[PATCH 1/5] iov_iter: introduce iov_iter_pin_user_pages*() routines

[John Hubbard]

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()

Unlike the iov_iter_get_pages*() routines, the
iov_iter_pin_user_pages*() routines assert that only ITER_IOVEC items
are passed in. They then call pin_user_pages_fast(), instead of
get_user_pages_fast().

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@xxxxxxxxxx>
---
 include/linux/uio.h |  5 +++
 lib/iov_iter.c      | 80 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

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..d818b16d136b 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1309,6 +1309,44 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
 	return __pipe_get_pages(i, min(maxsize, capacity), pages, iter_head, start);
 }
 
+ssize_t iov_iter_pin_user_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 (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_user_pages);
+
 ssize_t iov_iter_get_pages(struct iov_iter *i,
 		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
@@ -1388,6 +1426,48 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 	return n;
 }
 
+ssize_t iov_iter_pin_user_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 (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_user_pages_alloc);
+
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
 		   size_t *start)
-- 
2.28.0