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

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On Tue, Jun 28, 2022 at 03:49:28PM +0800, Ming Lei wrote:
> 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.

But that's an assumption that you simply can't make!

We allow block device drivers to be stacked in _any_ combination. After a bio is
completed it may have been partially advanced, fully advanced, trimmed, not
trimmed, anything - and bi_sector and thus also bio_end_sector() may have
changed, and will have if there's partition tables involved.

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