The patch below does not apply to the 4.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git
commit
id to [off-list ref].

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 17024ad0a0fdfcfe53043afb969b813d3e020c21 Mon Sep 17 00:00:00
2001
From: Omar Sandoval <redacted>
Date: Thu, 20 Jul 2017 15:10:35 -0700
Subject: [PATCH] Btrfs: fix early ENOSPC due to delalloc

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, as
well
as early ENOSPCs Christoph saw when doing a big cp -r onto Btrfs.

Fixes: 957780eb2788 ("Btrfs: introduce ticketed enospc
infrastructure")
Tested-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.na
me>
Cc: stable@vger.kernel.org
Reviewed-by: Josef Bacik <redacted>
Signed-off-by: Omar Sandoval <redacted>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a6635f07b8f1..e3b0b4196d3d 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c

@@ -4825,10 +4825,6 @@ static void shrink_delalloc(struct

btrfs_fs_info *fs_info, u64 to_reclaim,
 		else
 			flush = BTRFS_RESERVE_NO_FLUSH;
 		spin_lock(&space_info->lock);
-		if (can_overcommit(fs_info, space_info, orig, flush,
false)) {
-			spin_unlock(&space_info->lock);
-			break;
-		}
 		if (list_empty(&space_info->tickets) &&
 		    list_empty(&space_info->priority_tickets)) {
 			spin_unlock(&space_info->lock);