Re: [PATCH] Btrfs: fix early ENOSPC due to delalloc

[Omar Sandoval <osandov@osandov.com>]

On Thu, Jul 06, 2017 at 10:59:27PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> If a lot of metadata is reserved for outstanding delayed allocations, we
> rely on shrink_delalloc() to reclaim metadata space in order to fulfill
> reservation tickets. However, shrink_delalloc() has a shortcut where if
> it determines that space can be overcommitted, it will stop early. This
> made sense before the ticketed enospc system, but now it means that
> shrink_delalloc() will often not reclaim enough space to fulfill any
> tickets, leading to an early ENOSPC. (Reservation tickets don't care
> about being able to overcommit, they need every byte accounted for.)
> 
> Fix it by getting rid of the shortcut so that shrink_delalloc() reclaims
> all of the metadata it is supposed to. This fixes early ENOSPCs we were
> seeing when doing a btrfs receive to populate a new filesystem.

Jeff, Nikolay, did either of you get a chance to test this yet?

> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> I don't have a good reproducer for this except for the btrfs send stream
> I was given by someone internally, unfortunately.
> 
>  fs/btrfs/extent-tree.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 33d979e9ea2a..83eecd33ad96 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4776,10 +4776,6 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig,
>  		else
>  			flush = BTRFS_RESERVE_NO_FLUSH;
>  		spin_lock(&space_info->lock);
> -		if (can_overcommit(root, space_info, orig, flush)) {
> -			spin_unlock(&space_info->lock);
> -			break;
> -		}
>  		if (list_empty(&space_info->tickets) &&
>  		    list_empty(&space_info->priority_tickets)) {
>  			spin_unlock(&space_info->lock);
> -- 
> 2.13.2
>