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

["Liu, Jingqi" <jingqi.liu@intel.com>]

On 4/8/2020 5:42 PM, Joao Martins wrote:
> On 4/8/20 3:25 AM, Liu, Jingqi wrote:
>> On 4/8/2020 2:28 AM, Joao Martins wrote:
>>> 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:
>>>>> 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.
>> Hi Dan,
>>
>> Thanks for your comments.
>>
>> Seems it is a mistake.
>>
>> It should be: $(readlink -f /sys/dev/char/253\:263)/../../align
>>
> Hmm, perhaps you have an extra '../' in the path? This works for me:
>
> # ls $(readlink -f /sys/dev/char/252\:0/../align)
> /sys/devices/platform/e820_pmem/ndbus0/region0/dax0.0/dax0.0/../align
> # cat $(readlink -f /sys/dev/char/252\:0)/../align
> 2097152
> # cat /sys/dev/char/252\:0/../align
> 2097152

Hi Joao,

Hmm, I need to have an extra '../' in the path. The details are as follows:

# ll /dev/dax2.0
crw------- 1 root root 251, 5 Mar 20 13:35 /dev/dax2.0
# uname -r
5.6.0-rc1-00044-gb19e8c684703
# readlink -f /sys/dev/char/251\:5/
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus0/region2/dax2.1/dax/dax2.0
# ls $(readlink -f /sys/dev/char/251\:5)/../align
ls: cannot access 
'/sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus0/region2/dax2.1/dax/dax2.0/../align': 
No such file or directory
# ls $(readlink -f /sys/dev/char/251\:5)/../dax_region/align
ls: cannot access 
'/sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus0/region2/dax2.1/dax/dax2.0/../dax_region/align': 
No such file or directory
# ls $(readlink -f /sys/dev/char/251\:5)/../../align
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus0/region2/dax2.1/dax/dax2.0/../../align
# ls $(readlink -f /sys/dev/char/251\:5)/../../dax_region/align
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/ACPI0012:00/ndbus0/region2/dax2.1/dax/dax2.0/../../dax_region/align
# lsmod|grep pmem
dax_pmem_compat        16384  0
device_dax             20480  1 dax_pmem_compat
dax_pmem_core          16384  1 dax_pmem_compat
# lsmod|grep dax
dax_pmem_compat        16384  0
device_dax             20480  1 dax_pmem_compat
dax_pmem_core          16384  1 dax_pmem_compat

Seems some configurations are different ?

Can you share your info as above ? Thanks.

>>> 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)
>> Seems that you mean to use $(readlink -f
>> /sys/dev/char/253\:263)/../../dax_region/align.
>>
>> Right ?
>>
> An extra '../' ?
>
> # ls $(readlink -f /sys/dev/char/252\:0/../dax_region/align)
> /sys/devices/platform/e820_pmem/ndbus0/region0/dax0.0/dax0.0/../align
> # cat $(readlink -f /sys/dev/char/252\:0)/../dax_region/align
> 2097152
> # cat /sys/dev/char/252\:0/../dax_region/align
> 2097152
>
> For HMAT/hmem devdax, though, only 'dax_region/align' is available for now:
>
> # ls $(readlink -f /sys/dev/char/252:0)/../align
> ls: cannot access /sys/devices/platform/hmem.0/dax0.0/../align: No such file or
> directory
> # ls $(readlink -f /sys/dev/char/252:0)/../dax_region/align
> /sys/devices/platform/hmem.0/dax0.0/../dax_region/align
> # cat $(readlink -f /sys/dev/char/252:0)/../dax_region/align
> 2097152
>
> The 'dax_region/align' was just an idea mainly because it's common to both
> device-dax devices -- not sure how others feel about it.

Seems it's reasonable. I need to sync the above path with yours.

Thanks,

Jingqi

>
> 	Joao
>
>> Thanks,
>>
>> Jingqi
>>
>>>> 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