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

[Qu Wenruo <wqu@suse.com>]

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
>>
>