dax: Report bytes remaining in dax_iomap_actor()

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: <x86@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files 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);