reservations.  This is a preparation patch for upcoming changes,
> technically this shouldn't happen today because of the way we add bytes
> to tickets before adding them to the space_info in most cases.

nit: IMO this changelog should be a bit more explicit since this commit
really makes reservations in FIFO order. Have you also quantified what's
the latency impact as I suspect this will introduce such latencies?


> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/space-info.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index e9406b2133d1..d671d6476eed 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -938,6 +938,7 @@ static int __reserve_metadata_bytes(struct btrfs_fs_info *fs_info,
>  	u64 used;
>  	u64 reclaim_bytes = 0;
>  	int ret = 0;
> +	bool pending_tickets;
>  
>  	ASSERT(orig_bytes);
>  	ASSERT(!current->journal_info || flush != BTRFS_RESERVE_FLUSH_ALL);
> @@ -945,14 +946,17 @@ static int __reserve_metadata_bytes(struct btrfs_fs_info *fs_info,
>  	spin_lock(&space_info->lock);
>  	ret = -ENOSPC;
>  	used = btrfs_space_info_used(space_info, true);
> +	pending_tickets = !list_empty(&space_info->tickets) ||
> +		!list_empty(&space_info->priority_tickets);
>  
>  	/*
>  	 * Carry on if we have enough space (short-circuit) OR call
>  	 * can_overcommit() to ensure we can overcommit to continue.
>  	 */
> -	if ((used + orig_bytes <= space_info->total_bytes) ||
> -	    can_overcommit(fs_info, space_info, orig_bytes, flush,
> -			   system_chunk)) {
> +	if (!pending_tickets &&
> +	    ((used + orig_bytes <= space_info->total_bytes) ||
> +	     can_overcommit(fs_info, space_info, orig_bytes, flush,
> +			   system_chunk))) {
>  		btrfs_space_info_update_bytes_may_use(fs_info, space_info,
>  						      orig_bytes);
>  		trace_btrfs_space_reservation(fs_info, "space_info",
>