[PATCH 2/3] hotplug-memory.c: enhance dlpar_memory_remove* LMB checks

[Daniel Henrique Barboza]

On 5/3/21 10:02 PM, David Gibson wrote:
> On Fri, Apr 30, 2021 at 09:09:16AM -0300, Daniel Henrique Barboza wrote:
>> dlpar_memory_remove_by_ic() validates the amount of LMBs to be removed
>> by checking !DRCONF_MEM_RESERVED, and in the following loop before
>> dlpar_remove_lmb() a check for DRCONF_MEM_ASSIGNED is made before
>> removing it. This means that a LMB that is both !DRCONF_MEM_RESERVED and
>> !DRCONF_MEM_ASSIGNED will be counted as valid, but then not being
>> removed.  The function will end up not removing all 'lmbs_to_remove'
>> LMBs while also not reporting any errors.
>>
>> Comparing it to dlpar_memory_remove_by_count(), the validation is done
>> via lmb_is_removable(), which checks for DRCONF_MEM_ASSIGNED and fadump
>> constraints. No additional check is made afterwards, and
>> DRCONF_MEM_RESERVED is never checked before dlpar_remove_lmb(). The
>> function doesn't have the same 'check A for validation, then B for
>> removal' issue as remove_by_ic(), but it's not checking if the LMB is
>> reserved.
>>
>> There is no reason for these functions to validate the same operation in
>> two different manners.
> 
> Actually, I think there is: remove_by_ic() is handling a request to
> remove a specific range of LMBs.  If any are reserved, they can't be
> removed and so this needs to fail.  But if they are !ASSIGNED, that
> essentially means they're *already* removed (or never added), so
> "removing" them is, correctly, a no-op.

I guess that makes sense. Although I am not aware of any situation, at least
thinking about how QEMU adds/removes LMBs, where some LMBs would be removed
'ad-hoc' in the middle of a LMB range that maps to a QEMU DIMM, I can't say
that this wouldn't never happen either. It is sensible to make remove_by_ic()
resilient to this situation.

I'll re-send this patch just with the remove_by_count() change.


Thanks,


Daniel

> 
> remove_by_count(), in contrast, is being asked to remove a fixed
> number of LMBs from wherever they can be found, and for that it needs
> to find LMBs that haven't already been removed.
> 
> Basically remove_by_ic() is an absolute request: "make this set of
> LMBs be not-plugged", whereas remove_by_count() is a relative request
> "make N less LMBs be plugged".
> 
> 
> So I think remove_by_ic()s existing handling is correct.  I'm less
> sure if remove_by_count() ignoring RESERVED is correct - I couldn't
> quickly find under what circumstances RESERVED gets set.
> 
> 
>> This patch addresses that by changing
>> lmb_is_removable() to also check for DRCONF_MEM_RESERVED to tell if a
>> lmb is removable, making dlpar_memory_remove_by_count() take the
>> reservation state into account when counting the LMBs.
>> lmb_is_removable() is then used in the validation step of
>> dlpar_memory_remove_by_ic(), which is already checking for both states
>> but in different stages, to avoid counting a LMB that is not assigned as
>> eligible for removal. We can then skip the check before
>> dlpar_remove_lmb() since we're validating all LMBs beforehand.
>>
>> Signed-off-by: Daniel Henrique Barboza <danielhb413 at gmail.com>
>> ---
>>   arch/powerpc/platforms/pseries/hotplug-memory.c | 8 +++-----
>>   1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
>> index bb98574a84a2..4e6d162c3f1a 100644
>> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
>> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
>> @@ -348,7 +348,8 @@ static int pseries_remove_mem_node(struct device_node *np)
>>   
>>   static bool lmb_is_removable(struct drmem_lmb *lmb)
>>   {
>> -	if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
>> +	if ((lmb->flags & DRCONF_MEM_RESERVED) ||
>> +		!(lmb->flags & DRCONF_MEM_ASSIGNED))
>>   		return false;
>>   
>>   #ifdef CONFIG_FA_DUMP
>> @@ -523,7 +524,7 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
>>   
>>   	/* Validate that there are enough LMBs to satisfy the request */
>>   	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
>> -		if (lmb->flags & DRCONF_MEM_RESERVED)
>> +		if (!lmb_is_removable(lmb))
>>   			break;
>>   
>>   		lmbs_available++;
>> @@ -533,9 +534,6 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
>>   		return -EINVAL;
>>   
>>   	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
>> -		if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
>> -			continue;
>> -
>>   		rc = dlpar_remove_lmb(lmb);
>>   		if (rc)
>>   			break;
>