[PATCH 05/10] dax: provide an iomap based dax read/write path

[Christoph Hellwig <hch@lst.de>]

This is a much simpler implementation of the DAX read/write path that makes
use of the iomap infrastructure.  It does not try to mirror the direct I/O
calling conventions and thus doesn't have to deal with i_dio_count or the
end_io handler, but instead leaves locking and filesystem-specific I/O
completion to the caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/dax.c              | 103 ++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/iomap.h |   2 +
 2 files changed, 105 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index 84343ce..57ad456 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -31,6 +31,8 @@
 #include <linux/vmstat.h>
 #include <linux/pfn_t.h>
 #include <linux/sizes.h>
+#include <linux/iomap.h>
+#include "internal.h"
 
 /*
  * We use lowest available bit in exceptional entry for locking, other two
@@ -1241,3 +1243,104 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
 	return dax_zero_page_range(inode, from, length, get_block);
 }
 EXPORT_SYMBOL_GPL(dax_truncate_page);
+
+#ifdef CONFIG_FS_IOMAP
+static loff_t
+iomap_dax_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
+		struct iomap *iomap)
+{
+	struct iov_iter *iter = data;
+	loff_t end = pos + length, done = 0;
+	ssize_t ret = 0;
+
+	if (iov_iter_rw(iter) == READ) {
+		end = min(end, i_size_read(inode));
+		if (pos >= end)
+			return 0;
+
+		if (iomap->type == IOMAP_HOLE || iomap->type == IOMAP_UNWRITTEN)
+			return iov_iter_zero(min(length, end - pos), iter);
+	}
+
+	if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
+		return -EIO;
+
+	while (pos < end) {
+		unsigned offset = pos & (PAGE_SIZE - 1);
+		struct blk_dax_ctl dax = { 0 };
+		ssize_t map_len;
+
+		dax.sector = iomap->blkno +
+			(((pos & PAGE_MASK) - iomap->offset) >> 9);
+		dax.size = (length + offset + PAGE_SIZE - 1) & PAGE_MASK;
+		map_len = dax_map_atomic(iomap->bdev, &dax);
+		if (map_len < 0) {
+			ret = map_len;
+			break;
+		}
+
+		dax.addr += offset;
+		map_len -= offset;
+		if (map_len > end - pos)
+			map_len = end - pos;
+
+		if (iov_iter_rw(iter) == WRITE)
+			map_len = copy_from_iter_pmem(dax.addr, map_len, iter);
+		else
+			map_len = copy_to_iter(dax.addr, map_len, iter);
+		dax_unmap_atomic(iomap->bdev, &dax);
+		if (map_len <= 0) {
+			ret = map_len ? map_len : -EFAULT;
+			break;
+		}
+
+		pos += map_len;
+		length -= map_len;
+		done += map_len;
+	}
+
+	return done ? done : ret;
+}
+
+/**
+ * iomap_dax_rw - Perform I/O to a DAX file
+ * @iocb: The control block for this I/O
+ * @iter: The addresses to do I/O from or to
+ * @ops: iomap ops passed from the file system
+ *
+ * This funtions performs read and write operations to directly mapped
+ * persistent memory.  The callers needs to take care of read/write exclusion
+ * and evicting any page cache pages in the region under I/O.
+ */
+ssize_t
+iomap_dax_rw(struct kiocb *iocb, struct iov_iter *iter,
+		struct iomap_ops *ops)
+{
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	loff_t pos = iocb->ki_pos, ret = 0, done = 0;
+	size_t count = iov_iter_count(iter);
+	unsigned flags = 0;
+
+	if (!count)
+		return 0;
+
+	if (iov_iter_rw(iter) == WRITE)
+		flags |= IOMAP_WRITE;
+
+	do {
+		ret = iomap_apply(inode, pos, count, flags, ops, iter,
+				  iomap_dax_actor);
+		if (ret <= 0)
+			break;
+		pos += ret;
+		done += ret;
+	} while ((count = iov_iter_count(iter)));
+
+	if (!done)
+		return ret;
+
+	iocb->ki_pos += done;
+	return done;
+}
+EXPORT_SYMBOL_GPL(iomap_dax_rw);
+#endif /* CONFIG_FS_IOMAP */
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 14d7067..3d5f785 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -65,6 +65,8 @@ struct iomap_ops {
 
 ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
 		struct iomap_ops *ops);
+ssize_t iomap_dax_rw(struct kiocb *iocb, struct iov_iter *iter,
+		struct iomap_ops *ops);
 int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
 		bool *did_zero, struct iomap_ops *ops);
 int iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
-- 
2.1.4