Re: [PATCH 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[David Hildenbrand <david@redhat.com>]

On 30.05.25 11:52, Baolin Wang wrote:
> 
> 
> On 2025/5/30 17:16, David Hildenbrand wrote:
>> On 30.05.25 11:10, David Hildenbrand wrote:
>>> On 30.05.25 10:59, Ryan Roberts wrote:
>>>> On 30/05/2025 09:44, David Hildenbrand wrote:
>>>>> On 30.05.25 10:04, Ryan Roberts wrote:
>>>>>> On 29/05/2025 09:23, Baolin Wang wrote:
>>>>>>> As we discussed in the previous thread [1], the MADV_COLLAPSE will
>>>>>>> ignore
>>>>>>> the system-wide anon/shmem THP sysfs settings, which means that
>>>>>>> even though
>>>>>>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE
>>>>>>> will still
>>>>>>> attempt to collapse into a anon/shmem THP. This violates the rule
>>>>>>> we have
>>>>>>> agreed upon: never means never. This patch set will address this
>>>>>>> issue.
>>>>>>
>>>>>> This is a drive-by comment from me without having the previous
>>>>>> context, but...
>>>>>>
>>>>>> Surely MADV_COLLAPSE *should* ignore the THP sysfs settings? It's a
>>>>>> deliberate
>>>>>> user-initiated, synchonous request to use huge pages for a range of
>>>>>> memory.
>>>>>> There is nothing *transparent* about it, it just happens to be
>>>>>> implemented using
>>>>>> the same logic that THP uses.
>>>>>>
>>>>>> I always thought this was a deliberate design decision.
>>>>>
>>>>> If the admin said "never", then why should a user be able to
>>>>> overwrite that?
>>>>
>>>> Well my interpretation would be that the admin is saying never
>>>> *transparently*
>>>> give anyone any hugepages; on balance it does more harm than good for my
>>>> workloads. The toggle is called transparent_hugepage/enabled, after all.
>>>
>>> I'd say it's "enabling transparent huge pages" not "transparently
>>> enabling huge pages". After all, these things are ... transparent huge
>>> pages.
>>>
>>> But yeah, it's confusing.
>>>
>>>>
>>>> Whereas MADV_COLLAPSE is deliberately applied to a specific region at an
>>>> opportune moment in time, presumably because the user knows that the
>>>> region
>>>> *will* benefit and because that point in the execution is not
>>>> sensitive to latency.
>>>
>>> Not sure if MADV_HUGEPAGE is really *that* different.
>>>
>>>>
>>>> I see them as logically separate.
>>>>
>>>>>
>>>>> The design decision I recall is that if VM_NOHUGEPAGE is set, we'll
>>>>> ignore that.
>>>>> Because that was set by the app itself (MADV_NOHUEPAGE).
> 
> IIUC, MADV_COLLAPSE does not ignore the VM_NOHUGEPAGE setting, if we set
> VM_NOHUGEPAGE, then MADV_COLLAPSE will not be allowed to collapse a THP.
> See:
> __thp_vma_allowable_orders() ---> vma_thp_disabled()

Interesting, maybe I misremember things.

Maybe because process_madvise() could try MADV_COLLAPSE on a different 
process. And if that process as VM_NOHUGEPAGE set, it could be problematic.

-- 
Cheers,

David / dhildenb