Re: [PATCH POC] prctl: extend PR_SET_THP_DISABLE to optionally exclude VM_HUGEPAGE

[David Hildenbrand <david@redhat.com>]

On 25.07.25 00:27, Usama Arif wrote:
> 
>> Hi!
>>
>>>
>>> Over here, with MMF_DISABLE_THP_EXCEPT_ADVISED, MADV_HUGEPAGE will succeed as vm_flags has
>>> VM_HUGEPAGE set, but MADV_COLLAPSE will fail to give a hugepage (as VM_HUGEPAGE is not set
>>> and MMF_DISABLE_THP_EXCEPT_ADVISED is set) which I feel might not be the right behaviour
>>> as MADV_COLLAPSE is "advise" and the prctl flag is PR_THP_DISABLE_EXCEPT_ADVISED?
>>
>> THPs are disabled for these regions, so it's at least consistent with the "disable all", but ...
>>
>>>
>>> This will be checked in multiple places in madvise_collapse: thp_vma_allowable_order,
>>> hugepage_vma_revalidate which calls thp_vma_allowable_order and hpage_collapse_scan_pmd
>>> which also ends up calling hugepage_vma_revalidate.
>>>> A hacky way would be to save and overwrite vma->vm_flags with VM_HUGEPAGE at the start of madvise_collapse
>>> if VM_NOHUGEPAGE is not set, and reset vma->vm_flags to its original value at the end of madvise_collapse
>>> (Not something I am recommending, just throwing it out there).
>>
>> Gah.
>>
>>>
>>> Another possibility is to pass the fact that you are in madvise_collapse to these functions
>>> as an argument, this might look ugly, although maybe not as ugly as hugepage_vma_revalidate
>>> already has collapse control arg, so just need to take care of thp_vma_allowable_orders.
>>
>> Likely this.
>>
>>>
>>> Any preference or better suggestions?
>>
>> What you are asking for is not MMF_DISABLE_THP_EXCEPT_ADVISED as I planned it, but MMF_DISABLE_THP_EXCEPT_ADVISED_OR_MADV_COLLAPSE.
>>
>> Now, one could consider MADV_COLLAPSE an "advise". (I am not opposed to that change)
>>
> 
> lol yeah I always think of MADV_COLLAPSE as an extreme version of MADV_HUGE (more of a demand
> than an advice :)), eventhough its not persistant.
> Which is why I think might be unexpected if MADV_HUGE gives hugepages but MADV_COLLAPSE doesn't
> (But could just be my opinion).
> 
>> Indeed, the right way might be telling vma_thp_disabled() whether we are in collapse.
>>
>> Can you try implementing that on top of my patch to see how it looks?
>>
> 
> My reasoning is that a process that is running with system policy always but with
> PR_THP_DISABLE_EXCEPT_ADVISED gets THPs in exactly the same behaviour as a process that is running
> with system policy madvise. This will help us achieve (3) that you mentioned in the
> commit message:
> (3) Switch from THP=madvise to THP=always, but keep the old behavior
>       (THP only when advised) for selected workloads.
> 
> 
> I have written quite a few selftests now for prctl SET_THP_DISABLE, both with and without
> PR_THP_DISABLE_EXCEPT_ADVISED set incorporating your feedback on it. I have all of them passing
> with the below diff. The diff is slightly ugly, but very simple and hopefully acceptable. If it
> looks good, I can send a series with everything. Probably make the below diff as a separate patch
> on top of this patch as its mostly adding an extra arg to functions and would keep the review easier?

Yes, we should do it as a separate patch, makes our life easier, because 
that requires more work.

We require a cleanup first, the boolean parameter for 
__thp_vma_allowable_orders() is no good.

I just pushed something untested to my branch (slightly adjusted patch#1 
+ 2 more patches), can you have a look at that? (untested ... :) )

-- 
Cheers,

David / dhildenb