Re: [PATCH] Btrfs: fix regression when running delayed references

[Filipe Manana <fdmanana@kernel.org>]

On Thu, Oct 22, 2015 at 10:32 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>
>
>  wrote on 2015/10/22 09:47 +0100:
>>
>> From: Filipe Manana <fdmanana@suse.com>
>>
>> In the kernel 4.2 merge window we had a refactoring/rework of the delayed
>> references implementation in order to fix certain problems with qgroups.
>> However that rework introduced one more regression that leads to the
>> following trace when running delayed references for metadata:
>>
>> [35908.064664] kernel BUG at fs/btrfs/extent-tree.c:1832!
>> [35908.065201] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>> [35908.065201] Modules linked in: dm_flakey dm_mod btrfs crc32c_generic
>> xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache
>> sunrpc loop fuse parport_pc psmouse i2
>> [35908.065201] CPU: 14 PID: 15014 Comm: kworker/u32:9 Tainted: G        W
>> 4.3.0-rc5-btrfs-next-17+ #1
>> [35908.065201] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>> rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
>> [35908.065201] Workqueue: btrfs-extent-refs btrfs_extent_refs_helper
>> [btrfs]
>> [35908.065201] task: ffff880114b7d780 ti: ffff88010c4c8000 task.ti:
>> ffff88010c4c8000
>> [35908.065201] RIP: 0010:[<ffffffffa04928b5>]  [<ffffffffa04928b5>]
>> insert_inline_extent_backref+0x52/0xb1 [btrfs]
>> [35908.065201] RSP: 0018:ffff88010c4cbb08  EFLAGS: 00010293
>> [35908.065201] RAX: 0000000000000000 RBX: ffff88008a661000 RCX:
>> 0000000000000000
>> [35908.065201] RDX: ffffffffa04dd58f RSI: 0000000000000001 RDI:
>> 0000000000000000
>> [35908.065201] RBP: ffff88010c4cbb40 R08: 0000000000001000 R09:
>> ffff88010c4cb9f8
>> [35908.065201] R10: 0000000000000000 R11: 000000000000002c R12:
>> 0000000000000000
>> [35908.065201] R13: ffff88020a74c578 R14: 0000000000000000 R15:
>> 0000000000000000
>> [35908.065201] FS:  0000000000000000(0000) GS:ffff88023edc0000(0000)
>> knlGS:0000000000000000
>> [35908.065201] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>> [35908.065201] CR2: 00000000015e8708 CR3: 0000000102185000 CR4:
>> 00000000000006e0
>> [35908.065201] Stack:
>> [35908.065201]  ffff88010c4cbb18 0000000000000f37 ffff88020a74c578
>> ffff88015a408000
>> [35908.065201]  ffff880154a44000 0000000000000000 0000000000000005
>> ffff88010c4cbbd8
>> [35908.065201]  ffffffffa0492b9a 0000000000000005 0000000000000000
>> 0000000000000000
>> [35908.065201] Call Trace:
>> [35908.065201]  [<ffffffffa0492b9a>] __btrfs_inc_extent_ref+0x8b/0x208
>> [btrfs]
>> [35908.065201]  [<ffffffffa0497117>] ?
>> __btrfs_run_delayed_refs+0x4d4/0xd33 [btrfs]
>> [35908.065201]  [<ffffffffa049773d>] __btrfs_run_delayed_refs+0xafa/0xd33
>> [btrfs]
>> [35908.065201]  [<ffffffffa04a976a>] ? join_transaction.isra.10+0x25/0x41f
>> [btrfs]
>> [35908.065201]  [<ffffffffa04a97ed>] ? join_transaction.isra.10+0xa8/0x41f
>> [btrfs]
>> [35908.065201]  [<ffffffffa049914d>] btrfs_run_delayed_refs+0x75/0x1dd
>> [btrfs]
>> [35908.065201]  [<ffffffffa04992f1>] delayed_ref_async_start+0x3c/0x7b
>> [btrfs]
>> [35908.065201]  [<ffffffffa04d4b4f>] normal_work_helper+0x14c/0x32a
>> [btrfs]
>> [35908.065201]  [<ffffffffa04d4e93>] btrfs_extent_refs_helper+0x12/0x14
>> [btrfs]
>> [35908.065201]  [<ffffffff81063b23>] process_one_work+0x24a/0x4ac
>> [35908.065201]  [<ffffffff81064285>] worker_thread+0x206/0x2c2
>> [35908.065201]  [<ffffffff8106407f>] ? rescuer_thread+0x2cb/0x2cb
>> [35908.065201]  [<ffffffff8106407f>] ? rescuer_thread+0x2cb/0x2cb
>> [35908.065201]  [<ffffffff8106904d>] kthread+0xef/0xf7
>> [35908.065201]  [<ffffffff81068f5e>] ? kthread_parkme+0x24/0x24
>> [35908.065201]  [<ffffffff8147d10f>] ret_from_fork+0x3f/0x70
>> [35908.065201]  [<ffffffff81068f5e>] ? kthread_parkme+0x24/0x24
>> [35908.065201] Code: 6a 01 41 56 41 54 ff 75 10 41 51 4d 89 c1 49 89 c8 48
>> 8d 4d d0 e8 f6 f1 ff ff 48 83 c4 28 85 c0 75 2c 49 81 fc ff 00 00 00 77 02
>> <0f> 0b 4c 8b 45 30 8b 4d 28 45 31
>> [35908.065201] RIP  [<ffffffffa04928b5>]
>> insert_inline_extent_backref+0x52/0xb1 [btrfs]
>> [35908.065201]  RSP <ffff88010c4cbb08>
>> [35908.310885] ---[ end trace fe4299baf0666457 ]---
>>
>> This happens because the new delayed references code no longer merges
