From: Josef Bacik <josef@toxicpanda.com>
Date: Wed, 28 Apr 2021 13:38:45 -0400
Subject: btrfs: use the global rsv size in the preemptive thresh calculation
Git-commit: 1239e2da16bf85e13063de7d2e9638219efca984
Patch-mainline: v5.14-rc1
References: bsc#1202528

We calculate the amount of "free" space available for normal
reservations by taking the total space and subtracting out the hard used
space, which is readonly, used, and reserved space.

However we weren't taking into account the global block rsv, which is
essentially hard used space.  Handle this by subtracting it from the
available free space, so that our threshold more closely mirrors
reality.

We need to do the check because it's possible that the global_rsv_size +
used is > total_bytes, sometimes the global reserve can end up being
calculated as larger than the available size (think small filesystems
where we only have the original 8MiB chunk of metadata).  It doesn't
usually happen, but that can get us into trouble so this is safer.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Acked-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/space-info.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index e341f995a7dd..fedf3440145a 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -840,8 +840,10 @@ static bool need_preemptive_reclaim(struct btrfs_fs_info *fs_info,
 
 	thresh = calc_available_free_space(fs_info, space_info,
 					   BTRFS_RESERVE_FLUSH_ALL);
-	thresh += (space_info->total_bytes - space_info->bytes_used -
-		   space_info->bytes_reserved - space_info->bytes_readonly);
+	used = space_info->bytes_used + space_info->bytes_reserved +
+	       space_info->bytes_readonly + global_rsv_size;
+	if (used < space_info->total_bytes)
+		thresh += space_info->total_bytes - used;
 	thresh >>= space_info->clamp;
 
 	used = space_info->bytes_pinned;