[PATCH v06 3/5] migration/memory: Add hotplug READD_MULTIPLE

[Michael Ellerman]

Michael Bringmann <mwb at linux.vnet.ibm.com> writes:
> On 10/16/2018 02:57 PM, Tyrel Datwyler wrote:
>> On 10/15/2018 05:39 PM, Michael Ellerman wrote:
>>> Michael Bringmann <mwb at linux.vnet.ibm.com> writes:
>>>> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
>>>> index 2b796da..9c76345 100644
>>>> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
>>>> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
>>>> @@ -541,6 +549,23 @@ static int dlpar_memory_readd_by_index(u32 drc_index)
>>>>  	return rc;
>>>>  }
>>>>
>>>> +static int dlpar_memory_readd_multiple(void)
>>>> +{
>>>> +	struct drmem_lmb *lmb;
>>>> +	int rc;
>>>> +
>>>> +	pr_info("Attempting to update multiple LMBs\n");
>>>> +
>>>> +	for_each_drmem_lmb(lmb) {
>>>> +		if (drmem_lmb_update(lmb)) {
>>>> +			rc = dlpar_memory_readd_helper(lmb);
>>>> +			drmem_remove_lmb_update(lmb);
>>>> +		}
>>>> +	}
>>>> +
>>>> +	return rc;
>>>> +}
>>>
>>> This leaves rc potentially uninitialised.
>>>
>>> What should the result be in that case, -EINVAL ?
>> 
>> On another note if there are multiple LMBs to update the value of rc only reflects the final dlpar_memory_readd_helper() call.
>
> Correct.  But that is what happens when we compress common code
> between two disparate uses i.e. updating memory association after
> a migration event with no reporting mechanism other than the console
> log, vs re-adding a single LMB by index for the purposes of DLPAR / drmgr.
>
> I could discard the return value from dlpar_memory_readd_helper entirely
> in this function and just return 0, but in my experience, once errors start
> to occur in memory dlpar ops, they tend to keep on occurring, so I was
> returning the last one.  We could also make the code smart enough to
> capture and return the first/last non-zero return code.  I didn't believe
> that the frequency of errors for this operation warranted the overhead.

The actual error value is probably not very relevant.

But dropping errors entirely is almost always a bad idea.

So I think you should at least return an error if any error occurred,
that way at least an error will be returned up to the caller(s).

Something like:

	int rc;

	rc = 0;
	for_each_drmem_lmb(lmb) {
		if (drmem_lmb_update(lmb)) {
			rc |= dlpar_memory_readd_helper(lmb);
			drmem_remove_lmb_update(lmb);
		}
	}

	if (rc)
		return -EIO;


cheers