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
>
