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

[David Hildenbrand <david@redhat.com>]

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).
>>
>> Hmm, ok. My instinct would have been the opposite; MADV_NOHUGEPAGE means "I
>> don't want the risk of latency spikes and memory bloat that THP can cause". Not
>> "ignore my explicit requests to MADV_COLLAPSE".
>>
>> But if that descision was already taken and that's the current behavior then I
>> agree we have an inconsistency with respect to the sysfs control.
>>
>> Perhaps we should be guided by real world usage - AIUI there is a cloud that
>> disables THP at system level today (Google?).
> The use case I am aware of for disabling it for debugging purposes.
> Saved us quite some headake in the past at customer sites for
> troubleshooting + workarounds ...
> 
> 
> Let's take a look at the man page:
> 
> MADV_COLLAPSE is  independent  of  any  sysfs  (see  sysfs(5))  setting
> under  /sys/kernel/mm/transparent_hugepage, both in terms of determining
> THP eligibility, and allocation semantics.
> 
> I recall we discussed that it should ignore the max_ptes_none/swap/shared.
> 
> But "any" setting would include "enable" ...

It kind-of contradicts the linked 
Documentation/admin-guide/mm/transhuge.rst, where we have this 
*beautiful* comment

"Transparent Hugepage Support for anonymous memory can be entirely 
disable (mostly for debugging purposes".

I mean, "entirely" is also pretty clear to me.

I would assume that the man page of MADV_COLLAPSE should have talked 
about ignoring *khugepaged* toggles (max_ptes_none ...), at least that's 
what I recall from the discussions back then.

-- 
Cheers,

David / dhildenb