On 03/01/2019 07:25 AM, Alex Ghiti wrote:
> On 2/28/19 5:26 PM, Mike Kravetz wrote:
>> On 2/28/19 12:23 PM, Dave Hansen wrote:
>>> On 2/28/19 11:50 AM, Mike Kravetz wrote:
>>>> On 2/28/19 11:13 AM, Dave Hansen wrote:
>>>>>> + if (hstate_is_gigantic(h) &&
>>>>>> !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
>>>>>> + spin_lock(&hugetlb_lock);
>>>>>> + if (count > persistent_huge_pages(h)) {
>>>>>> + spin_unlock(&hugetlb_lock);
>>>>>> + return -EINVAL;
>>>>>> + }
>>>>>> + goto decrease_pool;
>>>>>> + }
>>>>> This choice confuses me. The "Decrease the pool size" code already
>>>>> works and the code just falls through to it after skipping all the
>>>>> "Increase the pool size" code.
>>>>>
>>>>> Why did did you need to add this case so early? Why not just let it
>>>>> fall through like before?
>>>> I assume you are questioning the goto, right? You are correct in that
>>>> it is unnecessary and we could just fall through.
>>> Yeah, it just looked odd to me.
>
> (Dave I do not receive your answers, I don't know why).
I collected mistakes here: domain name expired and no mailing list added :)
Really sorry about that, I missed the whole discussion (if any).
Could someone forward it to me (if any) ? Thanks !
> I'd rather avoid useless checks when we already know they won't
> be met and I think that makes the code more understandable.
>
> But that's up to you for the next version.
>
> Thanks
>>>
>>>> However, I wonder if we might want to consider a wacky condition
>>>> that the
>>>> above check would prevent. Consider a system/configuration with 5
>>>> gigantic
>>>> pages allocated at boot time. Also CONFIG_CONTIG_ALLOC is not
>>>> enabled, so
>>>> it is not possible to allocate gigantic pages after boot.
>>>>
>>>> Suppose the admin decreased the number of gigantic pages to 3.
>>>> However, all
>>>> gigantic pages were in use. So, 2 gigantic pages are now 'surplus'.
>>>> h->nr_huge_pages == 5 and h->surplus_huge_pages == 2, so
>>>> persistent_huge_pages() == 3.
>>>>
>>>> Now suppose the admin wanted to increase the number of gigantic
>>>> pages to 5.
>>>> The above check would prevent this. However, we do not need to really
>>>> 'allocate' two gigantic pages. We can simply convert the surplus
>>>> pages.
>>>>
>>>> I admit this is a wacky condition. The ability to 'free' gigantic
>>>> pages
>>>> at runtime if !CONFIG_CONTIG_ALLOC makes it possible. I don't
>>>> necessairly
>>>> think we should consider this. hugetlbfs code just makes me think of
>>>> wacky things. :)
>>> I think you're saying that the newly-added check is overly-restrictive.
>>> If we "fell through" like I was suggesting we would get better
>>> behavior.
>> At first, I did not think it overly restrictive. But, I believe we can
>> just eliminate that check for gigantic pages. If
>> !CONFIG_CONTIG_ALLOC and
>> this is a request to allocate more gigantic pages,
>> alloc_pool_huge_page()
>> should return NULL.
>>
>> The only potential issue I see is that in the past we have returned
>> EINVAL
>> if !CONFIG_CONTIG_ALLOC and someone attempted to increase the pool size.
>> Now, we will not increase the pool and will not return an error. Not
>> sure
>> if that is an acceptable change in user behavior.
>
> If I may, I think that this is the kind of info the user wants to have
> and we should
> return an error when it is not possible to allocate runtime huge pages.
> I already noticed that if someone asks for 10 huge pages, and only 5
> are allocated,
> no error is returned to the user and I found that surprising.
>
>>
>> If we go down this path, then we could remove this change as well:
>
> I agree that in that path, we do not need the following change neither.
>
>>
>>> @@ -2428,7 +2442,9 @@ static ssize_t
>>> __nr_hugepages_store_common(bool obey_mempolicy,
>>> } else
>>> nodes_allowed = &node_states[N_MEMORY];
>>> - h->max_huge_pages = set_max_huge_pages(h, count, nodes_allowed);
>>> + err = set_max_huge_pages(h, count, nodes_allowed);
>>> + if (err)
>>> + goto out;
>>> if (nodes_allowed != &node_states[N_MEMORY])
>>> NODEMASK_FREE(nodes_allowed);
>> Do note that I beleive there is a bug the above change. The code after
>> the out label is:
>>
>> out:
>> NODEMASK_FREE(nodes_allowed);
>> return err;
>> }
>>
>> With the new goto, we need the same
>> if (nodes_allowed != &node_states[N_MEMORY]) before NODEMASK_FREE().
