Re: [PATCH v4 4/8] ext4: introduce direct I/O read path using iomap infrastructure

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On 10/3/19 5:04 PM, Matthew Bobrowski wrote:
This patch introduces a new direct I/O read path 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 read operation
being performed on the inode does not pass the preliminary checks
performed within ext4_dio_supported(), then we simply fallback to
buffered I/O in order to fulfil the request.

Existing direct I/O read buffer_head code has been removed as it's now
redundant.

Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>
---
  fs/ext4/file.c  | 58 +++++++++++++++++++++++++++++++++++++++++++++----
  fs/ext4/inode.c | 32 +--------------------------
  2 files changed, 55 insertions(+), 35 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index ab75aee3e687..69ac042fb74b 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_supported(struct inode *inode)
+{
+	if (IS_ENABLED(CONFIG_FS_ENCRYPTION) && IS_ENCRYPTED(inode))
+		return false;
+	if (fsverity_active(inode))
+		return false;
+	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);
+	}
Same comments here.
Let's follow as per the discussion here
https://patchwork.kernel.org/patch/11141577/


+
+	if (!ext4_dio_supported(inode)) {
+		inode_unlock_shared(inode);
+		/*
+		 * Fallback to buffered I/O if the operation being
+		 * performed on the inode is not supported by direct
+		 * I/O. The IOCB_DIRECT flag needs to be cleared here
+		 * in order to ensure that the direct I/O path withiin
+		 * 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 +111,19 @@ 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);
  }

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 1dace576b8bd..159ffb92f82d 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -863,9 +863,6 @@ int ext4_dio_get_block(struct inode *inode, sector_t iblock,
  {
  	/* We don't expect handle for direct IO */
  	WARN_ON_ONCE(ext4_journal_current_handle());
-
-	if (!create)
-		return _ext4_get_block(inode, iblock, bh, 0);
  	return ext4_get_block_trans(inode, iblock, bh, EXT4_GET_BLOCKS_CREATE);
  }

@@ -3855,30 +3852,6 @@ static ssize_t ext4_direct_IO_write(struct kiocb *iocb, struct iov_iter *iter)
  	return ret;
  }

-static ssize_t ext4_direct_IO_read(struct kiocb *iocb, struct iov_iter *iter)
-{
-	struct address_space *mapping = iocb->ki_filp->f_mapping;
-	struct inode *inode = mapping->host;
-	size_t count = iov_iter_count(iter);
-	ssize_t ret;
-
-	/*
-	 * Shared inode_lock is enough for us - it protects against concurrent
-	 * writes & truncates and since we take care of writing back page cache,
-	 * we are protected against page writeback as well.
-	 */
-	inode_lock_shared(inode);
-	ret = filemap_write_and_wait_range(mapping, iocb->ki_pos,
-					   iocb->ki_pos + count - 1);
-	if (ret)
-		goto out_unlock;
-	ret = __blockdev_direct_IO(iocb, inode, inode->i_sb->s_bdev,
-				   iter, ext4_dio_get_block, NULL, NULL, 0);
-out_unlock:
-	inode_unlock_shared(inode);
-	return ret;
-}
-
  static ssize_t ext4_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
  {
  	struct file *file = iocb->ki_filp;
@@ -3905,10 +3878,7 @@ static ssize_t ext4_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
  		return 0;

  	trace_ext4_direct_IO_enter(inode, offset, count, iov_iter_rw(iter));
-	if (iov_iter_rw(iter) == READ)
-		ret = ext4_direct_IO_read(iocb, iter);
-	else
-		ret = ext4_direct_IO_write(iocb, iter);
+	ret = ext4_direct_IO_write(iocb, iter);
  	trace_ext4_direct_IO_exit(inode, offset, count, iov_iter_rw(iter), ret);
  	return ret;
  }





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