[PATCH v2 5/6] iov_iter: Introduce ITER_FLAG_FAST_ONLY flag

[Andreas Gruenbacher]

Introduce a new ITER_FLAG_FAST_ONLY flag to indicate to get_user_pages to use
the FOLL_FAST_ONLY flag.  This will cause get_user_pages to fail when it would
otherwise fault in a page.

Currently, the ITER_FLAG_FAST_ONLY flag is only checked in iov_iter_get_pages
and iov_iter_get_pages_alloc.  This is enough for iomaop_dio_rw(), but it may
make sense to check for this flag in other contexts as well.

Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
---
 include/linux/uio.h | 15 ++++++++++++---
 lib/iov_iter.c      | 20 +++++++++++++++-----
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 74f819c41735..d3d629c2153a 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -18,6 +18,9 @@ struct kvec {
 };
 
 enum iter_type {
+	/* set if get_user_pages should use FOLL_FAST_ONLY */
+	ITER_FLAG_FAST_ONLY = 2,
+
 	/* iter types */
 	ITER_IOVEC = 4,
 	ITER_KVEC = 8,
@@ -30,8 +33,9 @@ enum iter_type {
 struct iov_iter {
 	/*
 	 * Bit 0 is the read/write bit, set if we're writing.
-	 * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and
-	 * the caller isn't expecting to drop a page reference when done.
+	 * Bit 1 is the ITER_FLAG_FAST_ONLY bit, set if get_user_pages
+	 * should use the FOLL_FAST_ONLY flag when trying to fault in pages
+	 * (only useful for type ITER_IOVEC).
 	 */
 	unsigned int type;
 	size_t iov_offset;
@@ -55,7 +59,7 @@ struct iov_iter {
 
 static inline enum iter_type iov_iter_type(const struct iov_iter *i)
 {
-	return i->type & ~(READ | WRITE);
+	return i->type & ~(READ | WRITE | ITER_FLAG_FAST_ONLY);
 }
 
 static inline bool iter_is_iovec(const struct iov_iter *i)
@@ -93,6 +97,11 @@ static inline unsigned char iov_iter_rw(const struct iov_iter *i)
 	return i->type & (READ | WRITE);
 }
 
+static inline bool iov_iter_is_fast_only(const struct iov_iter *i)
+{
+	return i->type & ITER_FLAG_FAST_ONLY;
+}
+
 /*
  * Total number of bytes covered by an iovec.
  *
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 3beecf8f77de..182ff2afed19 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1538,6 +1538,8 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
 {
+	unsigned int gup_flags = 0;
+
 	if (maxsize > i->count)
 		maxsize = i->count;
 
@@ -1548,6 +1550,11 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
+	if (iov_iter_rw(i) != WRITE)
+		gup_flags |= FOLL_WRITE;
+	if (iov_iter_is_fast_only(i))
+		gup_flags |= FOLL_FAST_ONLY;
+
 	iterate_all_kinds(i, maxsize, v, ({
 		unsigned long addr = (unsigned long)v.iov_base;
 		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
@@ -1558,9 +1565,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 			len = maxpages * PAGE_SIZE;
 		addr &= ~(PAGE_SIZE - 1);
 		n = DIV_ROUND_UP(len, PAGE_SIZE);
-		res = get_user_pages_fast(addr, n,
-				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0,
-				pages);
+		res = get_user_pages_fast(addr, n, gup_flags, pages);
 		if (unlikely(res < 0))
 			return res;
 		return (res == n ? len : res * PAGE_SIZE) - *start;
@@ -1665,6 +1670,7 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
 		   size_t *start)
 {
+	unsigned int gup_flags = 0;
 	struct page **p;
 
 	if (maxsize > i->count)
@@ -1677,6 +1683,11 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
+	if (iov_iter_rw(i) != WRITE)
+		gup_flags |= FOLL_WRITE;
+	if (iov_iter_is_fast_only(i))
+		gup_flags |= FOLL_FAST_ONLY;
+
 	iterate_all_kinds(i, maxsize, v, ({
 		unsigned long addr = (unsigned long)v.iov_base;
 		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
@@ -1688,8 +1699,7 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		p = get_pages_array(n);
 		if (!p)
 			return -ENOMEM;
-		res = get_user_pages_fast(addr, n,
-				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0, p);
+		res = get_user_pages_fast(addr, n, gup_flags, p);
 		if (unlikely(res < 0)) {
 			kvfree(p);
 			return res;
-- 
2.26.3