On 07/22/2015 12:51 PM, Jens Axboe wrote:

On 07/20/2015 07:29 AM, Christoph Hellwig wrote:

quoted

Currently we have two different ways to signal an I/O error on a BIO:

  (1) by clearing the BIO_UPTODATE flag
  (2) by returning a Linux errno value to the bi_end_io callback

The first one has the drawback of only communicating a single possible
error (-EIO), and the second one has the drawback of not beeing
persistent
when bios are queued up, and are not passed along from child to parent
bio in the ever more popular chaining scenario.  Having both mechanisms
available has the additional drawback of utterly confusing driver authors
and introducing bugs where various I/O submitters only deal with one of
them, and the others have to add boilerplate code to deal with both kinds
of error returns.

So add a new bi_error field to store an errno value directly in struct
bio and remove the existing mechanisms to clean all this up.

I think this is a good change, the only part I _really_ dislike is that
this now bumps a struct bio from 2 cache lines to 3. Have you done any
perf testing?

One possible solution would be to shrink bi_flags to an unsigned int, no 
problems fitting that in. Then we could stuff bi_error in that (new) 
hole, and we would end up having the same size again.

-- 
Jens Axboe