Re: [PATCH] btrfs: repair all bad mirrors

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 21, 2022 at 06:19:19PM +0300, Nikolay Borisov wrote:
>> +
>> +	mirror = failrec->this_mirror;
>> +	do {
>> +		mirror = prev_mirror(failrec, mirror);
>> +		repair_io_failure(fs_info, ino, start, failrec->len,
>> +				  failrec->logical, page, pg_offset, mirror);
>> +	} while (mirror != failrec->orig_mirror);
>
> But does this work as intended? Say we have a raid1c4 and we read from 
> mirror 3 which is bad, in this case failrec->orig_mirror = 3 and 
> ->this_mirror = 4. The read from mirror 4 returns good data and 
> clean_io_failure is called with mirror= 3 in which case only mirror 3 is 
> repaired (assume 1/2 were also bad we don't know it yet, because the 
> original bio request didn't submit to them based on the PID policy).

Yes.  Although that is what I intended as we don't want to read
data we don't otherwise have to. Maybe it should state "all known bad
mirrors" instead of "all mirrors".  I think if we want to check all
mirror we need to defer to the scrub code.