btrfs: do not allow reservations if we have pending tickets

If we already have tickets on the list we don't want to steal their
reservations.  This is a preparation patch for upcoming changes,
technically this shouldn't happen today because of the way we add bytes
to tickets before adding them to the space_info in most cases.

This does not change the FIFO nature of reserve tickets, it simply
allows us to enforce it in a different way.  Previously it was enforced
because any new space would be added to the first ticket on the list,
which would result in new reservations getting a reserve ticket.  This
replaces that mechanism by simply checking to see if we have outstanding
reserve tickets and skipping straight to adding a ticket for our
reservation.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index bea7ae0a9739d4758dd7d7f8d3cee2ea1570ebbf..918bffb6ba30ad2e56d7512e254a741a4deab2d4 100644 (file)
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -993,6 +993,7 @@ static int __reserve_metadata_bytes(struct btrfs_fs_info *fs_info,
        struct reserve_ticket ticket;
        u64 used;
        int ret = 0;
+       bool pending_tickets;
 
        ASSERT(orig_bytes);
        ASSERT(!current->journal_info || flush != BTRFS_RESERVE_FLUSH_ALL);
@@ -1000,14 +1001,17 @@ static int __reserve_metadata_bytes(struct btrfs_fs_info *fs_info,
        spin_lock(&space_info->lock);
        ret = -ENOSPC;
        used = btrfs_space_info_used(space_info, true);
+       pending_tickets = !list_empty(&space_info->tickets) ||
+               !list_empty(&space_info->priority_tickets);
 
        /*
         * Carry on if we have enough space (short-circuit) OR call
         * can_overcommit() to ensure we can overcommit to continue.
         */
-       if ((used + orig_bytes <= space_info->total_bytes) ||
-           can_overcommit(fs_info, space_info, orig_bytes, flush,
-                          system_chunk)) {
+       if (!pending_tickets &&
+           ((used + orig_bytes <= space_info->total_bytes) ||
+            can_overcommit(fs_info, space_info, orig_bytes, flush,
+                          system_chunk))) {
                btrfs_space_info_update_bytes_may_use(fs_info, space_info,
                                                      orig_bytes);
                trace_btrfs_space_reservation(fs_info, "space_info",