xen/privcmd: Corrected error handling path

Previously, if lock_pages() end up partially mapping pages, it used
to return -ERRNO due to which unlock_pages() have to go through
each pages[i] till *nr_pages* to validate them. This can be avoided
by passing correct number of partially mapped pages & -ERRNO separately,
while returning from lock_pages() due to error.

With this fix unlock_pages() doesn't need to validate pages[i] till
*nr_pages* for error scenario and few condition checks can be ignored.

Signed-off-by: Souptick Joarder <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Reviewed-by: Paul Durrant <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: Paul Durrant <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/privcmd.c

···
 
 static int lock_pages(
 	struct privcmd_dm_op_buf kbufs[], unsigned int num,
-	struct page *pages[], unsigned int nr_pages)
+	struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
 {
 	unsigned int i;
 
 	for (i = 0; i < num; i++) {
 		unsigned int requested;
-		int pinned;
+		int page_count;
 
 		requested = DIV_ROUND_UP(
 			offset_in_page(kbufs[i].uptr) + kbufs[i].size,
···
 		if (requested > nr_pages)
 			return -ENOSPC;
 
-		pinned = get_user_pages_fast(
+		page_count = get_user_pages_fast(
 			(unsigned long) kbufs[i].uptr,
 			requested, FOLL_WRITE, pages);
-		if (pinned < 0)
-			return pinned;
+		if (page_count < 0)
+			return page_count;
 
-		nr_pages -= pinned;
-		pages += pinned;
+		*pinned += page_count;
+		nr_pages -= page_count;
+		pages += page_count;
 	}
 
 	return 0;
···
 {
 	unsigned int i;
 
-	if (!pages)
-		return;
-
-	for (i = 0; i < nr_pages; i++) {
-		if (pages[i])
-			put_page(pages[i]);
-	}
+	for (i = 0; i < nr_pages; i++)
+		put_page(pages[i]);
 }
 
 static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
···
 	struct xen_dm_op_buf *xbufs = NULL;
 	unsigned int i;
 	long rc;
+	unsigned int pinned = 0;
 
 	if (copy_from_user(&kdata, udata, sizeof(kdata)))
 		return -EFAULT;
···
 		goto out;
 	}
 
-	rc = lock_pages(kbufs, kdata.num, pages, nr_pages);
-	if (rc)
+	rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
+	if (rc < 0) {
+		nr_pages = pinned;
 		goto out;
+	}
 
 	for (i = 0; i < kdata.num; i++) {
 		set_xen_guest_handle(xbufs[i].h, kbufs[i].uptr);