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

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

Hey Wei,

On 2025/10/2 09:38, 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

IIRC, it was commit dafff3f4c850 ("mm: split underused THPs") that
started unconditionally adding all new anon THPs to _deferred_list :)

> 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.
> 
> Fixes: 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault")

Shouldn't this rather be the following?

Fixes: dafff3f4c850 ("mm: split underused THPs")

Thanks,
Lance

> 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>
> 
> ---
> v2:
>    * add fix, cc stable and put description about the flow of current
>      code
>    * move deferred_split_folio() into map_anon_folio_pmd()
> ---
>   mm/huge_memory.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1b81680b4225..f13de93637bf 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1232,6 +1232,7 @@ static void map_anon_folio_pmd(struct folio *folio, pmd_t *pmd,
>   	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)
> @@ -1272,7 +1273,6 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>   		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>   		map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
>   		mm_inc_nr_ptes(vma->vm_mm);
> -		deferred_split_folio(folio, false);
>   		spin_unlock(vmf->ptl);
>   	}
>