[PATCH v5 05/12] iov_iter: Introduce fault_in_iov_iter_writeable

[Andreas Gruenbacher <agruenba@redhat.com>]

Introduce a new fault_in_iov_iter_writeable helper for safely faulting in an
iterator for writing.  Uses get_user_pages() to fault in the pages without
actually writing to them, which would be destructive.

We'll use fault_in_iov_iter_writeable in gfs2 once we've determined that the
iterator passed to .read_iter isn't in memory.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 include/linux/pagemap.h |  1 +
 include/linux/uio.h     |  1 +
 lib/iov_iter.c          | 41 +++++++++++++++++++++++++++
 mm/gup.c                | 61 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 104 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 7c9edc9694d9..a629807edb8c 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -737,6 +737,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter);
  * Fault in userspace address range.
  */
 size_t fault_in_writeable(char __user *uaddr, size_t size);
+size_t fault_in_safe_writeable(const char __user *uaddr, size_t size);
 size_t fault_in_readable(const char __user *uaddr, size_t size);
 
 int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 12d30246c2e9..ffa431aeb067 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -120,6 +120,7 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
 void iov_iter_revert(struct iov_iter *i, size_t bytes);
 size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t bytes);
+size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t bytes);
 size_t iov_iter_single_seg_count(const struct iov_iter *i);
 size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index c0fa1618561c..4ffc76801eaa 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -471,6 +471,47 @@ size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size)
 }
 EXPORT_SYMBOL(fault_in_iov_iter_readable);
 
+/*
+ * fault_in_iov_iter_writeable - fault in iov iterator for writing
+ * @i: iterator
+ * @size: maximum length
+ *
+ * Faults in the iterator using get_user_pages(), i.e., without triggering
+ * hardware page faults.  This is primarily useful when we know that some or
+ * all of the pages in @i aren't in memory.
+ *
+ * Returns the number of bytes faulted in, or 0 if no bytes could be faulted in
+ * (i.e., because the address is invalid).
+ *
+ * Always returns the number of avaliable bytes for non-user space iterators.
+ */
+size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size)
+{
+	if (size > i->count)
+		size = i->count;
+
+	if (iter_is_iovec(i)) {
+		const struct iovec *p;
+		size_t bytes = size;
+		size_t skip;
+
+		for (p = i->iov, skip = i->iov_offset; bytes; p++, skip = 0) {
+			size_t len = min(bytes, p->iov_len - skip);
+			size_t ret;
+
+			if (unlikely(!len))
+				continue;
+			ret = fault_in_safe_writeable(p->iov_base + skip, len);
+			bytes -= ret;
+			if (ret != len)
+				break;
+		}
+		return size - bytes;
+	}
+	return size;
+}
+EXPORT_SYMBOL(fault_in_iov_iter_writeable);
+
 void iov_iter_init(struct iov_iter *i, unsigned int direction,
 			const struct iovec *iov, unsigned long nr_segs,
 			size_t count)
diff --git a/mm/gup.c b/mm/gup.c
index d04984d5d93c..7218e27c2481 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1694,6 +1694,67 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
 }
 EXPORT_SYMBOL(fault_in_writeable);
 
+/**
+ * fault_in_safe_writeable - fault in an address range for writing
+ * @uaddr: start of address range
+ * @size: length of address range
+ *
+ * Faults in an address range using get_user_pages, i.e., without triggering
+ * hardware page faults.  This is primarily useful when we know that some or
+ * all of the pages in the address range aren't in memory.
+ *
+ * Other than fault_in_writeable(), this function is non-destructive.
+ *
+ * Note that we don't pin or otherwise hold the pages referenced that we fault
+ * in.  There's no guarantee that they'll stay in memory for any duration of
+ * time.
+ *
+ * Returns the number of bytes faulted in from @uaddr.
+ */
+size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
+{
+	unsigned long start = (unsigned long)uaddr;
+	unsigned long end, nstart, nend;
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma = NULL;
+	int locked = 0;
+
+	/* FIXME: Protect against overflow! */
+
+	end = PAGE_ALIGN(start + size);
+	for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
+		unsigned long nr_pages;
+		long ret;
+
+		if (!locked) {
+			locked = 1;
+			mmap_read_lock(mm);
+			vma = find_vma(mm, nstart);
+		} else if (nstart >= vma->vm_end)
+			vma = vma->vm_next;
+		if (!vma || vma->vm_start >= end)
+			break;
+		nend = min(end, vma->vm_end);
+		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+			continue;
+		if (nstart < vma->vm_start)
+			nstart = vma->vm_start;
+		nr_pages = (nend - nstart) / PAGE_SIZE;
+		ret = __get_user_pages_locked(mm, nstart, nr_pages,
+					      NULL, NULL, &locked,
+					      FOLL_TOUCH | FOLL_WRITE);
+		if (ret <= 0)
+			break;
+		nend = nstart + ret * PAGE_SIZE;
+	}
+	if (locked)
+		mmap_read_unlock(mm);
+	if (nstart > start)
+		return min(nstart - start, size);
+	return 0;
+}
+EXPORT_SYMBOL(fault_in_safe_writeable);
+
 size_t fault_in_readable(const char __user *uaddr, size_t size)
 {
 	const char __user *start = uaddr, *end;
-- 
2.26.3