Wire the read-only nilfs_iomap_ops added in the previous patch into
the O_DIRECT read path, and eliminate blockdev_direct_IO() from
nilfs2 entirely:

 - nilfs_file_open() now sets FMODE_CAN_ODIRECT explicitly, since
   permission to open the file O_DIRECT was previously implied by
   aops->direct_IO being non-NULL.
 - nilfs_file_read_iter() dispatches O_DIRECT reads to iomap_dio_rw()
   using nilfs_iomap_ops; everything else still goes through
   generic_file_read_iter() as before.
 - nilfs_file_write_iter() strips IOCB_DIRECT and falls through to
   generic_file_write_iter()'s ordinary buffered path. NILFS2 cannot
   perform true direct I/O writes: new blocks are delay-allocated and
   only given a real disk address by the segment constructor, which
   works on buffer_head lists, not iomap. This reproduces today's
   actual behavior: the old nilfs_direct_IO() already just returned 0
   for WRITE.
 - nilfs_direct_IO() and the .direct_IO callback on nilfs_aops are
   removed.
 - drop the unnecessary "select LEGACY_DIRECT_IO" from Kconfig
   in favor of "select FS_IOMAP".

Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
cc: linux-nilfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
---
 fs/nilfs2/Kconfig |  2 +-
 fs/nilfs2/file.c  | 40 +++++++++++++++++++++++++++++++++++++---
 fs/nilfs2/inode.c | 13 -------------
 3 files changed, 38 insertions(+), 17 deletions(-)

diff --git a/fs/nilfs2/Kconfig b/fs/nilfs2/Kconfig
index 7dae168e346e..0a5ace60e6ab 100644
--- a/fs/nilfs2/Kconfig
+++ b/fs/nilfs2/Kconfig
@@ -3,7 +3,7 @@ config NILFS2_FS
 	tristate "NILFS2 file system support"
 	select BUFFER_HEAD
 	select CRC32
-	select LEGACY_DIRECT_IO
+	select FS_IOMAP
 	help
 	  NILFS2 is a log-structured file system (LFS) supporting continuous
 	  snapshotting.  In addition to versioning capability of the entire
diff --git a/fs/nilfs2/file.c b/fs/nilfs2/file.c
index f93b68c4877c..ad2e87c049c9 100644
--- a/fs/nilfs2/file.c
+++ b/fs/nilfs2/file.c
@@ -10,9 +10,12 @@
 #include <linux/fs.h>
 #include <linux/filelock.h>
 #include <linux/mm.h>
+#include <linux/uio.h>
+#include <linux/iomap.h>
 #include <linux/writeback.h>
 #include "nilfs.h"
 #include "segment.h"
+#include "iomap.h"
 
 int nilfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 {
@@ -133,20 +136,51 @@ static int nilfs_file_mmap_prepare(struct vm_area_desc *desc)
 	return 0;
 }
 
+static int nilfs_file_open(struct inode *inode, struct file *file)
+{
+	file->f_mode |= FMODE_CAN_ODIRECT;
+	return generic_file_open(inode, file);
+}
+
+static ssize_t nilfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	if (iocb->ki_flags & IOCB_DIRECT) {
+		return iomap_dio_rw(iocb, to, &nilfs_iomap_ops,
+				    NULL, 0, NULL, 0);
+	} else
+		return generic_file_read_iter(iocb, to);
+}
+
+static ssize_t nilfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
+{
+	/*
+	 * NILFS2 cannot perform true direct I/O writes: new blocks are
+	 * delay-allocated and are only given a real disk address when
+	 * the segment constructor writes them out as part of a log,
+	 * which works directly on buffer_head lists rather than
+	 * through iomap. Fall back to the ordinary buffered write path
+	 * for O_DIRECT writes.
+	 */
+	if (iocb->ki_flags & IOCB_DIRECT)
+		iocb->ki_flags &= ~IOCB_DIRECT;
+
+	return generic_file_write_iter(iocb, from);
+}
+
 /*
  * We have mostly NULL's here: the current defaults are ok for
  * the nilfs filesystem.
  */
 const struct file_operations nilfs_file_operations = {
 	.llseek		= generic_file_llseek,
-	.read_iter	= generic_file_read_iter,
-	.write_iter	= generic_file_write_iter,
+	.read_iter	= nilfs_file_read_iter,
+	.write_iter	= nilfs_file_write_iter,
 	.unlocked_ioctl	= nilfs_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= nilfs_compat_ioctl,
 #endif	/* CONFIG_COMPAT */
 	.mmap_prepare	= nilfs_file_mmap_prepare,
-	.open		= generic_file_open,
+	.open		= nilfs_file_open,
 	/* .release	= nilfs_release_file, */
 	.fsync		= nilfs_sync_file,
 	.splice_read	= filemap_splice_read,
diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
index 51f7e125a311..f4a9d9ea9c3f 100644
--- a/fs/nilfs2/inode.c
+++ b/fs/nilfs2/inode.c
@@ -257,18 +257,6 @@ static int nilfs_write_end(const struct kiocb *iocb,
 	return err ? : copied;
 }
 
-static ssize_t
-nilfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
-{
-	struct inode *inode = file_inode(iocb->ki_filp);
-
-	if (iov_iter_rw(iter) == WRITE)
-		return 0;
-
-	/* Needs synchronization with the cleaner */
-	return blockdev_direct_IO(iocb, inode, iter, nilfs_get_block);
-}
-
 const struct address_space_operations nilfs_aops = {
 	.read_folio		= nilfs_read_folio,
 	.writepages		= nilfs_writepages,
@@ -277,7 +265,6 @@ const struct address_space_operations nilfs_aops = {
 	.write_begin		= nilfs_write_begin,
 	.write_end		= nilfs_write_end,
 	.invalidate_folio	= block_invalidate_folio,
-	.direct_IO		= nilfs_direct_IO,
 	.migrate_folio		= buffer_migrate_folio_norefs,
 	.is_partially_uptodate  = block_is_partially_uptodate,
 };
-- 
2.43.0