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

[Dan Williams <dan.j.williams@intel.com>]

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? 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. 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.