linux-ext4 - Re: [PATCH v5 4/6] dax: check synchronous mapping is supported

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Date:   Wed, 10 Apr 2019 10:25:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-acpi@...r.kernel.org,
        qemu-devel@...gnu.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, dan.j.williams@...el.com,
        zwisler@...nel.org, vishal.l.verma@...el.com, dave.jiang@...el.com,
        mst@...hat.com, jasowang@...hat.com, willy@...radead.org,
        rjw@...ysocki.net, hch@...radead.org, lenb@...nel.org,
        jack@...e.cz, tytso@....edu, adilger.kernel@...ger.ca,
        darrick.wong@...cle.com, lcapitulino@...hat.com, kwolf@...hat.com,
        imammedo@...hat.com, jmoyer@...hat.com, nilal@...hat.com,
        riel@...riel.com, stefanha@...hat.com, aarcange@...hat.com,
        david@...hat.com, david@...morbit.com, cohuck@...hat.com,
        xiaoguangrong.eric@...il.com, pbonzini@...hat.com,
        kilobyte@...band.pl, yuval.shaia@...cle.com
Subject: Re: [PATCH v5 4/6] dax: check synchronous mapping is supported

On Wed 10-04-19 09:38:24, Pankaj Gupta wrote:
> This patch introduces 'daxdev_mapping_supported' helper
> which checks if 'MAP_SYNC' is supported with filesystem
> mapping. It also checks if corresponding dax_device is
> synchronous. Virtio pmem device is asynchronous and
> does not not support VM_SYNC. 
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> ---
>  include/linux/dax.h | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b896706a5ee9..4a2a60ffec86 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -38,6 +38,24 @@ void kill_dax(struct dax_device *dax_dev);
>  void dax_write_cache(struct dax_device *dax_dev, bool wc);
>  bool dax_write_cache_enabled(struct dax_device *dax_dev);
>  bool dax_synchronous(struct dax_device *dax_dev);
> +
> +/*
> + * Callers check if synchronous mapping is enabled for DAX file
> + * and attached dax device is also synchronous.
> + *
> + * dax_synchronous function verifies if dax device is synchronous.
> + * Currently, only virtio pmem device supports asynchronous device
> + * flush.
> + */

Thanks for the patch! I'd restructure this comment like:

/*
 * Check if given mapping is supported by the file / underlying device.
 */
> +static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
> +				struct dax_device *dax_dev)
> +{

	/* Everyone supports non-sync mappings */
> +	if (!(vma->vm_flags & VM_SYNC))
> +		return true;
	/* Sync mappings are supported only for files using DAX */
> +	if (!IS_DAX(file_inode(vma->vm_file)))
> +		return false;
	/* Underlying device must support persisting through CPU instructions */
> +	return dax_synchronous(dax_dev);
> +}
>  #else
>  static inline struct dax_device *dax_get_by_host(const char *host)
>  {
> @@ -69,6 +87,11 @@ static inline bool dax_synchronous(struct dax_device *dax_dev)
>  {
>  	return true;
>  }
> +static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
> +				struct dax_device *dax_dev)
> +{
> +	return true;

This looks wrong. Shouldn't it rather be:

	return !(vma->flags & VM_SYNC);

?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR