Re: [PATCH] exec: fetch the alignment of Linux devdax pmem character device nodes

[Joao Martins <joao.m.martins@oracle.com>]

On 4/7/20 5:55 PM, Dan Williams wrote:
> On Tue, Apr 7, 2020 at 4:01 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>> On 4/1/20 4:13 AM, Jingqi Liu wrote:
>>> If the backend file is devdax pmem character device, the alignment
>>> specified by the option 'align=NUM' in the '-object memory-backend-file'
>>> needs to match the alignment requirement of the devdax pmem character device.
>>>
>>> This patch fetches the devdax pmem file 'align', so that we can compare
>>> it with the NUM of 'align=NUM'.
>>> The NUM needs to be larger than or equal to the devdax pmem file 'align'.
>>>
>>> It also fixes the problem that mmap() returns failure in qemu_ram_mmap()
>>> when the NUM of 'align=NUM' is less than the devdax pmem file 'align'.
>>>
>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>> Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
>>> ---
>>>  exec.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
>>>  1 file changed, 45 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/exec.c b/exec.c
>>> index de9d949902..8221abffec 100644
>>> --- a/exec.c
>>> +++ b/exec.c
>>> @@ -1736,6 +1736,42 @@ static int64_t get_file_size(int fd)
>>>      return size;
>>>  }
>>>
>>> +static int64_t get_file_align(int fd)
>>> +{
>>> +    int64_t align = -1;
>>> +#if defined(__linux__)
>>> +    struct stat st;
>>> +
>>> +    if (fstat(fd, &st) < 0) {
>>> +        return -errno;
>>> +    }
>>> +
>>> +    /* Special handling for devdax character devices */
>>> +    if (S_ISCHR(st.st_mode)) {
>>> +        g_autofree char *subsystem_path = NULL;
>>> +        g_autofree char *subsystem = NULL;
>>> +
>>> +        subsystem_path = g_strdup_printf("/sys/dev/char/%d:%d/subsystem",
>>> +                                         major(st.st_rdev), minor(st.st_rdev));
>>> +        subsystem = g_file_read_link(subsystem_path, NULL);
>>> +
>>> +        if (subsystem && g_str_has_suffix(subsystem, "/dax")) {
>>> +            g_autofree char *align_path = NULL;
>>> +            g_autofree char *align_str = NULL;
>>> +
>>> +            align_path = g_strdup_printf("/sys/dev/char/%d:%d/device/align",
>>> +                                    major(st.st_rdev), minor(st.st_rdev));
>>> +
>>
>> Perhaps, you meant instead:
>>
>>         /sys/dev/char/%d:%d/align
>>
> 
> Hmm, are you sure that's working? 

It is, except that I made the slight mistake of testing with a bunch of wip
patches on top which one of them actually adds the 'align' to child dax device.

Argh, my apologies - and thanks for noticing.

> I expect the alignment to be found
> in the region device:
> 
> /sys/class/dax:
> /sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus1/region1/dax1.1/dax1.0
> $(readlink -f /sys/dev/char/253\:263)/../align
> $(readlink -f /sys/dev/char/253\:263)/device/align
> 
> 
> /sys/bus/dax:
> /sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus1/region1/dax1.0/dax1.0
> $(readlink -f /sys/dev/char/253\:265)/../align
> $(readlink -f /sys/dev/char/253\:265)/device/align <-- No such file
> 
> The use of the /sys/dev/char/%d:%d/device is only supported by the
> deprecated /sys/class/dax. 

I don't have the deprecated dax class enabled as could you tell, so the second
case is what I was testing. Except it wasn't a namespace/nvdimm but rather an
hmem device-dax.

'../align' though covers only one case? What about hmem which '../align' returns
ENOENT; perhaps using '../dax_region/align' instead which is common to both?
Albeit that wouldn't address the sub-division devices (that I mention above)

> The current /sys/bus/dax device-model can
> be a drop in replacement as long as software is not written to the
> /sys/class sysfs layout, i.e. it uses ../ instead of device/ to walk
> to the region properties.
> 
/nods