On Sun, Dec 13, 2015 at 10:51 AM, Alex Lyakas <a...@zadarastorage.com> wrote:
> Hi Filipe Manana,
>
> My understanding of selecting delayed refs to run or merging them is
> far from complete. Can you please explain what will happen in the
> following scenario:
>
> 1) Ref1 is created, as you explain
> 2) Somebody calls __btrfs_run_delayed_refs() and runs Ref1, and we end
> up with an EXTENT_ITEM and an inline extent back ref
> 3) Ref2 and Ref3 are added
> 4) Somebody calls __btrfs_run_delayed_refs()
>
> At this point, we cannot merge Ref2 and Ref3, because they might be
> referencing tree blocks of completely different trees, thus
> comp_tree_refs() will return 1 or -1. But we will select Ref3 to be
> run, because we prefer BTRFS_ADD_DELAYED_REF over
> BTRFS_DROP_DELAYED_REF, as you explained. So we hit the same BUG_ON
> now, because we already have Ref1 in the extent tree.
No, that won't happen. If the ref (Ref3) is for a different tree, than
it has a different inline extent from Ref1
(lookup_inline_extent_backref returns -ENOENT and not 0).
If they are all for the same tree it means Ref3 is not merged with
Ref2 because they have different seq numbers and a seq value exist in
fs_info->tree_mod_seq_list, and we skip Ref3 through
btrfs_check_delayed_seq() until such seq number goes away from
tree_mod_seq_list. If no seq number exists in tree_mod_seq_list then
we merge it (Ref3) through btrfs_merge_delayed_refs(), called when
running delayed refs, with Ref2 (which removes both refs since one is
"-1" and the other "+1").
Iow, after this regression fix, no behaviour changed from releases before 4.2.
>
> So something should prevent us from running Ref3 before running Ref2.
> We should run Ref2 first, which should get rid of the EXTENT_ITEM and
> the inline backref, and then run Ref3 to create a new backref with a
> proper owner. What is that something?
>
> Can you please point me at what am I missing?
>
> Also, can such scenario happen in 3.18 kernel, which still has an
> rbtree per ref-head? Looking at the code, I don't see anything
> preventing that from happening.
>
> Thanks,
> Alex.
>
>
> On Sun, Oct 25, 2015 at 8:51 PM, <fdman...@kernel.org> wrote:
>> From: Filipe Manana <fdman...@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;
>
> What happens if Ref1 is run at this point?
>
>>
>> 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;
> Can Ref3 be merged with Ref1? They might be referencing blocks of
> completely different trees (although using the same bytenr). So they
> will have different root/parent and thus cannot be merged, I think.
>
>>
>> 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+.
>>
>> 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).
>>
>> Fixes: c6fc24549960 ("btrfs: delayed-ref: Use list to replace the ref_root
>> in ref_head.")
>> Reported-by: Peter Becker <floyd....@gmail.com>
>> Reported-by: Stéphane Lesimple <stephane_bt...@lesimple.fr>
>> Tested-by: Stéphane Lesimple <stephane_bt...@lesimple.fr>
>> Reported-by: Malte Schröder <ma...@tnxip.de>
>> Reported-by: Derek Dongray <de...@valedon.co.uk>
>> Reported-by: Erkki Seppala <flux-bt...@inside.org>
>> Cc: sta...@vger.kernel.org # 4.2+
>> Signed-off-by: Filipe Manana <fdman...@suse.com>
>> ---
>>
>> V2: No code changes. Added Stéphane's Tested-by tag and made patch
>> part of a 2 fixes series, to reflect the other remaining fix
>> depends on this one.
>> V3: No changes. Updated only the 2nd patch in the series to v3 (change log
>> updated).
>>
>> 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 bd9b63b..2f41580 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 92fdbc6..c0f30f5 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -2502,7 +2502,21 @@ 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
>> --
>> 2.1.3
>>
>> --
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