linux-kernel - Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default

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Message-ID: <20151103003234.GM10656@dastard>
Date:	Tue, 3 Nov 2015 11:32:34 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	axboe@...com, jack@...e.cz, linux-nvdimm@...ts.01.org,
	linux-kernel@...r.kernel.org, ross.zwisler@...ux.intel.com,
	hch@....de
Subject: Re: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default

On Sun, Nov 01, 2015 at 11:30:53PM -0500, Dan Williams wrote:
> Now that we have the ability to dynamically enable DAX for a raw block
> inode, make the behavior opt-in by default.  DAX does not have feature
> parity with pagecache backed mappings, so applications should knowingly
> enable DAX semantics.
> 
> Note, this is only for mappings returned to userspace.  For the
> synchronous usages of DAX, dax_do_io(), there is no semantic difference
> with the bio submission path, so that path remains default enabled.
> 
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  block/ioctl.c      |    3 +--
>  fs/block_dev.c     |   33 +++++++++++++++++++++++----------
>  include/linux/fs.h |    8 ++++++++
>  3 files changed, 32 insertions(+), 12 deletions(-)
> 
> diff --git a/block/ioctl.c b/block/ioctl.c
> index 205d57612fbd..c4c3a09d9ca9 100644
> --- a/block/ioctl.c
> +++ b/block/ioctl.c
> @@ -298,13 +298,12 @@ static inline int is_unrecognized_ioctl(int ret)
>  #ifdef CONFIG_FS_DAX
>  static int blkdev_set_dax(struct block_device *bdev, int n)
>  {
> -	struct gendisk *disk = bdev->bd_disk;
>  	int rc = 0;
>  
>  	if (n)
>  		n = S_DAX;
>  
> -	if (n && !disk->fops->direct_access)
> +	if (n && !blkdev_dax_capable(bdev))
>  		return -ENOTTY;
>  
>  	mutex_lock(&bdev->bd_inode->i_mutex);
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 13ce6d0ff7f6..ee34a31e6fa4 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -152,16 +152,37 @@ static struct inode *bdev_file_inode(struct file *file)
>  	return file->f_mapping->host;
>  }
>  
> +#ifdef CONFIG_FS_DAX
> +bool blkdev_dax_capable(struct block_device *bdev)
> +{
> +	struct gendisk *disk = bdev->bd_disk;
> +
> +	if (!disk->fops->direct_access)
> +		return false;
> +
> +	/*
> +	 * If the partition is not aligned on a page boundary, we can't
> +	 * do dax I/O to it.
> +	 */
> +	if ((bdev->bd_part->start_sect % (PAGE_SIZE / 512))
> +			|| (bdev->bd_part->nr_sects % (PAGE_SIZE / 512)))
> +		return false;
> +
> +	return true;

Where do you check that S_DAX has been enabled on the block device
now?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
--
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