>> delayed references that have different sequence values. The following
>> steps are an example sequence leading to this issue:
>>
>> 1) Transaction N starts, fs_info->tree_mod_seq has value 0;
>>
>> 2) Extent buffer (btree node) A is allocated, delayed reference Ref1 for
>>     bytenr A is created, with a value of 1 and a seq value of 0;
>>
>> 3) fs_info->tree_mod_seq is incremented to 1;
>>
>> 4) Extent buffer A is deleted through btrfs_del_items(), which calls
>>     btrfs_del_leaf(), which in turn calls btrfs_free_tree_block(). The
>>     later returns the metadata extent associated to extent buffer A to
>>     the free space cache (the range is not pinned), because the extent
>>     buffer was created in the current transaction (N) and writeback never
>>     happened for the extent buffer (flag BTRFS_HEADER_FLAG_WRITTEN not set
>>     in the extent buffer).
>>     This creates the delayed reference Ref2 for bytenr A, with a value
>>     of -1 and a seq value of 1;
>>
>> 5) Delayed reference Ref2 is not merged with Ref1 when we create it,
>>     because they have different sequence numbers (decided at
>>     add_delayed_ref_tail_merge());
>>
>> 6) fs_info->tree_mod_seq is incremented to 2;
>>
>> 7) Some task attempts to allocate a new extent buffer (done at
>>     extent-tree.c:find_free_extent()), but due to heavy fragmentation
>>     and running low on metadata space the clustered allocation fails
>>     and we fall back to unclustered allocation, which finds the
>>     extent at offset A, so a new extent buffer at offset A is allocated.
>>     This creates delayed reference Ref3 for bytenr A, with a value of -1
>>     and a seq value of 2;
>>
>> 8) Ref3 is not merged neither with Ref2 nor Ref1, again because they
>>     all have different seq values;
>>
>> 9) We start running the delayed references (__btrfs_run_delayed_refs());
>>
>> 10) The delayed Ref1 is the first one being applied, which ends up
>>      creating an inline extent backref in the extent tree;
>>
>> 10) Next the delayed reference Ref3 is selected for execution, and not
>>      Ref2, because select_delayed_ref() always gives a preference for
>>      positive references (that have an action of BTRFS_ADD_DELAYED_REF);
>>
>> 11) When running Ref3 we encounter alreay the inline extent backref
>>      in the extent tree at insert_inline_extent_backref(), which makes
>>      us hit the following BUG_ON:
>>
>>          BUG_ON(owner < BTRFS_FIRST_FREE_OBJECTID);
>>
>>      This is always true because owner corresponds to the level of the
>>      extent buffer/btree node in the btree.
>>
>> For the scenario described above we hit the BUG_ON because we never merge
>> references that have different seq values.
>>
>> We used to do the merging before the 4.2 kernel, more specifically, before
>> the commmits:
>>
>>    c6fc24549960 ("btrfs: delayed-ref: Use list to replace the ref_root in
>> ref_head.")
>>    c43d160fcd5e ("btrfs: delayed-ref: Cleanup the unneeded functions.")
>>
>> This issue became more exposed after the following change that was added
>> to 4.2 as well:
>>
>>    cffc3374e567 ("Btrfs: fix order by which delayed references are run")
>>
>> Which in turn fixed another regression by the two commits previously
>> mentioned.
>>
>> So fix this by bringing back the delayed reference merge code, with the
>> proper adaptations so that it operates against the new data structure
>> (linked list vs old red black tree implementation).
>>
>> This issue was hit running fstest btrfs/063 in a loop. Several people have
>> reported this issue in the mailing list when running on kernels 4.2+.
>
>
> Thanks Filipe,
>
> My fault again. :(
> But I'm not completely sure about if tree_mod_seq is still needed now.
>
> IIRC, with the new qgroup accounting happen at commit_transaction time,
> btrfs_find_all_roots() should either searching commit tree for old roots, or
> search current tree for new roots.
> No need to search using tree_mod_seq.
>
> If so, I'd like just allow merging refs without checking tree_mod_seq.
>
> I was going to do it but not completely sure is there any other user of
> tree_mod_seq.
> And if it's possible to get rid of tree_mod_seq and merge with last
> delayed_ref, things should get cleaner without new codes.

