[PATCH 1/5] ext4: introduce direct IO read code path using iomap infrastructure

[Matthew Bobrowski <mbobrowski@mbobrowski.org>]

This patch introduces a new direct IO read code path implementation
that makes use of the iomap infrastructure.

The new function ext4_dio_read_iter() is responsible for calling into the
iomap infrastructure via iomap_dio_rw(). If the inode in question does not
pass preliminary checks in ext4_dio_checks(), then we simply fallback to
buffered IO and try to take that path. Prior to doing so, we drop the
IOCB_DIRECT flag from iocb->ki_flags to prevent generic_file_read_iter() from
taking the direct IO code path once again.

Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
---
 fs/ext4/file.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 70b0438dbc94..360eff7b6aa2 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -34,6 +34,53 @@
 #include "xattr.h"
 #include "acl.h"
 
+static bool ext4_dio_checks(struct inode *inode)
+{
+#ifdef CONFIG_FS_ENCRYPTION
+	if (IS_ENCRYPTED(inode))
+		return false;
+#endif
+	if (ext4_should_journal_data(inode))
+		return false;
+	if (ext4_has_inline_data(inode))
+		return false;
+	return true;
+}
+
+static ssize_t ext4_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	ssize_t ret;
+	struct inode *inode = file_inode(iocb->ki_filp);
+
+	/*
+	 * Get exclusion from truncate and other inode operations.
+	 */
+	if (!inode_trylock_shared(inode)) {
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return -EAGAIN;
+		inode_lock_shared(inode);
+	}
+
+	if (!ext4_dio_checks(inode)) {
+		inode_unlock_shared(inode);
+		/*
+		 * Fallback to buffered IO if the operation being
+		 * performed on the inode is not supported by direct
+		 * IO. The IOCB_DIRECT flag from iocb->ki_flags needs
+		 * to be cleared here to ensure that the direct IO
+		 * code path in generic_file_read_iter() is not taken.
+		 */
+		iocb->ki_flags &= ~IOCB_DIRECT;
+		return generic_file_read_iter(iocb, to);
+	}
+
+	ret = iomap_dio_rw(iocb, to, &ext4_iomap_ops, NULL);
+	inode_unlock_shared(inode);
+
+	file_accessed(iocb->ki_filp);
+	return ret;
+}
+
 #ifdef CONFIG_FS_DAX
 static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
@@ -64,16 +111,20 @@ static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
 
 static ssize_t ext4_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
-	if (unlikely(ext4_forced_shutdown(EXT4_SB(file_inode(iocb->ki_filp)->i_sb))))
+	struct inode *inode = file_inode(iocb->ki_filp);
+
+	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
 		return -EIO;
 
 	if (!iov_iter_count(to))
 		return 0; /* skip atime */
 
 #ifdef CONFIG_FS_DAX
-	if (IS_DAX(file_inode(iocb->ki_filp)))
+	if (IS_DAX(inode))
 		return ext4_dax_read_iter(iocb, to);
 #endif
+	if (iocb->ki_flags & IOCB_DIRECT)
+		return ext4_dio_read_iter(iocb, to);
 	return generic_file_read_iter(iocb, to);
 }
 
-- 
2.16.4


-- 
Matthew Bobrowski