[DTrace-devel] [PATCH] Convert dvars to use the default block of zeros

[Eugene Loh]

On 8/4/22 12:25, Kris Van Hees wrote:
> On Thu, Aug 04, 2022 at 10:03:13AM -0700, Eugene Loh via DTrace-devel wrote:
>> Reviewed-by: Eugene Loh <eugene.loh at oracle.com>
>> I'm getting confused about the order of patches, but in dt_bpf.c I thought
>> there was a block of code that computed dt_zerosize, bumping it up depending
>> on dt_maxtuplesize, dt_aggsiz, etc.  That would seem to be the right place
>> to bump it up per strsize + 1 and dt_maxdvarsize as well.  As it is, the
>> current patch seems to compute dt_zerosize based only on dt_maxdvarsize and
>> strsize+1, ignoring any previous information about dt_zerosize (which would
>> have incorporated info on dt_maxtuplesize and dt_aggsiz).
> No, the patch that introduces the block of zeros never assigns a value to
> zerosize (not in your patch, and not in my version).  Since this patch is the
> first use of that block, it introduces determining the value of zerosize.
>
> Later uses (in your patches) will need to add further logic to determine the
> size needed.

Okay, but if this patch is going to go in before the others, how about 
putting the dt_zerosize= statement earlier by itself.  For this 
particular patch, maybe it's okay to insert that one-liner into the 
middle of some other dense code, but since we already know how this code 
will evolve we might as well put the dt_zerosize= line earlier, by 
itself, so that the code can be patched more cleanly. It is often hard 
to predict how code will evolve.  In this case, we already can look 
several patches ahead.

>> On 8/4/22 06:55, Kris Van Hees via DTrace-devel wrote:
>>> diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
>>> @@ -431,7 +431,8 @@ dt_bpf_gmap_create(dtrace_hdl_t *dtp)
>>>    	 */
>>>    	dtp->dt_strlen = dt_strtab_size(dtp->dt_ccstab);
>>>    	dtp->dt_zerooffset = P2ROUNDUP(dtp->dt_strlen, 8);
>>> -	sz = dtp->dt_zerooffset + MAX(strsize + 1, dtp->dt_zerosize);
>>> +	dtp->dt_zerosize = MAX(dtp->dt_maxdvarsize, strsize + 1);
>>> +	sz = dtp->dt_zerooffset + dtp->dt_zerosize;
>>>    	strtab = dt_zalloc(dtp, sz);
>>>    	if (strtab == NULL)
>>>    		return dt_set_errno(dtp, EDT_NOMEM);