Re: [Patch v2] mm/huge_memory: add pmd folio to ds_queue in do_huge_zero_wp_pmd()

[David Hildenbrand <david@redhat.com>]

On 02.10.25 05:17, Wei Yang wrote:
> On Thu, Oct 02, 2025 at 10:31:53AM +0800, Lance Yang wrote:
>>
>>
>> On 2025/10/2 09:46, Wei Yang wrote:
>>> On Thu, Oct 02, 2025 at 01:38:25AM +0000, Wei Yang wrote:
>>>> We add pmd folio into ds_queue on the first page fault in
>>>> __do_huge_pmd_anonymous_page(), so that we can split it in case of
>>>> memory pressure. This should be the same for a pmd folio during wp
>>>> page fault.
>>>>
>>>> Commit 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault") miss
>>>> to add it to ds_queue, which means system may not reclaim enough memory
>>>> in case of memory pressure even the pmd folio is under used.
>>>>
>>>> Move deferred_split_folio() into map_anon_folio_pmd() to make the pmd
>>>> folio installation consistent.
>>>>
>>>
>>> Since we move deferred_split_folio() into map_anon_folio_pmd(), I am thinking
>>> about whether we can consolidate the process in collapse_huge_page().
>>>
>>> Use map_anon_folio_pmd() in collapse_huge_page(), but skip those statistic
>>> adjustment.
>>
>> Yeah, that's a good idea :)
>>
>> We could add a simple bool is_fault parameter to map_anon_folio_pmd()
>> to control the statistics.
>>
>> The fault paths would call it with true, and the collapse paths could
>> then call it with false.
>>
>> Something like this:
>>
>> ```
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 1b81680b4225..9924180a4a56 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1218,7 +1218,7 @@ static struct folio *vma_alloc_anon_folio_pmd(struct
>> vm_area_struct *vma,
>> }
>>
>> static void map_anon_folio_pmd(struct folio *folio, pmd_t *pmd,
>> -		struct vm_area_struct *vma, unsigned long haddr)
>> +		struct vm_area_struct *vma, unsigned long haddr, bool is_fault)
>> {
>> 	pmd_t entry;
>>
>> @@ -1228,10 +1228,15 @@ static void map_anon_folio_pmd(struct folio *folio,
>> pmd_t *pmd,
>> 	folio_add_lru_vma(folio, vma);
>> 	set_pmd_at(vma->vm_mm, haddr, pmd, entry);
>> 	update_mmu_cache_pmd(vma, haddr, pmd);
>> -	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>> -	count_vm_event(THP_FAULT_ALLOC);
>> -	count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
>> -	count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>> +
>> +	if (is_fault) {
>> +		add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>> +		count_vm_event(THP_FAULT_ALLOC);
>> +		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
>> +		count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>> +	}
>> +
>> +	deferred_split_folio(folio, false);
>> }
>>
>> static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d0957648db19..2eddd5a60e48 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1227,17 +1227,10 @@ static int collapse_huge_page(struct mm_struct *mm,
>> unsigned long address,
>> 	__folio_mark_uptodate(folio);
>> 	pgtable = pmd_pgtable(_pmd);
>>
>> -	_pmd = folio_mk_pmd(folio, vma->vm_page_prot);
>> -	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
>> -
>> 	spin_lock(pmd_ptl);
>> 	BUG_ON(!pmd_none(*pmd));
>> -	folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
>> -	folio_add_lru_vma(folio, vma);
>> 	pgtable_trans_huge_deposit(mm, pmd, pgtable);
>> -	set_pmd_at(mm, address, pmd, _pmd);
>> -	update_mmu_cache_pmd(vma, address, pmd);
>> -	deferred_split_folio(folio, false);
>> +	map_anon_folio_pmd(folio, pmd, vma, address, false);
>> 	spin_unlock(pmd_ptl);
>>
>> 	folio = NULL;
>> ```
>>
>> Untested, though.
>>
> 
> This is the same as I thought.
> 
> Will prepare a patch for it.

Let's do that as an add-on patch, though.

-- 
Cheers

David / dhildenb