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

[Wei Yang <richard.weiyang@gmail.com>]

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.


-- 
Wei Yang
Help you, Help me