Well, the tree mod seq is what allows backref walkers (and possibly
other paths) to get a consistent view of all btrees and delayed refs
state while doing some processing - that's why we have calls to
btrfs_check_delayed_seq() when running delayed references - so that
any backref walker will not see changes that happen after it started,
i.e. it will see a consistent view of all the btrees (like an
in-memory snapshot of all btrees while the transaction is running).

I don't think you can get this level of consistency through any other
existing means.
So just adding back yet more code that was removed despite still being needed.

This is affecting way too many people now, I would like to get this
fixed and later, if there's a better (new) solution for this, we can
get it in.

thanks

>
> Thanks,
> Qu
>
>
>
>>
>> Fixes: c6fc24549960 ("btrfs: delayed-ref: Use list to replace the ref_root
>> in ref_head.")
>> Reported-by: Peter Becker <floyd.net@gmail.com>
>> Reported-by: Stéphane Lesimple <stephane_btrfs@lesimple.fr>
>> Reported-by: Malte Schröder <malte@tnxip.de>
>> Reported-by: Derek Dongray <derek@valedon.co.uk>
>> Reported-by: Erkki Seppala <flux-btrfs@inside.org>
>> Cc: stable@vger.kernel.org  # 4.2+
>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>> ---
>>   fs/btrfs/delayed-ref.c | 113
>> +++++++++++++++++++++++++++++++++++++++++++++++++
>>   fs/btrfs/extent-tree.c |  14 ++++++
>>   2 files changed, 127 insertions(+)
>>
>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
>> index ac3e81d..4832943 100644
>> --- a/fs/btrfs/delayed-ref.c
>> +++ b/fs/btrfs/delayed-ref.c
>> @@ -197,6 +197,119 @@ static inline void drop_delayed_ref(struct
>> btrfs_trans_handle *trans,
>>                 trans->delayed_ref_updates--;
>>   }
>>
>> +static bool merge_ref(struct btrfs_trans_handle *trans,
>> +                     struct btrfs_delayed_ref_root *delayed_refs,
>> +                     struct btrfs_delayed_ref_head *head,
>> +                     struct btrfs_delayed_ref_node *ref,
>> +                     u64 seq)
>> +{
>> +       struct btrfs_delayed_ref_node *next;
>> +       bool done = false;
>> +
>> +       next = list_first_entry(&head->ref_list, struct
>> btrfs_delayed_ref_node,
>> +                               list);
>> +       while (!done && &next->list != &head->ref_list) {
>> +               int mod;
>> +               struct btrfs_delayed_ref_node *next2;
>> +
>> +               next2 = list_next_entry(next, list);
>> +
>> +               if (next == ref)
>> +                       goto next;
>> +
>> +               if (seq && next->seq >= seq)
>> +                       goto next;
>> +
>> +               if (next->type != ref->type || next->no_quota !=
>> ref->no_quota)
>> +                       goto next;
>> +
>> +               if ((ref->type == BTRFS_TREE_BLOCK_REF_KEY ||
>> +                    ref->type == BTRFS_SHARED_BLOCK_REF_KEY) &&
>> +                   comp_tree_refs(btrfs_delayed_node_to_tree_ref(ref),
>> +                                  btrfs_delayed_node_to_tree_ref(next),
>> +                                  ref->type))
>> +                       goto next;
>> +               if ((ref->type == BTRFS_EXTENT_DATA_REF_KEY ||
>> +                    ref->type == BTRFS_SHARED_DATA_REF_KEY) &&
>> +                   comp_data_refs(btrfs_delayed_node_to_data_ref(ref),
>> +                                  btrfs_delayed_node_to_data_ref(next)))
>> +                       goto next;
>> +
>> +               if (ref->action == next->action) {
>> +                       mod = next->ref_mod;
>> +               } else {
>> +                       if (ref->ref_mod < next->ref_mod) {
>> +                               swap(ref, next);
>> +                               done = true;
>> +                       }
>> +                       mod = -next->ref_mod;
>> +               }
>> +
>> +               drop_delayed_ref(trans, delayed_refs, head, next);
>> +               ref->ref_mod += mod;
