Re: [PATCH 5.20 1/4] block: add bio_rewind() API

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jun 28, 2022 at 12:26:10AM -0400, Kent Overstreet wrote:
> On Mon, Jun 27, 2022 at 03:36:22PM +0800, Ming Lei wrote:
> > Not mention bio_iter, bvec_iter has been 32 bytes, which is too big to
> > hold in per-io data structure. With this patch, 8bytes is enough
> > to rewind one bio if the end sector is fixed.
> 
> And with rewind, you're making an assumption about the state the iterator is
> going to be in when the IO has completed.
> 
> What if the iterator was never advanced?

bio_rewind() works as expected if the iterator doesn't advance, since bytes
between the recorded position and the end position isn't changed, same
with the end position.

> 
> So say you check for that by saving some other part of the iterator - but that
> may have legitimately changed too, if the bio was redirected (bi_sector changes)
> or trimmed (bi_size changes)
> 
> I still think this is an inherently buggy interface, the way it's being proposed
> to be used.

The patch did mention that the interface should be for situation in which end
sector of bio won't change.


Thanks,
Ming