[PATCH 4/9] btrfs: rework btrfs_space_info_add_old_bytes

[Josef Bacik <josef@toxicpanda.com>]

If there are pending tickets and we are overcommitted we will simply
return free'd reservations to space_info->bytes_may_use if we cannot
overcommit any more.  This is problematic because we assume any free
space would have been added to the tickets, and so if we go from an
overcommitted state to not overcommitted we could have plenty of space
in our space_info but be unable to make progress on our tickets because
we only refill tickets from previous reservations.

Consider the following example.  We were allowed to overcommit to
space_info->total_bytes + 2mib.  Now we've allocated all of our chunks
and are no longer allowed to overcommit those extra 2mib.  Assume there
is a 3mib reservation we are now freeing.  Because we can no longer
overcommit we do not partially refill the ticket with the 3mib, instead
we subtract it from space_info->bytes_may_use.  Now the total reserved
space is 1mib less than total_bytes, meaning we have 1mib of space we
could reserve.  Now assume that our ticket is 2mib, and we only have
1mib of space to reclaim, so we have a partial refilling to 1mib.  We
keep trying to flush and eventually give up and ENOSPC the ticket, when
there was the remaining 1mib left in the space_info for usage.

Instead of doing this partial filling of tickets dance we need to simply
add our space to the space_info, and then do the normal check to see if
we can satisfy the whole reservation.  If we can then we wake up the
ticket and carry on.  This solves the above problem and makes it much
more straightforward to understand how the tickets are satisfied.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/space-info.c | 43 ++++++++++++++++---------------------------
 1 file changed, 16 insertions(+), 27 deletions(-)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index a0a36d5768e1..357fe7548e07 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -233,52 +233,41 @@ void btrfs_space_info_add_old_bytes(struct btrfs_fs_info *fs_info,
 				    struct btrfs_space_info *space_info,
 				    u64 num_bytes)
 {
-	struct reserve_ticket *ticket;
 	struct list_head *head;
-	u64 used;
 	enum btrfs_reserve_flush_enum flush = BTRFS_RESERVE_NO_FLUSH;
-	bool check_overcommit = false;
 
 	spin_lock(&space_info->lock);
 	head = &space_info->priority_tickets;
+	btrfs_space_info_update_bytes_may_use(fs_info, space_info, -num_bytes);
 
-	/*
-	 * If we are over our limit then we need to check and see if we can
-	 * overcommit, and if we can't then we just need to free up our space
-	 * and not satisfy any requests.
-	 */
-	used = btrfs_space_info_used(space_info, true);
-	if (used - num_bytes >= space_info->total_bytes)
-		check_overcommit = true;
 again:
-	while (!list_empty(head) && num_bytes) {
-		ticket = list_first_entry(head, struct reserve_ticket,
-					  list);
-		/*
-		 * We use 0 bytes because this space is already reserved, so
-		 * adding the ticket space would be a double count.
-		 */
-		if (check_overcommit &&
-		    !can_overcommit(fs_info, space_info, 0, flush, false))
-			break;
-		if (num_bytes >= ticket->bytes) {
+	while (!list_empty(head)) {
+		struct reserve_ticket *ticket;
+		u64 used = btrfs_space_info_used(space_info, true);
+
+		ticket = list_first_entry(head, struct reserve_ticket, list);
+
+		/* Check and see if our ticket can be satisified now. */
+		if ((used + ticket->bytes <= space_info->total_bytes) ||
+		    can_overcommit(fs_info, space_info, ticket->bytes, flush,
+				   false)) {
+			btrfs_space_info_update_bytes_may_use(fs_info,
+							      space_info,
+							      ticket->bytes);
 			list_del_init(&ticket->list);
-			num_bytes -= ticket->bytes;
 			ticket->bytes = 0;
 			space_info->tickets_id++;
 			wake_up(&ticket->wait);
 		} else {
-			ticket->bytes -= num_bytes;
-			num_bytes = 0;
+			break;
 		}
 	}
 
-	if (num_bytes && head == &space_info->priority_tickets) {
+	if (head == &space_info->priority_tickets) {
 		head = &space_info->tickets;
 		flush = BTRFS_RESERVE_FLUSH_ALL;
 		goto again;
 	}
-	btrfs_space_info_update_bytes_may_use(fs_info, space_info, -num_bytes);
 	spin_unlock(&space_info->lock);
 }
 
-- 
2.21.0