>> +               if (ref->ref_mod == 0) {
>> +                       drop_delayed_ref(trans, delayed_refs, head, ref);
>> +                       done = true;
>> +               } else {
>> +                       /*
>> +                        * Can't have multiples of the same ref on a tree
>> block.
>> +                        */
>> +                       WARN_ON(ref->type == BTRFS_TREE_BLOCK_REF_KEY ||
>> +                               ref->type == BTRFS_SHARED_BLOCK_REF_KEY);
>> +               }
>> +next:
>> +               next = next2;
>> +       }
>> +
>> +       return done;
>> +}
>> +
>> +void btrfs_merge_delayed_refs(struct btrfs_trans_handle *trans,
>> +                             struct btrfs_fs_info *fs_info,
>> +                             struct btrfs_delayed_ref_root *delayed_refs,
>> +                             struct btrfs_delayed_ref_head *head)
>> +{
>> +       struct btrfs_delayed_ref_node *ref;
>> +       u64 seq = 0;
>> +
>> +       assert_spin_locked(&head->lock);
>> +
>> +       if (list_empty(&head->ref_list))
>> +               return;
>> +
>> +       /* We don't have too many refs to merge for data. */
>> +       if (head->is_data)
>> +               return;
>> +
>> +       spin_lock(&fs_info->tree_mod_seq_lock);
>> +       if (!list_empty(&fs_info->tree_mod_seq_list)) {
>> +               struct seq_list *elem;
>> +
>> +               elem = list_first_entry(&fs_info->tree_mod_seq_list,
>> +                                       struct seq_list, list);
>> +               seq = elem->seq;
>> +       }
>> +       spin_unlock(&fs_info->tree_mod_seq_lock);
>> +
>> +       ref = list_first_entry(&head->ref_list, struct
>> btrfs_delayed_ref_node,
>> +                              list);
>> +       while (&ref->list != &head->ref_list) {
>> +               if (seq && ref->seq >= seq)
>> +                       goto next;
>> +
>> +               if (merge_ref(trans, delayed_refs, head, ref, seq)) {
>> +                       if (list_empty(&head->ref_list))
>> +                               break;
>> +                       ref = list_first_entry(&head->ref_list,
>> +                                              struct
>> btrfs_delayed_ref_node,
>> +                                              list);
>> +                       continue;
>> +               }
>> +next:
>> +               ref = list_next_entry(ref, list);
>> +       }
>> +}
>> +
>>   int btrfs_check_delayed_seq(struct btrfs_fs_info *fs_info,
>>                             struct btrfs_delayed_ref_root *delayed_refs,
>>                             u64 seq)
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 522fb45..42d9310 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -2433,7 +2433,21 @@ static noinline int __btrfs_run_delayed_refs(struct
>> btrfs_trans_handle *trans,
>>                         }
>>                 }
>>
>> +               /*
>> +                * We need to try and merge add/drops of the same ref
>> since we
>> +                * can run into issues with relocate dropping the implicit
>> ref
>> +                * and then it being added back again before the drop can
>> +                * finish.  If we merged anything we need to re-loop so we
>> can
>> +                * get a good ref.
>> +                * Or we can get node references of the same type that
>> weren't
>> +                * merged when created due to bumps in the tree mod seq,
>> and
>> +                * we need to merge them to prevent adding an inline
>> extent
>> +                * backref before dropping it (triggering a BUG_ON at
>> +                * insert_inline_extent_backref()).
>> +                */
>>                 spin_lock(&locked_ref->lock);
>> +               btrfs_merge_delayed_refs(trans, fs_info, delayed_refs,
>> +                                        locked_ref);
>>
>>                 /*
>>                  * locked_ref is the head node, so we have to go one
>>
>