Re: [BUG] btrfs: degraded RAID6 preallocated writes return EIO from fdatasync

[Qu Wenruo]

在 2026/9/29 20:04, Bartosz Fenski 写道:
> Hello,
>
> A fresh four-device Btrfs RAID6 filesystem returns EIO from fdatasync()
> when writing to a file preallocated by fio after one device is removed.
>
> The filesystem is clean before the device is removed, one missing device
> is within RAID6 tolerance, and all surviving-device error counters remain
> zero.
>
> I reproduced this twice on unmodified upstream Linux v7.3-rc5. Both
> runs failed after exactly the same number of writes and at the same
> fdatasync offset. Disabling fio's preallocation makes the same workload
> complete successfully.

I'll take a look.

[...]
> A guard similar to the following before accessing csum_buf may be needed:
>
>    if (!test_bit(rbio_sector_index(rbio, stripe_nr, sector_nr),
>                  rbio->csum_bitmap))
>            return 0;

It's already there, check verify_bio_data_sectors().

Anyway I'll try to reproduce and debug it here.

Thanks for the report,
Qu

> The relevant checksum verification was introduced by commit:
>
>    7a3150723061 ("btrfs: raid56: do data csum verification during RMW 
> cycle")
>
> Current Btrfs for-next still appears to lack the per-sector bitmap check.
>
> I searched the linux-btrfs archive and did not find an existing report
> with this combination of a clean filesystem, one missing RAID6 member,
> preallocated extents, and sync-time EIO.
>
> I can test a proposed patch on the same v7.3-rc5 VM.
>
> Regards,
> Bartosz Fenski