Re: [PATCH v6.20] io_uring/rsrc: refactor io_import_kbuf() to use single loop

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

On Thu, Dec 18, 2025 at 09:42:43AM +0800, Ming Lei wrote:
> On Wed, Dec 17, 2025 at 11:16:47PM +0800, huang-jl wrote:
> > The code looks correct to me.
> > 
> > > This simplifies the logic
> > 
> > I'm not an expert in Linux development, but from my perspective, the
> > original version seems simpler and more readable. The semantics of
> > iov_iter_advance() are clear and well-understood.
> > 
> > That said, I understand the appeal of merging them into a single loop.
> > 
> > > and avoids the overhead of iov_iter_advance()
> > 
> > Could you clarify what overhead you mean? If it's the function call
> > overhead, I think the compiler would inline it anyway. The actual
> > iteration work seems equivalent between both approaches.
> 
> iov_iter_advance() is global function, and it can't be inline.
> 
> Also single loop is more readable, cause ->iov_offset can be ignored easily.
> 
> In theory, re-calculating nr_segs isn't necessary, it is just for avoiding
> potential split, however not get idea how it is triggered. Nitesh didn't
> mention the exact reason:
> 
> https://lkml.org/lkml/2025/4/16/351
> 
> I will look at the reason and see if it can be avoided.

The reason is in both bio_iov_bvec_set() and bio_may_need_split().

->bi_vcnt doesn't make sense for cloned bio, and shouldn't be used as multiple
segment hint.

However, it also shows bio_split_rw() is too heavy.


Thanks,
Ming