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

[Dan Williams]

On Thu, Apr 9, 2020 at 7:33 AM Liu, Jingqi <address@hidden> wrote:
>
> 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 <address@hidden> 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.

Alternatively maybe you can use libdaxctl that automatically handles
the ABI differences between compat-dax-class and dax-bus layouts? I
didn't recommend it before because I was not sure about qemu's policy
about taking on new library dependencies, but with libdaxctl you could
do something like (untested):

path = g_strdup_printf("/sys/dev/char/%d:%d", major(st.st_rdev),
minor(st.st_rdev));
rpath = realpath(path, NULL);
daxctl_region_foreach(ctx, region)
    if (strstr(daxctl_region_get_path(region), rpath)) {
        align = daxctl_region_get_align(region);
        break;
    }