libceph: fix handling of short returns from get_user_pages

get_user_pages() can return fewer pages than we ask for.  We were returning
a bogus pointer/error code in that case.  Instead, loop until we get all
the pages we want or get an error we can return to the caller.
Signed-off-by: Sage Weil <sage@newdream.net>

net/ceph/pagevec.c

@@ -16,22 +16,30 @@ struct page **ceph_get_direct_page_vector(const char __user *data,
 					  int num_pages, bool write_page)
 {
 	struct page **pages;
-	int rc;
+	int got = 0;
+	int rc = 0;
 
 	pages = kmalloc(sizeof(*pages) * num_pages, GFP_NOFS);
 	if (!pages)
 		return ERR_PTR(-ENOMEM);
 
 	down_read(&current->mm->mmap_sem);
-	rc = get_user_pages(current, current->mm, (unsigned long)data,
-			    num_pages, write_page, 0, pages, NULL);
+	while (got < num_pages) {
+		rc = get_user_pages(current, current->mm,
+		    (unsigned long)data + ((unsigned long)got * PAGE_SIZE),
+		    num_pages - got, write_page, 0, pages + got, NULL);
+		if (rc < 0)
+			break;
+		BUG_ON(rc == 0);
+		got += rc;
+	}
 	up_read(&current->mm->mmap_sem);
-	if (rc < num_pages)
+	if (rc < 0)
 		goto fail;
 	return pages;
 
 fail:
-	ceph_put_page_vector(pages, rc > 0 ? rc : 0, false);
+	ceph_put_page_vector(pages, got, false);
 	return ERR_PTR(rc);
 }
 EXPORT_SYMBOL(ceph_get_direct_page_vector);