Re: Ext4 iomap warning during btrfs/136 (yes, it's from btrfs test cases)

[Qu Wenruo <wqu@suse.com>]

在 2025/8/8 21:46, Theodore Ts'o 写道:
> On Fri, Aug 08, 2025 at 06:20:56PM +0930, Qu Wenruo wrote:
>>
>> 在 2025/8/8 17:22, Qu Wenruo 写道:
>>> Hi,
>>>
>>> [BACKGROUND]
>>> Recently I'm testing btrfs with 16KiB block size.
>>>
>>> Currently btrfs is artificially limiting subpage block size to 4K.
>>> But there is a simple patch to change it to support all block sizes <=
>>> page size in my branch:
>>>
>>> https://github.com/adam900710/linux/tree/larger_bs_support
>>>
>>> [IOMAP WARNING]
>>> And I'm running into a very weird kernel warning at btrfs/136, with 16K
>>> block size and 64K page size.
>>>
>>> The problem is, the problem happens with ext3 (using ext4 modeule) with
>>> 16K block size, and no btrfs is involved yet.
> 
> 
> Thanks for the bug report!  This looks like it's an issue with using
> indirect block-mapped file with a 16k block size.  I tried your
> reproducer using a 1k block size on an x86_64 system, which is how I
> test problem caused by the block size < page size.  It didn't
> reproduce there, so it looks like it really needs a 16k block size.
> 
> Can you say something about what system were you running your testing
> on --- was it an arm64 system, or a powerpc 64 system (the two most
> common systems with page size > 4k)?  (I assume you're not trying to
> do this on an Itanic.  :-)   And was the page size 16k or 64k?

The architecture is aarch64, the host board is Rock5B (cheap and fast 
enough), the test machine is a VM on that board, with ovmf as the UEFI 
firmware.

The kernel is configured to use 64K page size, the *ext3* system is 
using 16K block size.

Currently I tried the following combination with 64K page size and ext3, 
the result looks like the following

- 2K block size
- 4K block size
   All fine

- 8K block size
- 16K block size
   All the same kernel warning and never ending fsstress

- 32K block size
- 64K block size
   All fine

I am surprised as you that, not all subpage block size are having 
problems, just 2 of the less common combinations failed.

And the most common ones (4K, page size) are all fine.

Finally, if using ext4 not ext3, all combinations above are fine again.

So I ran out of ideas why only 2 block sizes fail here...

Thanks,
Qu

> 
> Thanks,
> 
> 					- Ted
>