在 2025/10/8 01:28, Darrick J. Wong 写道:
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?
None, those are adhoc added trace_printk()s.
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?
Yep, that's the next step.
I'm guessing that you can't do sub-block directio writes to btrfs
either?
Exactly.
Thanks,
Qu
--D
Thanks,
Qu
