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

["Stéphane Lesimple" <stephane_btrfs@lesimple.fr>]

Le 2015-10-22 19:03, Filipe Manana a écrit :
> On Thu, Oct 22, 2015 at 3:58 PM, Stéphane Lesimple
> <stephane_btrfs@lesimple.fr> wrote:
>> Le 2015-10-22 11:47, Filipe Manana a écrit :
>>> 
>>> On Thu, Oct 22, 2015 at 10:43 AM, Filipe Manana <fdmanana@kernel.org>
>>> wrote:
>>>> 
>>>> 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.
>>> 
>>> 
>>> And to make it clear, that wouldn't work. If a backref walker starts
>>> iterating the btrees and then new delayed refs get merged
>>> independently of the current tree mod seq, the walker will see an
>>> inconsistent state in the extent tree if the delayed references are
>>> run (which can happen often before a transaction commit).
>>> 
>>> So either make delayed references continue using tree mod seq as
>>> before 4.2, or come with a whole new mechanism that replaces the tree
>>> mod seq while still giving the same consistency guarantees.
>> 
>> 
>> I used your patch at https://patchwork.kernel.org/patch/7463161/ to 
>> build a
>> 4.3.0-rc6 kernel.
>> I mounted my FS with skip_balance, cancelled the paused balance (just 
>> to be
>> sure), then started a new one with :
>> 
>> # btrfs balance start -dconvert=raid5,soft /tank
>> 
>> (half of my data blocks are RAID1, the other half is RAID5, the goal 
>> is to
>> be fully RAID5).
>> 
>> I got a different stacktrace than the usual one, via netconsole, after 
>> a few
>> minutes. It's still referencing btrfs_run_delayed_refs though:
>> 
>> [  822.461809] ------------[ cut here ]------------
>> [  822.461833] kernel BUG at fs/btrfs/extent-tree.c:2287!
>> [  822.461849] invalid opcode: 0000 [#1] SMP
>> [  822.461866] Modules linked in: nfnetlink_queue nfnetlink_log 
>> nfnetlink
>> xt_multiport xt_comment xt_conntrack xt_nat xt_tcpudp xts gf128mul 
>> drbg
>> ansi_cprng btrfs nf_conntrack_ftp nf_conntrack_sane iptable_security
>> iptable_filter iptable_mangle iptable_nat nf_conntrack_ipv4 
>> nf_defrag_ipv4
>> [  822.462365] CPU: 1 PID: 3810 Comm: btrfs-transacti Tainted: G       
>>  W
>> 4.3.0-rc6p7463161+ #3
>> [  822.462391] Hardware name: ASUS All Series/H87I-PLUS, BIOS 1005
>> 01/06/2014
>> [  822.462412] task: ffff88011a4a1a00 ti: ffff8800b6638000 task.ti:
>> ffff8800b6638000
>> [  822.462434] RIP: 0010:[<ffffffffc032310b>]  [<ffffffffc032310b>]
>> __btrfs_run_delayed_refs.constprop.73+0x108b/0x10d0 [btrfs]
>> [  822.462476] RSP: 0018:ffff8800b663bcb0  EFLAGS: 00010202
>> [  822.462495] RAX: 0000000000000001 RBX: ffff8800b3821888 RCX:
>> ffff8800b3a46cb8
>> [  822.462517] RDX: 0000000000000001 RSI: 00000000000001e1 RDI:
>> ffff8800b3a46cb0
>> [  822.462543] RBP: ffff8800b663bdb8 R08: 0000000000000000 R09:
>> ffff8800b3a46cb8
>> [  822.462565] R10: ffff8800b3a46cb8 R11: ffff8800b3a46cb8 R12:
>> 0000000000000000
>> [  822.462587] R13: 0000000000000000 R14: 00000afefd330000 R15:
>> ffff8800b3a46c38
>> [  822.462687] FS:  0000000000000000(0000) GS:ffff88011fb00000(0000)
>> knlGS:0000000000000000
>> [  822.462716] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [  822.462736] CR2: 00007ff7a21ca000 CR3: 0000000001c10000 CR4:
>> 00000000000406e0
>> [  822.462762] Stack:
>> [  822.462770]  ffffffffc037c8be ffff88011fb169f0 0000000000000001
>> 0000000000000001
>> [  822.462802]  0000000000000001 ffff88011a4a1a60 ffff8800b663bd38
>> ffff8800b663bd40
>> [  822.462831]  ffffffff810aded5 ffffffff8101fcc9 ffff8800b663bd20
>> 00000000000014d9
>> [  822.462859] Call Trace:
>> [  822.462877]  [<ffffffffc037c8be>] ?
>> try_merge_free_space.isra.26+0x12e/0x180 [btrfs]
>> [  822.462902]  [<ffffffff810aded5>] ? put_prev_entity+0x35/0x660