>>
>> Sorry, I missed this in previous versions.
>
> Oh right, I'm really sorry I missed that, thank you for noticing.
>
On 3/1/19 2:21 PM, Alexandre Ghiti wrote:
> I collected mistakes here: domain name expired and no mailing list added :)
> Really sorry about that, I missed the whole discussion (if any).
> Could someone forward it to me (if any) ? Thanks !
Bounced you David and Mike's discussion (4 messages total). AFAICS that
was all.
On 03/01/2019 02:33 PM, Vlastimil Babka wrote:
> On 3/1/19 2:21 PM, Alexandre Ghiti wrote:
>> I collected mistakes here: domain name expired and no mailing list added :)
>> Really sorry about that, I missed the whole discussion (if any).
>> Could someone forward it to me (if any) ? Thanks !
> Bounced you David and Mike's discussion (4 messages total). AFAICS that
> was all.
Thank you Vlastimil, I got them.
Thanks,
On 3/1/19 5:21 AM, Alexandre Ghiti wrote:
> On 03/01/2019 07:25 AM, Alex Ghiti wrote:
>> On 2/28/19 5:26 PM, Mike Kravetz wrote:
>>> On 2/28/19 12:23 PM, Dave Hansen wrote:
>>>> On 2/28/19 11:50 AM, Mike Kravetz wrote:
>>>>> On 2/28/19 11:13 AM, Dave Hansen wrote:
>>>>>>> + if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
>>>>>>> + spin_lock(&hugetlb_lock);
>>>>>>> + if (count > persistent_huge_pages(h)) {
>>>>>>> + spin_unlock(&hugetlb_lock);
>>>>>>> + return -EINVAL;
>>>>>>> + }
>>>>>>> + goto decrease_pool;
>>>>>>> + }
>>>>>> This choice confuses me. The "Decrease the pool size" code already
>>>>>> works and the code just falls through to it after skipping all the
>>>>>> "Increase the pool size" code.
>>>>>>
>>>>>> Why did did you need to add this case so early? Why not just let it
>>>>>> fall through like before?
>>>>> I assume you are questioning the goto, right? You are correct in that
>>>>> it is unnecessary and we could just fall through.
>>>> Yeah, it just looked odd to me.
>
>> I'd rather avoid useless checks when we already know they won't
>> be met and I think that makes the code more understandable.
>>
>> But that's up to you for the next version.
I too find some value in the goto. It tells me this !CONFIG_CONTIG_ALLOC
case is special and we are skipping the normal checks. But, removing the
goto is not a requirement for me.
>>>>> However, I wonder if we might want to consider a wacky condition that the
>>>>> above check would prevent. Consider a system/configuration with 5 gigantic
...
>>
>> If I may, I think that this is the kind of info the user wants to have and we should
>> return an error when it is not possible to allocate runtime huge pages.
>> I already noticed that if someone asks for 10 huge pages, and only 5 are allocated,
>> no error is returned to the user and I found that surprising.
Upon further thought, let's not consider this wacky permanent -> surplus ->
permanent case. I just can't see it being an actual use case.
IIUC, that 'no error' behavior is somewhat expected. I seem to recall previous
discussions about changing with the end result to leave as is.
>>>> @@ -2428,7 +2442,9 @@ static ssize_t __nr_hugepages_store_common(bool obey_mempolicy,
>>>> } else
>>>> nodes_allowed = &node_states[N_MEMORY];
>>>> - h->max_huge_pages = set_max_huge_pages(h, count, nodes_allowed);
>>>> + err = set_max_huge_pages(h, count, nodes_allowed);
>>>> + if (err)
>>>> + goto out;
>>>> if (nodes_allowed != &node_states[N_MEMORY])
>>>> NODEMASK_FREE(nodes_allowed);
>>> Do note that I beleive there is a bug the above change. The code after
>>> the out label is:
>>>
>>> out:
>>> NODEMASK_FREE(nodes_allowed);
>>> return err;
>>> }
>>>
>>> With the new goto, we need the same
>>> if (nodes_allowed != &node_states[N_MEMORY]) before NODEMASK_FREE().
>>>
>>> Sorry, I missed this in previous versions.
>>
>> Oh right, I'm really sorry I missed that, thank you for noticing.
This is the only issue I have with the code in hugetlb.c. For me, the
goto can stay or go. End result is the same.
--
Mike Kravetz