wrong value from DTRACE (uint32 for int64)

[Mark Johnston]

On Mon, Dec 02, 2019 at 08:44:59PM +0100, Peter wrote:
> Hi @all,
> 
> I felt the need to look into my ZFS ARC, but DTRACE provided misleading  
> (i.e., wrong) output (on i386, 11.3-RELEASE):
> 
> # dtrace -Sn 'arc-available_memory { printf("%x %x", arg0, arg1); }'
> DIFO 0x286450a0 returns D type (integer) (size 8)
> OFF OPCODE      INSTRUCTION
> 00: 29010601    ldgs DT_VAR(262), %r1           ! DT_VAR(262) = "arg0"
> 01: 23000001    ret  %r1
> 
> NAME             ID   KND SCP FLAG TYPE
> arg0             262  scl glb r    D type (integer) (size 8)
> 
> DIFO 0x286450f0 returns D type (integer) (size 8)
> OFF OPCODE      INSTRUCTION
> 00: 29010701    ldgs DT_VAR(263), %r1           ! DT_VAR(263) = "arg1"
> 01: 23000001    ret  %r1
> 
> NAME             ID   KND SCP FLAG TYPE
> arg1             263  scl glb r    D type (integer) (size 8)
> dtrace: description 'arc-available_memory ' matched 1 probe
>    0     14        none:arc-available_memory 2fb000 2
>    0     14        none:arc-available_memory 4e000 2
>    1     14        none:arc-available_memory ffffb000 2
>    1     14        none:arc-available_memory ffffb000 2
>    1     14        none:arc-available_memory ffffb000 2
>    1     14        none:arc-available_memory 19000 2
>    0     14        none:arc-available_memory d38000 2
> 
> # dtrace -n 'arc-available_memory { printf("%d %d", arg0, arg1); }'
>    1     14        none:arc-available_memory 81920 5
>    1     14        none:arc-available_memory 69632 5
>    1     14        none:arc-available_memory 4294955008 5
>    1     14        none:arc-available_memory 4294955008 5
> 
> 
> The arg0 Variable is shown here obviousely as an unsigned int32 value. But  
> in fact, the probe in the sourcecode in arc.c is a signed int64:
> 
>          DTRACE_PROBE2(arc__available_memory, int64_t, lowest, int, r);
> 
> 
> User @shkhin in the forum pointed me to check the bare dtrace program,  
> unattached to the kernel code:
> https://forums.freebsd.org/threads/dtrace-treats-int64_t-as-uint32_t-on-i386.73223/post-446517
> 
> And there everything appears correct.
> 
> So two questions:
> 1. can anybody check and confirm this happening?
> 2. any idea what could be wrong here? (The respective variable in arc.c  
> bears the correct 64bit negative value, I checked that - and otherwise the  
> ARC couldn't shrink.)

The DTRACE_PROBE* macros cast their parameters to uintptr_t, which
will be 32 bits wide on i386.  You might be able to work around the
problem by casting arg0 to uint32_t in the script.