>> [  822.462924]  [<ffffffff8101fcc9>] ? sched_clock+0x9/0x10
>> [  822.462949]  [<ffffffffc0325dcd>] btrfs_run_delayed_refs+0x7d/0x2b0
>> [btrfs]
>> [  822.462972]  [<ffffffff817aa7ab>] ? schedule_timeout+0x16b/0x2a0
>> [  822.462999]  [<ffffffffc033a6b3>] 
>> btrfs_commit_transaction+0x43/0xb10
>> [btrfs]
>> [  822.463028]  [<ffffffffc0335c19>] transaction_kthread+0x1a9/0x230 
>> [btrfs]
>> [  822.463056]  [<ffffffffc0335a70>] ? 
>> btrfs_cleanup_transaction+0x550/0x550
>> [btrfs]
>> [  822.463080]  [<ffffffff81097099>] kthread+0xc9/0xe0
>> [  822.463096]  [<ffffffff81096fd0>] ? kthread_park+0x60/0x60
>> [  822.463116]  [<ffffffff817aba8f>] ret_from_fork+0x3f/0x70
>> [  822.463134]  [<ffffffff81096fd0>] ? kthread_park+0x60/0x60
>> [  822.463153] Code: c0 48 8b bd 40 ff ff ff 31 c0 e8 31 94 fe ff 0f 
>> 0b be
>> d3 00 00 00 48 c7 c7 3b c6 3b c0 e8 6e 63 d5 c0 e9 64 f9 ff ff 0f 0b 
>> 0f 0b
>> <0f> 0b be d3 00 00 00 48 c7 c7 3b c6 3b c0 e8 52 63 d5 c0 e9 60
>> [  822.463300] RIP  [<ffffffffc032310b>]
>> __btrfs_run_delayed_refs.constprop.73+0x108b/0x10d0 [btrfs]
>> [  822.463335]  RSP <ffff8800b663bcb0>
>> [  822.472131] ---[ end trace f1e21f38cb0ea144 ]---
>> 
>> A couple other stacktraces follow after some seconds, then the system 
>> dies
>> completely. sysrqd doesn't even work to reboot it remotely using sysrq
>> logic.
>> 
>> fs/btrfs/extent-tree.c:2287 is the line you get from a vanilla 4.3-rc6 
>> +
>> your patch. I'll post it as soon as I can get somebody to manually 
>> reboot
>> this remote machine (my kernel build machine is the same than the one
>> hosting the btrfs FS).
>> 
>> Don't hesitate to ask if you need me to debug or even ftrace 
>> something.
> 
> Thanks Stéphane. I haven't seen that crash yet (still running tests
> for 2 consecutive days now).
> Can you please try the following patch, which works on top of mine,
> and enable ftrace before running balance:
> 
> Debug patch:  https://friendpaste.com/5s3dItRpcpq3dH1E4KUJor
> 
> Enable ftrace:
> 
>     $ echo > /sys/kernel/debug/tracing/trace
>     $ echo "nop" > /sys/kernel/debug/tracing/current_tracer
>     $ echo 100000 > /sys/kernel/debug/tracing/buffer_size_kb   # if
> you can use larger buffer size, even better
>     $ echo > /sys/kernel/debug/tracing/set_ftrace_filter
>     $ echo 1 > /sys/kernel/debug/tracing/tracing_on
> 
>     $ run balance... wait until it finishes with IO error or the
> patch's printk message shows up in dmesg/syslog
> 
>     $ echo 0 > /sys/kernel/debug/tracing/tracing_on
> 
>     $ cat /sys/kernel/debug/tracing/trace > some_file.txt
> 
> Then send is some_file.txt for debugging, hopefully it will give some
> useful information. Note that it might produce tons of messages,
> depending on how long it takes for you to hit the BUG_ON.
> 
> Thanks a lot for this.

I'm compiling it now (using your v2 of the friendpaste diff).

I took the liberty to add a tracing_off() right before the return -EIO 
so that the trace tail ends exactly at the right place.

Last time I tried to use ftrace to diagnose the bug we're trying to fix, 
the system crashes so hard that usually it's complicated to get the 
trace contents written somewhere before the system is unusable. But I'll 
eventually work around it by using /sys/kernel/debug/tracing/trace_pipe 
to send the trace live to another machine over the LAN.

This series of bugs are so easy to trigger on my system that we'll 
hopefully get something useful out of the trace. I guess that's a good 
thing !

>> 
>> Thanks,
>> 
>> --
>> Stéphane.
>> 
>>>>> 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
>>>>>> 
>>>>> 
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