linux-kernel - Re: [PATCH v4 2/3] khugepaged: Optimize __collapse_huge_page_copy

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Message-ID: <0a70058f-e2fa-4f83-ace5-f58df589ac46@redhat.com>
Date: Thu, 24 Jul 2025 19:57:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 baohua@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] khugepaged: Optimize
 __collapse_huge_page_copy_succeeded() by PTE batching


>> +			if (folio_test_large(src)) {
>> +				unsigned int max_nr_ptes = (end - address) >> PAGE_SHIFT;
>> +
>> +				nr_ptes = folio_pte_batch(src, _pte, pteval, max_nr_ptes);
>> +			} else {
>>   				release_pte_folio(src);
>> +			}
>> +
>>   			/*
>>   			 * ptl mostly unnecessary, but preempt has to
>>   			 * be disabled to update the per-cpu stats
>>   			 * inside folio_remove_rmap_pte().
>>   			 */
>>   			spin_lock(ptl);
>> -			ptep_clear(vma->vm_mm, address, _pte);
>> -			folio_remove_rmap_pte(src, src_page, vma);
>> +			clear_ptes(vma->vm_mm, address, _pte, nr_ptes);
>> +			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
>>   			spin_unlock(ptl);
>> -			free_folio_and_swap_cache(src);
>> +			free_swap_cache(src);
>> +			folio_put_refs(src, nr_ptes);
> 
> Hm one thing here though is the free_folio_and_swap_cache() does:
> 
>          free_swap_cache(folio);
>          if (!is_huge_zero_folio(folio))
>                  folio_put(folio);
> 
> Whereas here you unconditionally reduce the reference count. Might this
> cause issues with the shrinker version of the huge zero folio?
> 
> Should this be:
> 
>                          if (!is_huge_zero_folio(src))
>                                  folio_put_refs(src, nr_ptes);
> 
> Or do we otherwise avoid issues with this?

(resending my reply)

The huge zero folio is never PTE-mapped.

-- 
Cheers,

David / dhildenb