On Fri, Jun 15, 2018 at 2:35 AM, Qu Wenruo <wqu@xxxxxxxx> 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 that can be fixed in a separate patch, to add it back to the list
of block groups to be deleted once everything is unpinned and passes
all other necessary criteria.
>
> But please note that, there are more problems related to extent
> allocator with block group auto removal.
The above isn't a problem of the allocator itself but rather in the
way we manage COW, commit roots and unpinning.
>
> Even a block group is marked unused, extent allocator can still allocate
> new extents from unused block group.
Why is that a problem?
It's ok (with some good benefits), as long as the cleaner thread (or
any thing that attempts to delete block groups in the unused list),
doesn't delete it.
> Thus delaying block group to next transaction won't work.
> (Extents get allocated in current transaction, and removed again in next
> transaction).
>
> So the root fix need to co-operate with extent allocator.
What do you mean by co-operation with the extent allocator? I don't
think the problem is there.
> But current fix should be enough for this particular bug.
>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> 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)) {
> --
> 2.17.1
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
