Re: [PATCH v2 2/3] iov_iter: introduce iov_iter_pin_user_pages*() routines

[Christoph Hellwig <hch@infradead.org>]

On Sat, Aug 29, 2020 at 01:08:52AM -0700, John Hubbard wrote:
> The new routines are:
>     iov_iter_pin_user_pages()
>     iov_iter_pin_user_pages_alloc()
> 
> and those correspond to these pre-existing routines:
>     iov_iter_get_pages()
>     iov_iter_get_pages_alloc()
> 
> Also, pipe_get_pages() and related are changed so as to pass
> down a "use_pup" (use pin_user_page() instead of get_page()) bool
> argument.
> 
> Unlike the iov_iter_get_pages*() routines, the
> iov_iter_pin_user_pages*() routines assert that only ITER_IOVEC or
> ITER_PIPE items are passed in. They then call pin_user_page*(), instead
> of get_user_pages_fast() or get_page().
> 
> Why: In order to incrementally change Direct IO callers from calling
> get_user_pages_fast() and put_page(), over to calling
> pin_user_pages_fast() and unpin_user_page(), there need to be mid-level
> routines that specifically call one or the other systems, for both page
> acquisition and page release.
> 
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  include/linux/uio.h |   5 ++
>  lib/iov_iter.c      | 110 ++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 107 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 3835a8a8e9ea..29b0504a27cc 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -229,6 +229,11 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages);
>  
>  const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
>  
> +ssize_t iov_iter_pin_user_pages(struct iov_iter *i, struct page **pages,
> +			size_t maxsize, unsigned int maxpages, size_t *start);
> +ssize_t iov_iter_pin_user_pages_alloc(struct iov_iter *i, struct page ***pages,
> +			size_t maxsize, size_t *start);
> +
>  static inline size_t iov_iter_count(const struct iov_iter *i)
>  {
>  	return i->count;
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 5e40786c8f12..f25555eb3279 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1269,7 +1269,8 @@ static inline ssize_t __pipe_get_pages(struct iov_iter *i,
>  				size_t maxsize,
>  				struct page **pages,
>  				int iter_head,
> -				size_t *start)
> +				size_t *start,
> +				bool use_pup)
>  {
>  	struct pipe_inode_info *pipe = i->pipe;
>  	unsigned int p_mask = pipe->ring_size - 1;
> @@ -1280,7 +1281,11 @@ static inline ssize_t __pipe_get_pages(struct iov_iter *i,
>  	maxsize = n;
>  	n += *start;
>  	while (n > 0) {
> -		get_page(*pages++ = pipe->bufs[iter_head & p_mask].page);
> +		if (use_pup)
> +			pin_user_page(*pages++ = pipe->bufs[iter_head & p_mask].page);
> +		else
> +			get_page(*pages++ = pipe->bufs[iter_head & p_mask].page);

Maybe this would become a little more readable with a local variable
and a little more verbosity:

		struct page *page = pipe->bufs[iter_head & p_mask].page;

		if (use_pup)
			pin_user_page(page);
		else
			get_page(page);

		*pages++ = page;