Any way to ensure minimal folio size and alignment for iomap based direct IO?

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

Hi,

Recently I'm implementing block size > page size support for btrfs.

One core point of the bs > ps support is:

- No fs block is allowed to cross (large) folio boundaries
   This ensure that the btrfs checksum routine needs no multi-shot calls
   for a single data block, and ensures we can use a lot of
   bio_advance_iter_single() calls to move to the next block.

This is pretty easy since filemap is already supporting a minimal folio 
order, thus most part it's a very smooth sail for buffered IOs.


But things are going crazy for iomap based direct IOs.

I'm getting the following bio during my local tests, which is using 8K 
fs block size with 4K page size:

[  130.957366] root=5 inode=2464 logical=15974400 length=8192 index=0 
bv_offset=0 bv_len=4096 is not aligned to 8192
[  130.957376] i=0 page=0xffff8cc616e96000 offset=0 size=4096
[  130.961977] i=1 page=0xffff8cc61730e000 offset=0 size=4096

The bio initially looks fine, the length is 8K, properly aligned.

But the dump of the bio shows it's not the case, instead of a large 
folio, it's two page sized folios.

This will not pass the btrfs requirement, but weirdly the alignment 
check for the iov_iter at check_direct_IO() shows no problem.

But unfortunately I can not find any folio allocation for the direct IO 
routine except the zero_page...

Any clue on the iomap part, or is the btrfs requirement incompatible 
with iomap in the first place?

Thanks,
Qu