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

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

On Mon, Dec 22, 2025 at 02:56:02PM -0500, Caleb Sander Mateos wrote:
> On Wed, Dec 17, 2025 at 9:28 PM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > 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().
> 
> nr_segs is not just a performance optimization, it's part of the
> struct iov_iter API and used by iov_iter_extract_bvec_pages(), as
> huang-jl pointed out. I don't think it's a good idea to assume that
> nr_segs isn't going to be used and doesn't need to be calculated
> correctly.

It doesn't have to be exact if the bytes covered by `count` won't cross
`nr_segs`.

The `nr_segs` re-calculation is added only for fixing performance regression
in the following link:

https://lkml.org/lkml/2025/4/16/351

because bio_iov_bvec_set() takes iter->nr_segs for setting bio->bi_vcnt.

> 
> I think this patch is a definite improvement as it reduces the number
> of assumptions about the internal structure of a bvec iov_iter. The
> remaining assignment to iter->iov_offset is unfortunate, but I don't
> see a great way around it.
> 

The re-calculation can be removed, please see the following patches:

https://lore.kernel.org/linux-block/20251218093146.1218279-1-ming.lei@redhat.com/

Nitesh has verified that it won't cause perf regression by replacing
bio->bi_vcnt with __bvec_iter_bvec() in bio_may_need_split().


Thanks,
Ming