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

[Usama Arif <usamaarif642@gmail.com>]

On 03/10/2025 15:08, Zi Yan wrote:
> On 3 Oct 2025, at 9:49, Lance Yang wrote:
> 
>> 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")
> 
> Yes, I agree. In this case, this patch looks more like an optimization
> for split underused THPs.
> 
> One observation on this change is that right after zero pmd wp, the
> deferred split queue could be scanned, the newly added pmd folio will
> split since it is all zero except one subpage. This means we probably
> should allocate a base folio for zero pmd wp and map the rest to zero
> page at the beginning if split underused THP is enabled to avoid
> this long trip. The downside is that user app cannot get a pmd folio
> if it is intended to write data into the entire folio.
> 
> Usama might be able to give some insight here.
> 

Thanks for CCing me Zi!

hmm I think the downside of not having PMD folio probably outweights the cost of splitting
a zer-filled page?
ofcourse I dont have any numbers to back that up, but that would be my initial guess.

Also:

Acked-by: Usama Arif <usamaarif642@gmail.com>


> 
>>
>> 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);
>>>   	}
>>>
> 
> 
> Best Regards,
> Yan, Zi