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Date:   Wed, 9 Jan 2019 08:26:54 -0800
From:   "Darrick J. Wong" <darrick.wong@oracle.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        qemu-devel@nongnu.org, linux-nvdimm@ml01.01.org,
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Subject: Re: [PATCH v3 5/5] xfs: disable map_sync for virtio pmem
Message-ID: <20190109162654.GW12689@magnolia>
References: <20190109144736.17452-1-pagupta@redhat.com>
 <20190109144736.17452-6-pagupta@redhat.com>
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On Wed, Jan 09, 2019 at 08:17:36PM +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 e474250..eae4aa4 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1190,6 +1190,14 @@ xfs_file_mmap(
>  	if (!IS_DAX(file_inode(filp)) && (vma->vm_flags & VM_SYNC))
>  		return -EOPNOTSUPP;
>  
> +	/* We don't support synchronous mappings with guest direct access
> +	 * and virtio based host page cache mechanism.
> +	 */
> +	if (IS_DAX(file_inode(filp)) && virtio_pmem_host_cache_enabled(

Echoing what Jan said, this ought to be some sort of generic function
that tells us whether or not memory mapped from the dax device will
always still be accessible even after a crash (i.e. supports MAP_SYNC).

What if the underlying file on the host is itself on pmem and can be
MAP_SYNC'd?  Shouldn't the guest be able to use MAP_SYNC as well?

--D

> +		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)))
> -- 
> 2.9.3
>