Re: [PATCH v4] btrfs: Don't remove block group still has pinned down bytes

[Qu Wenruo <wqu@suse.de>]

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On 2018年06月22日 17:52, David Sterba wrote:
> On Fri, Jun 22, 2018 at 12:35:00PM +0800, Qu Wenruo wrote:
>> [BUG]
>> Under certain KVM load and LTP tests, we are possible to hit the
>> following calltrace if quota is enabled:
>> ------
>> BTRFS critical (device vda2): unable to find logical 8820195328 length 4096
>> BTRFS critical (device vda2): unable to find logical 8820195328 length 4096
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 49 at ../block/blk-core.c:172 blk_status_to_errno+0x1a/0x30
>> CPU: 0 PID: 49 Comm: kworker/u2:1 Not tainted 4.12.14-15-default #1 SLE15 (unreleased)
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
>> Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
>> task: ffff9f827b340bc0 task.stack: ffffb4f8c0304000
>> RIP: 0010:blk_status_to_errno+0x1a/0x30
>> Call Trace:
>>  submit_extent_page+0x191/0x270 [btrfs]
>>  ? btrfs_create_repair_bio+0x130/0x130 [btrfs]
>>  __do_readpage+0x2d2/0x810 [btrfs]
>>  ? btrfs_create_repair_bio+0x130/0x130 [btrfs]
>>  ? run_one_async_done+0xc0/0xc0 [btrfs]
>>  __extent_read_full_page+0xe7/0x100 [btrfs]
>>  ? run_one_async_done+0xc0/0xc0 [btrfs]
>>  read_extent_buffer_pages+0x1ab/0x2d0 [btrfs]
>>  ? run_one_async_done+0xc0/0xc0 [btrfs]
>>  btree_read_extent_buffer_pages+0x94/0xf0 [btrfs]
>>  read_tree_block+0x31/0x60 [btrfs]
>>  read_block_for_search.isra.35+0xf0/0x2e0 [btrfs]
>>  btrfs_search_slot+0x46b/0xa00 [btrfs]
>>  ? kmem_cache_alloc+0x1a8/0x510
>>  ? btrfs_get_token_32+0x5b/0x120 [btrfs]
>>  find_parent_nodes+0x11d/0xeb0 [btrfs]
>>  ? leaf_space_used+0xb8/0xd0 [btrfs]
>>  ? btrfs_leaf_free_space+0x49/0x90 [btrfs]
>>  ? btrfs_find_all_roots_safe+0x93/0x100 [btrfs]
>>  btrfs_find_all_roots_safe+0x93/0x100 [btrfs]
>>  btrfs_find_all_roots+0x45/0x60 [btrfs]
>>  btrfs_qgroup_trace_extent_post+0x20/0x40 [btrfs]
>>  btrfs_add_delayed_data_ref+0x1a3/0x1d0 [btrfs]
>>  btrfs_alloc_reserved_file_extent+0x38/0x40 [btrfs]
>>  insert_reserved_file_extent.constprop.71+0x289/0x2e0 [btrfs]
>>  btrfs_finish_ordered_io+0x2f4/0x7f0 [btrfs]
>>  ? pick_next_task_fair+0x2cd/0x530
>>  ? __switch_to+0x92/0x4b0
>>  btrfs_worker_helper+0x81/0x300 [btrfs]
>>  process_one_work+0x1da/0x3f0
>>  worker_thread+0x2b/0x3f0
>>  ? process_one_work+0x3f0/0x3f0
>>  kthread+0x11a/0x130
>>  ? kthread_create_on_node+0x40/0x40
>>  ret_from_fork+0x35/0x40
>> Code: 00 00 5b c3 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 40 80 ff 0c 40 0f b6 c7 77 0b 48 c1 e0 04 8b 80 00 bf c8 bd c3 <0f> 0b b8 fb ff ff ff c3 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00
>> ---[ end trace f079fb809e7a862b ]---
>> BTRFS critical (device vda2): unable to find logical 8820195328 length 16384
>> BTRFS: error (device vda2) in btrfs_finish_ordered_io:3023: errno=-5 IO failure
>> BTRFS info (device vda2): forced readonly
>> BTRFS error (device vda2): pending csums is 2887680
>> ------
>>
>> [CAUSE]
>> It's caused by race with block group auto removal like the following
>> case:
>> - There is a meta block group X, which has only one tree block
>>   The tree block belongs to fs tree 257.
>> - In current transaction, some operation modified fs tree 257
>>   The tree block get CoWed, so the block group X is empty, and marked as
>>   unused, queued to be deleted.
>> - Some workload (like fsync) wakes up cleaner_kthread()
>>   Which will call btrfs_deleted_unused_bgs() to remove unused block
>>   groups.
>>   So block group X along its chunk map get removed.
>> - Some delalloc work finished for fs tree 257
>>   Quota needs to get the original reference of the extent, which will
>>   reads tree blocks of commit root of 257.
>>   Then since the chunk map get removed, above warning get triggered.
>>
>> [FIX]
>> Just teach btrfs_delete_unused_bgs() to skip block group who still has
>> pinned bytes.
>>
>> However there is a minor side effect, since currently we only queue
>> empty blocks at update_block_group(), and such empty block group with
>> pinned bytes won't go through update_block_group() again, such block
>> group won't be removed, until it get new extent allocated and removed.
> 
> So this cannot lead to free block groups that will not get cleaned for
> some longer period of time, right? After the bytes are unpinned and
> cleaner thread has a chance to run, the bg will not be blocked for
> deletion anymore.

Not exactly.

For this case, it's still possible we missed some empty block to cleanup:

btrfs_update_block_group()
|- mark one block group as unused

<While that bg still has pinned bytes>

btrfs_delete_unused_bgs()
|- Find that block group has pinned bytes
|- Remove it from unused_bgs list (ignore it)

Now that empty bg with pinned bytes will not be deleted any more.
Even after the pinned bytes are freed. (since it's no longer traced by
unused_bgs list)

In short, empty block group (with or without pinned bytes) will either
1) Get reused (have reserved/used)
   And will not be traced.
2) Ignored (have pinned) <<< This is the new behavior
   Will not be traced just as 1)
3) Removed


Thanks,
Qu
> 
> The visible effect of that is that the deletion might be slightly
> delayed.
> 


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