[PATCH v2 7/9] dax: report bytes remaining in dax_iomap_actor()

[Dan Williams]

In preparation for protecting the dax read(2) path from media errors
with copy_to_iter_mcsafe() (via dax_copy_to_iter()), convert the
implementation to report the bytes successfully transferred.

Cc: <x...@kernel.org>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Tony Luck <tony.l...@intel.com>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Andy Lutomirski <l...@amacapital.net>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
 fs/dax.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index a64afdf7ec0d..34a2d435ae4b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -991,6 +991,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t 
length, void *data,
        struct iov_iter *iter = data;
        loff_t end = pos + length, done = 0;
        ssize_t ret = 0;
+       size_t xfer;
        int id;
 
        if (iov_iter_rw(iter) == READ) {
@@ -1054,19 +1055,20 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t 
length, void *data,
                 * vfs_write(), depending on which operation we are doing.
                 */
                if (iov_iter_rw(iter) == WRITE)
-                       map_len = dax_copy_from_iter(dax_dev, pgoff, kaddr,
+                       xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
                                        map_len, iter);
                else
-                       map_len = dax_copy_to_iter(dax_dev, pgoff, kaddr,
+                       xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr,
                                        map_len, iter);
-               if (map_len <= 0) {
-                       ret = map_len ? map_len : -EFAULT;
-                       break;
-               }
 
-               pos += map_len;
-               length -= map_len;
-               done += map_len;
+               pos += xfer;
+               length -= xfer;
+               done += xfer;
+
+               if (xfer == 0)
+                       ret = -EFAULT;
+               if (xfer < map_len)
+                       break;
        }
        dax_read_unlock(id);