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

[Lance Yang <lance.yang@linux.dev>]

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.

> 
>> Fixes: 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault")
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Lance Yang <lance.yang@linux.dev>
>> Cc: Dev Jain <dev.jain@arm.com>
>> Cc: <stable@vger.kernel.org>
>>
>