Re: Direct IO reads being split unexpected at page boundary, but in the middle of a fs block (bs > ps cases)

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Oct 07, 2025 at 01:00:58PM +1030, Qu Wenruo wrote:
> 
> 
> 在 2025/10/7 01:37, Matthew Wilcox 写道:
> > On Wed, Oct 01, 2025 at 10:59:18AM +0930, Qu Wenruo wrote:
> > > Recently during the btrfs bs > ps direct IO enablement, I'm hitting a case
> > > where:
> > > 
> > > - The direct IO iov is properly aligned to fs block size (8K, 2 pages)
> > >    They do not need to be large folio backed, regular incontiguous pages
> > >    are supported.
> > > 
> > > - The btrfs now can handle sub-block pages
> > >    But still require the bi_size and (bi_sector << 9) to be block size
> > >    aligned.
> > > 
> > > - The bio passed into iomap_dio_ops::submit_io is not block size
> > >    aligned
> > >    The bio only contains one page, not 2.
> > 
> > That seems like a bug in the VFS/iomap somewhere.  Maybe try cc'ing the
> > people who know this code?
> > 
> 
> Add xfs and bcachefs subsystem into CC.
> 
> The root cause is that, function __bio_iov_iter_get_pages() can split the
> iov.
> 
> In my case, I hit the following dio during iomap_dio_bio_iter();
> 
>  fsstress-1153      6..... 68530us : iomap_dio_bio_iter: length=81920
> nr_pages=20 enter
>  fsstress-1153      6..... 68539us : iomap_dio_bio_iter: length=81920
> realsize=69632(17 pages)
>  fsstress-1153      6..... 68540us : iomap_dio_bio_iter: nr_pages=3 for next
> 
> Which bio_iov_iter_get_pages() split the 20 pages into two segments (17 + 3
> pages).
> That 17/3 split is not meeting the btrfs' block size requirement (in my case
> it's 8K block size).

Just out of curiosity, what are the corresponding
iomap_iter_{src,dst}map tracepoints for these iomap_dio_bio_iters?

I'm assuming there's one mapping for all 80k of data?

> I'm seeing XFS having a comment related to bio_iov_iter_get_pages() inside
> xfs_file_dio_write(), but there is no special checks other than
> iov_iter_alignment() check, which btrfs is also doing.
> 
> I guess since XFS do not need to bother data checksum thus such split is not
> a big deal?

I think so too.  The bios all point to the original iomap_dio so the
ioend only gets called once for the the full write IO, so a completion
of an out of place write will never see sub-block ranges.

> On the other hand, bcachefs is doing reverting to the block boundary instead
> thus solved the problem.
> However btrfs is using iomap for direct IOs, thus we can not manually revert
> the iov/bio just inside btrfs.
> 
> So I guess in this case we need to add a callback for iomap, to get the fs
> block size so that at least iomap_dio_bio_iter() can revert to the fs block
> boundary?

Or add a flags bit to iomap_dio_ops to indicate that the fs requires
block sized bios?

I'm guessing that you can't do sub-block directio writes to btrfs
either?

--D

> Thanks,
> Qu
>