linux-ext4 - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

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Date:	Mon, 2 May 2016 07:56:06 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Vishal Verma <vishal.l.verma@...el.com>
Cc:	linux-nvdimm@...1.01.org, linux-fsdevel@...r.kernel.org,
	linux-block@...r.kernel.org, xfs@....sgi.com,
	linux-ext4@...r.kernel.org, linux-mm@...ck.org,
	Matthew Wilcox <matthew@....cx>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...com>, Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

> index 79defba..97a1f5f 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -167,12 +167,21 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, loff_t offset)
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = bdev_file_inode(file);
>  
> -	if (IS_DAX(inode))
> +	if (iocb_is_direct(iocb))
> +		return __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter,
> +					    offset, blkdev_get_block, NULL,
> +					    NULL, DIO_SKIP_DIO_COUNT);
> +	else if (iocb_is_dax(iocb))
>  		return dax_do_io(iocb, inode, iter, offset, blkdev_get_block,
>  				NULL, DIO_SKIP_DIO_COUNT);
> -	return __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter, offset,
> -				    blkdev_get_block, NULL, NULL,
> -				    DIO_SKIP_DIO_COUNT);
> +	else {
> +		/*
> +		 * If we're in the direct_IO path, either the IOCB_DIRECT or
> +		 * IOCB_DAX flags must be set.
> +		 */
> +		WARN_ONCE(1, "Kernel Bug with iocb flags\n");
> +		return -ENXIO;
> +	}

DAX should not even end up in ->direct_IO.

> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -300,7 +300,7 @@ xfs_file_read_iter(
>  
>  	XFS_STATS_INC(mp, xs_read_calls);
>  
> -	if (unlikely(iocb->ki_flags & IOCB_DIRECT))
> +	if (unlikely(iocb->ki_flags & (IOCB_DIRECT | IOCB_DAX)))
>  		ioflags |= XFS_IO_ISDIRECT;

please also add a XFS_IO_ISDAX flag to propagate the information
properly and allow tracing to display the actual I/O type.

> +static inline bool iocb_is_dax(struct kiocb *iocb)
>  {
> +	return IS_DAX(file_inode(iocb->ki_filp)) &&
> +		(iocb->ki_flags & IOCB_DAX);
> +}
> +
> +static inline bool iocb_is_direct(struct kiocb *iocb)
> +{
> +	return iocb->ki_flags & IOCB_DIRECT;
>  }

No need for these helpers - especially as IOCB_DAX should never be set
if IS_DAX is false.
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