Re: [PATCH v2 3/5] mm/huge_memory: treat MADV_COLLAPSE as an advise with PR_THP_DISABLE_EXCEPT_ADVISED

[David Hildenbrand <david@redhat.com>]

On 31.07.25 16:38, Lorenzo Stoakes wrote:
> Nits on subject:
> 
> - It's >75 chars

No big deal. If we cna come up with something shorter, good.

> - advise is the verb, advice is the noun.

Yeah.

> 
> On Thu, Jul 31, 2025 at 01:27:20PM +0100, Usama Arif wrote:
>> From: David Hildenbrand <david@redhat.com>
>>
>> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
>> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
>> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
> 
> Hmm, I'm not sure about this.
> 
> So far this prctl() has been the only way to override MADV_COLLAPSE
> behaviour, but now we're allowing for this one case to not.

This is not an override really. prctl() disallowed MADV_COLLAPSE, but in 
the new mode we don't want that anymore.

 > > I suppose the precedent is that MADV_COLLAPSE overrides 'madvise' sysfs
> behaviour.
 > > I suppose what saves us here is 'advised' can be read to mean either
> MADV_HUGEPAGE or MADV_COLLAPSE.
 > > And yes, MADV_COLLAPSE is clearly the user requesting this behaviour.

Exactly.

> 
> I think the vagueness here is one that already existed, because one could
> perfectly one have expected MADV_COLLAPSE to obey sysfs and require
> MADV_HUGEPAGE to have been applied, but of course this is not the case.

Yes.

> 
> OK so fine.
> 
> BUT.
> 
> I think the MADV_COLLAPSE man page will need to be updated to mention this.
> 

Yes.

> And I REALLY think we should update the THP doc too to mention all these
> prctl() modes.
> 
> I'm not sure we cover that right now _at all_ and obviously we should
> describe the new flags.
> 
> Usama - can you add a patch to this series to do that?

Good point, let's document the interaction with prctl().

> 
>>
>> MADV_COLLAPSE is a clear advise that we want to collapse.
> 
> advise -> advice.
> 
>>
>> Note that we still respect the VM_NOHUGEPAGE flag, just like
>> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
>> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED.
> 
> You also need to mention the shmem change you've made I think.

Yes.

 > >>
>> Co-developed-by: Usama Arif <usamaarif642@gmail.com>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   include/linux/huge_mm.h    | 8 +++++++-
>>   include/uapi/linux/prctl.h | 2 +-
>>   mm/huge_memory.c           | 5 +++--
>>   mm/memory.c                | 6 ++++--
>>   mm/shmem.c                 | 2 +-
>>   5 files changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index b0ff54eee81c..aeaf93f8ac2e 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -329,7 +329,7 @@ struct thpsize {
>>    * through madvise or prctl.
>>    */
>>   static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>> -		vm_flags_t vm_flags)
>> +		vm_flags_t vm_flags, bool forced_collapse)
>>   {
>>   	/* Are THPs disabled for this VMA? */
>>   	if (vm_flags & VM_NOHUGEPAGE)
>> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>>   	 */
>>   	if (vm_flags & VM_HUGEPAGE)
>>   		return false;
>> +	/*
>> +	 * Forcing a collapse (e.g., madv_collapse), is a clear advise to
> 
> advise -> advice.
> 
>> +	 * use THPs.
>> +	 */
>> +	if (forced_collapse)
>> +		return false;
>>   	return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
>>   }
>>
>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>> index 9c1d6e49b8a9..ee4165738779 100644
>> --- a/include/uapi/linux/prctl.h
>> +++ b/include/uapi/linux/prctl.h
>> @@ -185,7 +185,7 @@ struct prctl_mm_map {
>>   #define PR_SET_THP_DISABLE	41
>>   /*
>>    * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
>> - * VM_HUGEPAGE).
>> + * VM_HUGEPAGE / MADV_COLLAPSE).
> 
> This is confusing you're mixing VMA flags with MADV ones... maybe just
> stick to madvise ones, or add extra context around VM_HUGEPAGE bit?

I don't see anything confusing here, really.

But if it helps you, we can do
	(e.g., MADV_HUGEPAGE / VM_HUGEPAGE, MADV_COLLAPSE).

(reason VM_HUGEPAGE is spelled out is that there might be code where we 
set VM_HUGEPAGE implicitly in the kernel)

> 
> Would need to be fixed up in a prior commit obviously.
> 
>>    */
>>   # define PR_THP_DISABLE_EXCEPT_ADVISED	(1 << 1)
>>   #define PR_GET_THP_DISABLE	42
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 85252b468f80..ef5ccb0ec5d5 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>   {
>>   	const bool smaps = type == TVA_SMAPS;
>>   	const bool in_pf = type == TVA_PAGEFAULT;
>> -	const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
>> +	const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
>> +	const bool enforce_sysfs = !forced_collapse;
> 
> Can we just get rid of this enforce_sysfs altogether in patch 2/5 and use
> forced_collapse?

Let's do that as a separate cleanup on top. I want least churn in that 
patch.

(had the same idea while writing that patch, but I have other things to 
focus on than cleaning up all this mess)

-- 
Cheers,

David / dhildenb