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

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@mbobrowski.org>
---
 fs/ext4/file.c  | 58 +++++++++++++++++++++++++++++++++++++++++++++----
 fs/ext4/inode.c | 32 +--------------------------
 2 files changed, 55 insertions(+), 35 deletions(-)

On Thu 03-10-19 21:34:00, 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@mbobrowski.org>

The patch looks good to me. Just one small nit below. With that fixed, you
can add:

Reviewed-by: Jan Kara <jack@suse.cz>

> +	/*
> +	 * 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);
> +	}

I've noticed here you actually introduce new trylock pattern - previously
we had unconditional inode_lock_shared() in ext4_direct_IO_read(). So the
cleanest would be to just use unconditional inode_lock_shared() here and
then fixup IOCB_NOWAIT handling (I agree that was missing in the original
code) in a separate patch. And the pattern should rather look like:

	if (iocb->ki_flags & IOCB_NOWAIT) {
		if (!inode_trylock_shared(inode))
			return -EAGAIN;
	} else {
		inode_lock_shared(inode);
	}

to avoid two atomical operations instead of one in the fast path. No need
to repeat old mistakes when we know better :).

								Honza

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@mbobrowski.org>
> ---
>   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;
>   }
>

On Tue, Oct 08, 2019 at 12:52:07PM +0200, Jan Kara wrote:
> On Thu 03-10-19 21:34:00, 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@mbobrowski.org>
> 
> The patch looks good to me. Just one small nit below. With that fixed, you
> can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>

Cool, I'll fix it!

> > +	/*
> > +	 * 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);
> > +	}
> 
> I've noticed here you actually introduce new trylock pattern - previously
> we had unconditional inode_lock_shared() in ext4_direct_IO_read(). So the
> cleanest would be to just use unconditional inode_lock_shared() here and
> then fixup IOCB_NOWAIT handling (I agree that was missing in the original
> code) in a separate patch.

Right, so I will just have an unconditional call to
inode_lock_shared() and in the patch that follows I will fix it up to
apply the new pattern.

> And the pattern should rather look like:
>
> 	if (iocb->ki_flags & IOCB_NOWAIT) {
> 		if (!inode_trylock_shared(inode))
> 			return -EAGAIN;
> 	} else {
> 		inode_lock_shared(inode);
> 	}
> 
> to avoid two atomical operations instead of one in the fast path. No need
> to repeat old mistakes when we know better :).

Yes, also agree.

--<M>--