Re: [PATCH v3] btrfs: Don't remove block group still has pinned down bytes

[Nikolay Borisov <nborisov@suse.com>]

On 15.06.2018 04:35, 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.
> 
> But please note that, there are more problems related to extent
> allocator with block group auto removal.
> 
> Even a block group is marked unused, extent allocator can still allocate
> new extents from unused block group.
> Thus delaying block group to next transaction won't work.
> (Extents get allocated in current transaction, and removed again in next
> transaction).

Isn't this easily solvable by making the allocator check whether the
block group it has chosen is part of the unused_bgs_list ?

> 
> So the root fix need to co-operate with extent allocator.
> But current fix should be enough for this particular bug.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

LGTM,

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
> changelog:
> v2:
>   Commit message update, to better indicate how pinned byte is used in
>   btrfs and why it's related to quota.
> v3:
>   Commit message update, further explaining the bug with an example.
>   And added the side effect of the fix, and possible further fix.
> ---
>  fs/btrfs/extent-tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index f190023386a9..7d14c4ca8232 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -10675,7 +10675,7 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
>  		/* Don't want to race with allocators so take the groups_sem */
>  		down_write(&space_info->groups_sem);
>  		spin_lock(&block_group->lock);
> -		if (block_group->reserved ||
> +		if (block_group->reserved || block_group->pinned ||
>  		    btrfs_block_group_used(&block_group->item) ||
>  		    block_group->ro ||
>  		    list_is_singular(&block_group->list)) {
>