Re: [RFC][PATCH] iov_iter: Add extraction functions

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Sep 10, 2022 at 12:18:10AM +0100, David Howells wrote:
> Hi Al, Jeff,
> 
> Here's a replacement for the extract_iter_to_iter() patch I had previously.
> It's a WIP, some bits aren't fully implemented, though some bits I have tested
> and got to work, but if you could take a look and see if you're okay with the
> interface.
> 
> I think I've addressed most of Al's comments.  The page-pinning is conditional
> on certain types of iterator, and a number of the iterator types just extract
> to the same thing.  It should now handle kvec-class iterators that refer to
> vmalloc'd data.
> 
> I've also added extraction to scatterlist (which I'll need for doing various
> crypto things) and extraction to ib_sge which could be used in cifs/smb RDMA,
> bypassing the conversion-to-scatterlist step.
> 
> As mentioned, there are bits that aren't fully implemented, let alone tested.

IDGI.  Essentially, you are passing a callback disguised as enum, only to lose
any type safety.  How is it better than "iov_iter_get_pages2() into a fixed-sized
array and handle the result" done in a loop?  No need to advance it (iov_iter_get_page2()
auto-advances), *way* fewer conditional branches and no need to share anything
between the 3 functions you are after...

> +ssize_t extract_iter_to_sg(struct iov_iter *iter, size_t len,
> +			   struct sg_table *sgtable, bool *pages_pinned)

Your *pages_pinned is user_backed_iter(iter), isn't it?