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

["Theodore Ts'o" <tytso@mit.edu>]

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?

Thanks,

					- Ted