[PATCH 2/5] btrfs: always reserve our entire size for the global reserve

[Josef Bacik <josef@toxicpanda.com>]

While messing with the overcommit logic I noticed that sometimes we'd
ENOSPC out when really we should have run out of space much earlier.  It
turns out it's because we'll only reserve up to the free amount left in
the space info for the global reserve, but that doesn't make sense with
overcommit because we could be well above our actual size.  This results
in the global reserve not carving out it's entire reservation, and thus
not putting enough pressure on the rest of the infrastructure to do the
right thing and ENOSPC out at a convenient time.  Fix this by always
taking our full reservation amount for the global reserve.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/block-rsv.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index 76be1d978c36..67071bb8e433 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -296,15 +296,10 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
 	block_rsv->size = min_t(u64, num_bytes, SZ_512M);
 
 	if (block_rsv->reserved < block_rsv->size) {
-		num_bytes = btrfs_space_info_used(sinfo, true);
-		if (sinfo->total_bytes > num_bytes) {
-			num_bytes = sinfo->total_bytes - num_bytes;
-			num_bytes = min(num_bytes,
-					block_rsv->size - block_rsv->reserved);
-			block_rsv->reserved += num_bytes;
-			btrfs_space_info_update_bytes_may_use(fs_info, sinfo,
-							      num_bytes);
-		}
+		num_bytes = block_rsv->size - block_rsv->reserved;
+		block_rsv->reserved += num_bytes;
+		btrfs_space_info_update_bytes_may_use(fs_info, sinfo,
+						      num_bytes);
 	} else if (block_rsv->reserved > block_rsv->size) {
 		num_bytes = block_rsv->reserved - block_rsv->size;
 		btrfs_space_info_update_bytes_may_use(fs_info, sinfo,
-- 
2.21.0