Re: WARNING: CPU: 3 PID: 439 at fs/btrfs/ctree.h:1559 btrfs_update_device+0x1c5/0x1d0 [btrfs]

[nborisov <nborisov@suse.de>]

On 2017-09-20 07:39, Qu Wenruo wrote:
> On 2017年09月20日 13:10, Qu Wenruo wrote:
>> 
>> 
>> On 2017年09月20日 12:59, Qu Wenruo wrote:
>>> 
>>> 
>>> On 2017年09月20日 12:49, Rich Rauenzahn wrote:
>>>> 
>>>> 
>>>> On 9/19/2017 5:31 PM, Qu Wenruo wrote:
>>>>> On 2017年09月19日 23:56, Rich Rauenzahn wrote:
>>>>>> [    4.747377] WARNING: CPU: 3 PID: 439 at fs/btrfs/ctree.h:1559
>>>>>> btrfs_update_device+0x1c5/0x1d0 [btrfs]
>>>>> 
>>>>> Is that line the following WARN_ON()?
>>>>> ---
>>>>> static inline void btrfs_set_device_total_bytes(struct 
>>>>> extent_buffer *eb,
>>>>>                         struct btrfs_dev_item *s,
>>>>>                         u64 val)
>>>>> {
>>>>>     BUILD_BUG_ON(sizeof(u64) !=
>>>>>              sizeof(((struct btrfs_dev_item *)0))->total_bytes);
>>>>>     WARN_ON(!IS_ALIGNED(val, eb->fs_info->sectorsize)); <<<
>>>>>     btrfs_set_64(eb, s, offsetof(struct btrfs_dev_item, 
>>>>> total_bytes), val);
>>>>> }
>>>>> ---
>>>>> 
>>>>> If so, that means your devices size is not aligned to 4K.
>>>>> 
>>>>> Is your block device still using old 512 block size?
>>>>> AFAIK nowadays most HDDs are using 4K blocksize and it's 
>>>>> recommended to use it.
>>>>> 
>>>>> It's not a big problem and one can easily remove the WARN_ON().
>>>>> But I think we'd better fix the caller to do round_down() before 
>>>>> calling this function.
>>>>> 
>>>> 
>>>> That's interesting!  I believe I made an effort to align them when I 
>>>> set it up years ago, but never knew how to verify.
>>> 
>>> Well, best verifying if that's the line causing the warning, since I 
>>> don't have the source of RedHat kernel.
>>> 
>>>> 
>>>> I have three mirrored filesystems:
>>>> 
>> [snip]
>>>> 
>>>> Number  Start (sector)    End (sector)  Size       Code Name
>>>>     1              40      3907029134   1.8 TiB     8300 BTRFS MEDIA
>>>> GPT fdisk (gdisk) version 0.8.6
>>> 
>>> At least this size is not aligned to 4K.
>>> 
>>>> 
>>>> Partition table scan:
>> [snip]
>>>> 
>>>> .....and one is aligned differently!
>>>> 
>>>> Could it be /dev/sdd that's the issue?  But it's aligned at 4096 -- 
>>>> so I'm not sure that's the issue after all.
>>> 
>>> Its start sector is aligned, but end point is not, so the size is not 
>>> aligned either.
>>> 
>>> BTW, is /dev/sdd added to btrfs using "btrfs device add"?
>>> In my test, if making btrfs on a unaligned file, it will round down 
>>> to its sectorsize boundary.
>> 
>> Confirmed that "btrfs device add" won't round down the size.
>> Check the btrfs-debug-tree output:
>> ------
>>          item 0 key (DEV_ITEMS DEV_ITEM 1) itemoff 16185 itemsize 98
>>                  devid 1 total_bytes 10737418240 bytes_used 2172649472
>>                  io_align 4096 io_width 4096 sector_size 4096 type 0
>>                  generation 0 start_offset 0 dev_group 0
>>                  seek_speed 0 bandwidth 0
>>                  uuid 243a1117-ca31-4d87-8656-81c5630aafb2
>>                  fsid 6452cde7-14d5-4541-aa07-b265a400bad0
>>          item 1 key (DEV_ITEMS DEV_ITEM 2) itemoff 16087 itemsize 98
>>                  devid 2 total_bytes 1073742336 bytes_used 0
>>                  io_align 4096 io_width 4096 sector_size 4096 type 0
>>                  generation 0 start_offset 0 dev_group 0
>>                  seek_speed 0 bandwidth 0
>>                  uuid 6bb07260-d230-4e22-88b1-1eabb46622ed
>>                  fsid 6452cde7-14d5-4541-aa07-b265a400bad0
>> ------
> 
> Sorry, the output is from v4.12.x, so no kernel warning nor the patch
> rounding down the value.
> 
>> 
>> Where first device is completely aligned, the 2nd device which is just 
>> 1G + 512, definitely not aligned.
>> 
>> So if you're using single device purely created by mkfs.btrfs, you're 
>> OK.
>> But if any new device added, you're not OK and causing the false 
>> alert.
>> 
>> Any way, it should not be hard to fix.
>> Just remove the WARN_ON() and add extra round_down when adding device.
> 
> In v4.13 kernel, the newly added devices are in fact rounded down.
> But existing device doesn't get the round down.

We got a report internally at Suse of this problem and it prevented a 
filesystem from being mounted due to the
following check failing: 
http://elixir.free-electrons.com/linux/latest/source/fs/btrfs/volumes.c#L6893.

Hence I added the rounding down fixes. And this warning was put 
specifically to catch future offenders and see
if I had missed a place to patch it. So removing the warning is the 
wrong solution to the problem. Generally if
balancing kicked in had to resize his disk everything would be back to 
normal.

> 
> So it's recommended to resize (shrink) your fs for very small size to
> fix it if you don't want to wait for the kernel fix.
> 
> Thanks,
> Qu
>> 
>> Thanks for the report,
>> Qu
>> 
>>> 
>>> So I'm wondering if it's caused by added new btrfs device.
>>> 
>>> Thanks,
>>> Qu
>>> 
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