bi_vcnt checks for repair

Hi all,

btrfs_check_repairable and bio_readpage_error have somewhat odd looking check for bi_vcnt, checking on how to proceed based on that.

Does anyone remember what they actually are checking for? Depending on the exact devices merging bi_vcnt can mean quite a few different things, and we should probably check for what people actually intended to check.

The suspect for adding these checks are in Cc, although the commit history is a bit murky.

(These two guys have been silent for some time. :) )

Try to explain it a bit, the following assumes that the data that's being read has at least one copy in another device,

if a read bio contains two pages and the second one is failing on checksum check, then we get into bio_readpage_error, in which it finds this bio has two pages, so it creats a new bio (so-called validation request) which only includes the fail-on-checksum victim page and issues it with REQ_FAILFAST_DEV to read the page again in hope of passing the checksum check, if it stills fails somehow, it comes into bio_readpage_error again to do another try but along with a different copy.

Overall, this bi_vcnt is to tell whether the bio is a normal one and a validation one. If the validation bio got merged somehow, then we need to do something to prevent that from happening.

Thanks,

-liubo

Can't we just set a different end_io handler for the two cases?