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Date:   Thu, 4 Apr 2019 09:09:12 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
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Subject: Re: [PATCH v4 5/5] xfs: disable map_sync for async flush
Message-ID: <20190403220912.GB26298@dastard>
References: <20190403104018.23947-1-pagupta@redhat.com>
 <20190403104018.23947-6-pagupta@redhat.com>
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On Wed, Apr 03, 2019 at 04:10:18PM +0530, Pankaj Gupta wrote:
> Virtio pmem provides asynchronous host page cache flush
> mechanism. we don't support 'MAP_SYNC' with virtio pmem 
> and xfs.
> 
> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> ---
>  fs/xfs/xfs_file.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 1f2e2845eb76..dced2eb8c91a 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1203,6 +1203,14 @@ xfs_file_mmap(
>  	if (!IS_DAX(file_inode(filp)) && (vma->vm_flags & VM_SYNC))
>  		return -EOPNOTSUPP;
>  
> +	/* We don't support synchronous mappings with DAX files if
> +	 * dax_device is not synchronous.
> +	 */
> +	if (IS_DAX(file_inode(filp)) && !dax_synchronous(
> +		xfs_find_daxdev_for_inode(file_inode(filp))) &&
> +					(vma->vm_flags & VM_SYNC))
> +		return -EOPNOTSUPP;
> +
>  	file_accessed(filp);
>  	vma->vm_ops = &xfs_file_vm_ops;
>  	if (IS_DAX(file_inode(filp)))

All this ad hoc IS_DAX conditional logic is getting pretty nasty.

xfs_file_mmap(
....
{
	struct inode	*inode = file_inode(filp);

	if (vma->vm_flags & VM_SYNC) {
		if (!IS_DAX(inode))
			return -EOPNOTSUPP;
		if (!dax_synchronous(xfs_find_daxdev_for_inode(inode))
			return -EOPNOTSUPP;
	}

	file_accessed(filp);
	vma->vm_ops = &xfs_file_vm_ops;
	if (IS_DAX(inode))
		vma->vm_flags |= VM_HUGEPAGE;
	return 0;
}


Even better, factor out all the "MAP_SYNC supported" checks into a
helper so that the filesystem code just doesn't have to care about
the details of checking for DAX+MAP_SYNC support....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com