Re: How to convert I/O iterators to iterators, sglists and RDMA lists

[Christoph Hellwig <hch@infradead.org>]

On Thu, Oct 20, 2022 at 03:03:56PM +0100, David Howells wrote:
> > What block file systems do is to take the pages from the iter and some flags
> > on what is pinned.  We can generalize this to store all extra state in a
> > flags word, or byte the bullet and allow cloning of the iter in one form or
> > another.
> 
> Yeah, I know.  A list of pages is not an ideal solution.  It can only handle
> contiguous runs of pages, possibly with a partial page at either end.  A bvec
> iterator would be of more use as it can handle a series of partial pages.
> 
> Note also that I would need to turn the pages *back* into an iterator in order
> to commune with sendmsg() in the nether reaches of some network filesystems.

Yes.  So I think the right thing here is to make sure we can send
the iter through the whole stack without a convesion.

> It would be nice to be able to pass an iterator to the crypto layer.  I'm not
> sure what the crypto people think of that.

Let's ask them..

> On the other hand, if you think the RDMA API should be taking scatterlists
> rather than sge lists, that would be fine.  Even better if I can just pass an
> iterator in directly - though neither scatterlist nor iterator has a place to
> put the RDMA local_dma_key - though I wonder if that's actually necessary for
> each sge element, or whether it could be handed through as part of the request
> as a hole.

Well, in the long run it should not take scatterlists either, as they
are a bad data structure.  But what should happen in the long run is
that the DMA mapping is only done in the hardware drivers, not the ULPs,
which is a really nasty layering violation.  This requires the strange
ib_dma_* stubs to disable DMA mapping for the software drivers, and it
also does complete unneeded DMA mappings for sends that are inline in
the SQE as supported by some Mellanox / Nvidia hardware.

> That's fine in principle.  However, I have some extraction code that can
> convert an iterator to another iterator, an sglist or an rdma sge list, using
> a common core of code to do all three.

So I think the iterator to iterator is a really bad idea and we should
not have it at all.  It just works around the issue about not being
able to easily keeping state after an iter based get_user_pages, but
that is beeing addressed at the moment.  The iter to ib_sge/scatterlist
are very much RDMA specific at the moment, so I guess that might be a
good place to keep them.  In fact I suspect the scatterlist conversion
should not be a public API at all, but hidden in rw.c and only be used
internally for the DMA mapping.