• Filipe Manana's avatar
    Btrfs: fix regression running delayed references when using qgroups · b06c4bf5
    Filipe Manana authored
    In the kernel 4.2 merge window we had a big changes to the implementation
    of delayed references and qgroups which made the no_quota field of delayed
    references not used anymore. More specifically the no_quota field is not
    used anymore as of:
    
      commit 0ed4792a ("btrfs: qgroup: Switch to new extent-oriented qgroup mechanism.")
    
    Leaving the no_quota field actually prevents delayed references from
    getting merged, which in turn cause the following BUG_ON(), at
    fs/btrfs/extent-tree.c, to be hit when qgroups are enabled:
    
      static int run_delayed_tree_ref(...)
      {
         (...)
         BUG_ON(node->ref_mod != 1);
         (...)
      }
    
    This happens on a scenario like the following:
    
      1) Ref1 bytenr X, action = BTRFS_ADD_DELAYED_REF, no_quota = 1, added.
    
      2) Ref2 bytenr X, action = BTRFS_DROP_DELAYED_REF, no_quota = 0, added.
         It's not merged with Ref1 because Ref1->no_quota != Ref2->no_quota.
    
      3) Ref3 bytenr X, action = BTRFS_ADD_DELAYED_REF, no_quota = 1, added.
         It's not merged with the reference at the tail of the list of refs
         for bytenr X because the reference at the tail, Ref2 is incompatible
         due to Ref2->no_quota != Ref3->no_quota.
    
      4) Ref4 bytenr X, action = BTRFS_DROP_DELAYED_REF, no_quota = 0, added.
         It's not merged with the reference at the tail of the list of refs
         for bytenr X because the reference at the tail, Ref3 is incompatible
         due to Ref3->no_quota != Ref4->no_quota.
    
      5) We run delayed references, trigger merging of delayed references,
         through __btrfs_run_delayed_refs() -> btrfs_merge_delayed_refs().
    
      6) Ref1 and Ref3 are merged as Ref1->no_quota = Ref3->no_quota and
         all other conditions are satisfied too. So Ref1 gets a ref_mod
         value of 2.
    
      7) Ref2 and Ref4 are merged as Ref2->no_quota = Ref4->no_quota and
         all other conditions are satisfied too. So Ref2 gets a ref_mod
         value of 2.
    
      8) Ref1 and Ref2 aren't merged, because they have different values
         for their no_quota field.
    
      9) Delayed reference Ref1 is picked for running (select_delayed_ref()
         always prefers references with an action == BTRFS_ADD_DELAYED_REF).
         So run_delayed_tree_ref() is called for Ref1 which triggers the
         BUG_ON because Ref1->red_mod != 1 (equals 2).
    
    So fix this by removing the no_quota field, as it's not used anymore as
    of commit 0ed4792a ("btrfs: qgroup: Switch to new extent-oriented
    qgroup mechanism.").
    
    The use of no_quota was also buggy in at least two places:
    
    1) At delayed-refs.c:btrfs_add_delayed_tree_ref() - we were setting
       no_quota to 0 instead of 1 when the following condition was true:
       is_fstree(ref_root) || !fs_info->quota_enabled
    
    2) At extent-tree.c:__btrfs_inc_extent_ref() - we were attempting to
       reset a node's no_quota when the condition "!is_fstree(root_objectid)
       || !root->fs_info->quota_enabled" was true but we did it only in
       an unused local stack variable, that is, we never reset the no_quota
       value in the node itself.
    
    This fixes the remainder of problems several people have been having when
    running delayed references, mostly while a balance is running in parallel,
    on a 4.2+ kernel.
    
    Very special thanks to Stéphane Lesimple for helping debugging this issue
    and testing this fix on his multi terabyte filesystem (which took more
    than one day to balance alone, plus fsck, etc).
    
    Also, this fixes deadlock issue when using the clone ioctl with qgroups
    enabled, as reported by Elias Probst in the mailing list. The deadlock
    happens because after calling btrfs_insert_empty_item we have our path
    holding a write lock on a leaf of the fs/subvol tree and then before
    releasing the path we called check_ref() which did backref walking, when
    qgroups are enabled, and tried to read lock the same leaf. The trace for
    this case is the following:
    
      INFO: task systemd-nspawn:6095 blocked for more than 120 seconds.
      (...)
      Call Trace:
        [<ffffffff86999201>] schedule+0x74/0x83
        [<ffffffff863ef64c>] btrfs_tree_read_lock+0xc0/0xea
        [<ffffffff86137ed7>] ? wait_woken+0x74/0x74
        [<ffffffff8639f0a7>] btrfs_search_old_slot+0x51a/0x810
        [<ffffffff863a129b>] btrfs_next_old_leaf+0xdf/0x3ce
        [<ffffffff86413a00>] ? ulist_add_merge+0x1b/0x127
        [<ffffffff86411688>] __resolve_indirect_refs+0x62a/0x667
        [<ffffffff863ef546>] ? btrfs_clear_lock_blocking_rw+0x78/0xbe
        [<ffffffff864122d3>] find_parent_nodes+0xaf3/0xfc6
        [<ffffffff86412838>] __btrfs_find_all_roots+0x92/0xf0
        [<ffffffff864128f2>] btrfs_find_all_roots+0x45/0x65
        [<ffffffff8639a75b>] ? btrfs_get_tree_mod_seq+0x2b/0x88
        [<ffffffff863e852e>] check_ref+0x64/0xc4
        [<ffffffff863e9e01>] btrfs_clone+0x66e/0xb5d
        [<ffffffff863ea77f>] btrfs_ioctl_clone+0x48f/0x5bb
        [<ffffffff86048a68>] ? native_sched_clock+0x28/0x77
        [<ffffffff863ed9b0>] btrfs_ioctl+0xabc/0x25cb
      (...)
    
    The problem goes away by eleminating check_ref(), which no longer is
    needed as its purpose was to get a value for the no_quota field of
    a delayed reference (this patch removes the no_quota field as mentioned
    earlier).
    Reported-by: default avatarStéphane Lesimple <stephane_btrfs@lesimple.fr>
    Tested-by: default avatarStéphane Lesimple <stephane_btrfs@lesimple.fr>
    Reported-by: default avatarElias Probst <mail@eliasprobst.eu>
    Reported-by: default avatarPeter Becker <floyd.net@gmail.com>
    Reported-by: default avatarMalte Schröder <malte@tnxip.de>
    Reported-by: default avatarDerek Dongray <derek@valedon.co.uk>
    Reported-by: default avatarErkki Seppala <flux-btrfs@inside.org>
    Cc: stable@vger.kernel.org  # 4.2+
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Reviewed-by: default avatarQu Wenruo <quwenruo@cn.fujitsu.com>
    b06c4bf5
tree-log.c 145 KB