iomap: Support partial direct I/O on user copy failures

In iomap_dio_rw, when iomap_apply returns an -EFAULT error and the
IOMAP_DIO_PARTIAL flag is set, complete the request synchronously and
return a partial result.  This allows the caller to deal with the page
fault and retry the remainder of the request.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index a2a368e..a434fb7 100644 (file)
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -581,6 +581,12 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
        if (iov_iter_rw(iter) == READ && iomi.pos >= dio->i_size)
                iov_iter_revert(iter, iomi.pos - dio->i_size);
 
+       if (ret == -EFAULT && dio->size && (dio_flags & IOMAP_DIO_PARTIAL)) {
+               if (!(iocb->ki_flags & IOCB_NOWAIT))
+                       wait_for_completion = true;
+               ret = 0;
+       }
+
        /* magic error code to fall back to buffered I/O */
        if (ret == -ENOTBLK) {
                wait_for_completion = true;

diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 24f8489..2a213b0 100644 (file)
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -330,6 +330,13 @@ struct iomap_dio_ops {
   */
 #define IOMAP_DIO_OVERWRITE_ONLY       (1 << 1)
 
+/*
+ * When a page fault occurs, return a partial synchronous result and allow
+ * the caller to retry the rest of the operation after dealing with the page
+ * fault.
+ */
+#define IOMAP_DIO_PARTIAL              (1 << 2)
+
 ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
                const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
                unsigned int dio_flags);