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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 1 Jun 2017 06:53:45 -0700
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Subject: Re: [Qemu-devel] [RESEND PATCH 1/2] nvdimm: warn if the backend is
 not a DAX device
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To: Xiao Guangrong <guangrong.xiao@gmail.com>
Cc: Haozhong Zhang <haozhong.zhang@intel.com>, qemu-devel@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, linux-fsdevel <linux-fsdevel@vger.kernel.org>

[ adding linux-fsdevel ]

On Thu, Jun 1, 2017 at 5:00 AM, Xiao Guangrong <guangrong.xiao@gmail.com> w=
rote:
>
>
> On 05/26/2017 10:32 AM, Haozhong Zhang wrote:
>
>> +static void nvdimm_check_dax(HostMemoryBackend *hostmem)
>> +{
>> +    char *mem_path =3D
>> +        object_property_get_str(OBJECT(hostmem), "mem-path", NULL);
>> +    char *dev_name =3D NULL, *sysfs_path =3D NULL;
>> +    bool is_dax =3D false;
>> +
>> +    if (!mem_path) {
>> +        goto out;
>> +    }
>> +
>> +    if (!g_str_has_prefix(mem_path, "/dev/dax")) {
>> +        goto out;
>> +    }
>> +
>> +    dev_name =3D mem_path + strlen("/dev/");
>> +    sysfs_path =3D g_strdup_printf("/sys/class/dax/%s", dev_name);
>> +    if (access(sysfs_path, F_OK)) {
>> +        goto out;
>> +    }
>> +
>> +    is_dax =3D true;
>> +
>
>
> So only dax raw disk has write-persistence guaranty=EF=BC=8C a pre-alloca=
ted
> file which locates on a DAX-enabled filesystem can not?

Correct, it is not guaranteed by any existing filesystem. It may work
by accident today on ext4 with a pre-allocated file, but a filesystem
is otherwise permitted to drop any writes that have not been synced.
Until we get an explicit MMAP_SYNC or METADATA_IMMUTABLE feature into
a filesystem the only interface that we can use to reliably pass
persistent memory through to a guest is device-dax.