Re: [PATCH 1/2 v2] Btrfs: fix regression when running delayed references

[Liu Bo <bo.li.liu@oracle.com>]

On 10/25/2015 06:04 PM, fdmanana@kernel.org wrote:
> 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

The value is 1 if we create an eb.

Others look good.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

thanks,

-liubo

>     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+.
>
> 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.net@gmail.com>
> Reported-by: Stéphane Lesimple <stephane_btrfs@lesimple.fr>
> Tested-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>
> ---
>
> 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.
>
>   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 2df4bc7..dd5756c 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
>