Re: [PATCH v2] sysinfo: include availram field in sysinfo struct

[Pintu Agarwal]

On Sat, 8 Jan 2022 at 03:52, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Pintu Kumar
> > Sent: 07 January 2022 18:08
> >
> > The sysinfo member does not have any "available ram" field and
> > the bufferram field is not much helpful either, to get a rough
> > estimate of available ram needed for allocation.
> >
> > One needs to parse MemAvailable field separately from /proc/meminfo
> > to get this info instead of directly getting if from sysinfo itself.
> >
> > Thus, this patch introduce a new field as availram in sysinfo
> > so that all the info total/free/available can be retrieved from
> > one place itself.
> >
> ...
> > diff --git a/include/uapi/linux/sysinfo.h b/include/uapi/linux/sysinfo.h
> > index 435d5c2..fe84c6a 100644
> > --- a/include/uapi/linux/sysinfo.h
> > +++ b/include/uapi/linux/sysinfo.h
> > @@ -19,7 +19,8 @@ struct sysinfo {
> >       __kernel_ulong_t totalhigh;     /* Total high memory size */
> >       __kernel_ulong_t freehigh;      /* Available high memory size */
> >       __u32 mem_unit;                 /* Memory unit size in bytes */
> > -     char _f[20-2*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses this.. */
>
> There are 4 pad bytes here on most 64bit architectures.
>
> > +     __kernel_ulong_t availram;      /* Memory available for allocation */
> > +     char _f[20-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses this.. */
> >  };
>
> You've not compile-time tested the size of the structure.
>
With "32" instead of "20" in padding I get these size of sysinfo:
In x86-64 kernel, with app 64-bit: Size of sysinfo = 128
In x86-64 kernel, with app 32-bit:: Size of sysinfo = 76
In arm-64 kernel, with app 32-bit: Size of sysinfo = 76

Okay the sys robot reported some issue in 64-bit build.
{{{
>> include/uapi/linux/sysinfo.h:23:14: error: size of array '_f' is too large
>>    23 |         char _f[20-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses this.. */
>>       |              ^~
}}}

Also, I got the same issue while building for arm64, so I tried to
adjust like this:
char _f[32-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];

With this the build works on both 32/64 but output fails when running
32-bit program on 64-bit kernel.
Also, the free command on 64-bit reports "stack smashing error"..

How do we resolve this issue to make it work on both arch ?
Also, I don't really understand the significance of that number "20"
in padding ?

Thanks,
Pintu