[PATCH v3 2/3] iov_iter: introduce iov_iter_pin_pages*() routines

[John Hubbard]

The new routines are:
    iov_iter_pin_pages()
    iov_iter_pin_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_page() instead of get_page()) bool
argument.

Unlike the iov_iter_get_pages*() routines, the iov_iter_pin_pages*()
routines assert that only ITER_IOVEC or ITER_PIPE items are passed in.
They then call pin_user_pages_fast() or pin_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.

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 include/linux/uio.h |   5 ++
 lib/iov_iter.c      | 113 ++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 110 insertions(+), 8 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 3835a8a8e9ea..e44eed12afdf 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_pages(struct iov_iter *i, struct page **pages,
+			size_t maxsize, unsigned int maxpages, size_t *start);
+ssize_t iov_iter_pin_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..2dc1f4812fa9 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 +1281,14 @@ 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);
+		struct page *page = pipe->bufs[iter_head & p_mask].page;
+
+		if (use_pup)
+			pin_page(page);
+		else
+			get_page(page);
+
+		*pages++ = page;
 		iter_head++;
 		n -= PAGE_SIZE;
 	}
@@ -1290,7 +1298,7 @@ static inline ssize_t __pipe_get_pages(struct iov_iter *i,
 
 static ssize_t pipe_get_pages(struct iov_iter *i,
 		   struct page **pages, size_t maxsize, unsigned maxpages,
-		   size_t *start)
+		   size_t *start, bool use_pup)
 {
 	unsigned int iter_head, npages;
 	size_t capacity;
@@ -1306,9 +1314,52 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
 	npages = pipe_space_for_user(iter_head, i->pipe->tail, i->pipe);
 	capacity = min(npages, maxpages) * PAGE_SIZE - *start;
 
-	return __pipe_get_pages(i, min(maxsize, capacity), pages, iter_head, start);
+	return __pipe_get_pages(i, min(maxsize, capacity), pages, iter_head,
+				start, use_pup);
 }
 
+ssize_t iov_iter_pin_pages(struct iov_iter *i,
+		   struct page **pages, size_t maxsize, unsigned int maxpages,
+		   size_t *start)
+{
+	size_t skip = i->iov_offset;
+	const struct iovec *iov;
+	struct iovec v;
+
+	if (unlikely(iov_iter_is_pipe(i)))
+		return pipe_get_pages(i, pages, maxsize, maxpages, start, true);
+	if (unlikely(iov_iter_is_discard(i)))
+		return -EFAULT;
+	if (WARN_ON_ONCE(!iter_is_iovec(i)))
+		return -EFAULT;
+
+	if (unlikely(!maxsize))
+		return 0;
+	maxsize = min(maxsize, i->count);
+
+	iterate_iovec(i, maxsize, v, iov, skip, ({
+		unsigned long addr = (unsigned long)v.iov_base;
+		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
+		int n;
+		int res;
+
+		if (len > maxpages * PAGE_SIZE)
+			len = maxpages * PAGE_SIZE;
+		addr &= ~(PAGE_SIZE - 1);
+		n = DIV_ROUND_UP(len, PAGE_SIZE);
+
+		res = pin_user_pages_fast(addr, n,
+				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0,
+				pages);
+		if (unlikely(res < 0))
+			return res;
+		return (res == n ? len : res * PAGE_SIZE) - *start;
+		0;
+	}))
+	return 0;
+}
+EXPORT_SYMBOL(iov_iter_pin_pages);
+
 ssize_t iov_iter_get_pages(struct iov_iter *i,
 		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
@@ -1317,7 +1368,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		maxsize = i->count;
 
 	if (unlikely(iov_iter_is_pipe(i)))
-		return pipe_get_pages(i, pages, maxsize, maxpages, start);
+		return pipe_get_pages(i, pages, maxsize, maxpages, start, false);
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
@@ -1357,7 +1408,7 @@ static struct page **get_pages_array(size_t n)
 
 static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
-		   size_t *start)
+		   size_t *start, bool use_pup)
 {
 	struct page **p;
 	unsigned int iter_head, npages;
@@ -1380,7 +1431,7 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 	p = get_pages_array(npages);
 	if (!p)
 		return -ENOMEM;
-	n = __pipe_get_pages(i, maxsize, p, iter_head, start);
+	n = __pipe_get_pages(i, maxsize, p, iter_head, start, use_pup);
 	if (n > 0)
 		*pages = p;
 	else
@@ -1388,6 +1439,52 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 	return n;
 }
 
+ssize_t iov_iter_pin_pages_alloc(struct iov_iter *i,
+		   struct page ***pages, size_t maxsize,
+		   size_t *start)
+{
+	struct page **p;
+	size_t skip = i->iov_offset;
+	const struct iovec *iov;
+	struct iovec v;
+
+	if (unlikely(iov_iter_is_pipe(i)))
+		return pipe_get_pages_alloc(i, pages, maxsize, start, true);
+	if (unlikely(iov_iter_is_discard(i)))
+		return -EFAULT;
+	if (WARN_ON_ONCE(!iter_is_iovec(i)))
+		return -EFAULT;
+
+	if (unlikely(!maxsize))
+		return 0;
+	maxsize = min(maxsize, i->count);
+
+	iterate_iovec(i, maxsize, v, iov, skip, ({
+		unsigned long addr = (unsigned long)v.iov_base;
+		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
+		int n;
+		int res;
+
+		addr &= ~(PAGE_SIZE - 1);
+		n = DIV_ROUND_UP(len, PAGE_SIZE);
+		p = get_pages_array(n);
+		if (!p)
+			return -ENOMEM;
+
+		res = pin_user_pages_fast(addr, n,
+				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0, p);
+		if (unlikely(res < 0)) {
+			kvfree(p);
+			return res;
+		}
+		*pages = p;
+		return (res == n ? len : res * PAGE_SIZE) - *start;
+		0;
+	}))
+	return 0;
+}
+EXPORT_SYMBOL(iov_iter_pin_pages_alloc);
+
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
 		   size_t *start)
@@ -1398,7 +1495,7 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		maxsize = i->count;
 
 	if (unlikely(iov_iter_is_pipe(i)))
-		return pipe_get_pages_alloc(i, pages, maxsize, start);
+		return pipe_get_pages_alloc(i, pages, maxsize, start, false);
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
-- 
2.28.0