Re: [PATCH v5 03/13] btrfs: introduce a new helper to submit read bio for scrub

[Christoph Hellwig <hch@infradead.org>]

On Tue, Mar 28, 2023 at 08:16:38AM +0800, Qu Wenruo wrote:
> For read part, as long as we skip all the csum verification and csum search,
> it can go the generic helper.

Yes, and we can just check for bbio->inode to decide that, like you did
in the earliest versions of the series.

> But in the future, mostly for the ZNS RST patches, we need special handling
> for the garbage range.
> 
> Johannes is already testing his RST branches upon this series, and he is
> hitting the ASSERT() on the mapped length due to RST changes.
> In that case, we need to skip certain ranges, and that's specific to scrub
> usage.

Can you explain what the issue is here?

> The btrfs_submit_bio() would duplicate the writes to all mirrors, which is
> exactly what I want to avoid for scrub usage.
> Thus the scrub specific write helper would always map the write to a single
> stripe, even for RAID56 writes.

That's basically what btrfs_repair_io_failure does.  Any chance we
could share the code for those instead of going to back to creating
multiple code paths for I/O submission?