Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[David Hildenbrand <david@redhat.com>]

On 26.06.25 14:44, Lance Yang wrote:
> 
> On 2025/6/26 17:29, Lance Yang wrote:
>> Before I send out the real patch, I'd like to get some quick feedback to
>> ensure I've understood the discussion correctly ;)
>>
>> Does this look like the right direction?
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index fb63d9256f09..5ebffe2137e4 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1845,23 +1845,37 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
>>    #endif
>>    }
>>    
>> -/* We support batch unmapping of PTEs for lazyfree large folios */
>> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>> -			struct folio *folio, pte_t *ptep)
>> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>> +			struct page_vma_mapped_walk *pvmw,
>> +			enum ttu_flags flags, pte_t pte)
>>    {
>>    	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> -	int max_nr = folio_nr_pages(folio);
>> -	pte_t pte = ptep_get(ptep);
>> +	unsigned long end_addr, addr = pvmw->address;
>> +	struct vm_area_struct *vma = pvmw->vma;
>> +	unsigned int max_nr;
>> +
>> +	if (flags & TTU_HWPOISON)
>> +		return 1;
>> +	if (!folio_test_large(folio))
>> +		return 1;
>>    
>> +	/* We may only batch within a single VMA and a single page table. */
>> +	end_addr = pmd_addr_end(addr, vma->vm_end);
>> +	max_nr = (end_addr - addr) >> PAGE_SHIFT;
>> +
>> +	/* We only support lazyfree batching for now ... */
>>    	if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
>> -		return false;
>> +		return 1;
>>    	if (pte_unused(pte))
>> -		return false;
>> -	if (pte_pfn(pte) != folio_pfn(folio))
>> -		return false;
>> +		return 1;
>> +
>> +	/* ... where we must be able to batch the whole folio. */
>> +	if (pte_pfn(pte) != folio_pfn(folio) || max_nr != folio_nr_pages(folio))
>> +		return 1;
>> +	max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
>> +				 NULL, NULL, NULL);
>>    
>> -	return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
>> -			       NULL, NULL) == max_nr;
>> +	return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
>>    }
>>    
>>    /*
>> @@ -2024,9 +2038,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>    			if (pte_dirty(pteval))
>>    				folio_mark_dirty(folio);
>>    		} else if (likely(pte_present(pteval))) {
>> -			if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
>> -			    can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
>> -				nr_pages = folio_nr_pages(folio);
>> +			nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
>>    			end_addr = address + nr_pages * PAGE_SIZE;
>>    			flush_cache_range(vma, address, end_addr);
>>    
>> @@ -2206,13 +2218,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>    			hugetlb_remove_rmap(folio);
>>    		} else {
>>    			folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
>> -			folio_ref_sub(folio, nr_pages - 1);
>>    		}
>>    		if (vma->vm_flags & VM_LOCKED)
>>    			mlock_drain_local();
>> -		folio_put(folio);
>> -		/* We have already batched the entire folio */
>> -		if (nr_pages > 1)
>> +		folio_put_refs(folio, nr_pages);
>> +
>> +		/*
>> +		 * If we are sure that we batched the entire folio and cleared
>> +		 * all PTEs, we can just optimize and stop right here.
>> +		 */
>> +		if (nr_pages == folio_nr_pages(folio))
>>    			goto walk_done;
>>    		continue;
>>    walk_abort:
>> --
> 
> Oops ... Through testing on my machine, I found that the logic doesn't
> behave as expected because I messed up the meaning of max_nr (the available
> scan room in the page table) with folio_nr_pages(folio) :(
> 
> With the following change:
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5ebffe2137e4..b1407348e14e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1850,9 +1850,9 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>   			enum ttu_flags flags, pte_t pte)
>   {
>   	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> +	unsigned int max_nr, nr_pages = folio_nr_pages(folio);
>   	unsigned long end_addr, addr = pvmw->address;
>   	struct vm_area_struct *vma = pvmw->vma;
> -	unsigned int max_nr;
>   
>   	if (flags & TTU_HWPOISON)
>   		return 1;
> @@ -1870,12 +1870,13 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>   		return 1;
>   
>   	/* ... where we must be able to batch the whole folio. */

Why is that still required? :)

> -	if (pte_pfn(pte) != folio_pfn(folio) || max_nr != folio_nr_pages(folio))
> +	if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages)
>   		return 1;
> -	max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
> -				 NULL, NULL, NULL);
>   
> -	return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
> +	max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages,
> +				 fpb_flags, NULL, NULL, NULL);
> +
> +	return (max_nr != nr_pages) ? 1 : max_nr;

Why is that still required? :)

-- 
Cheers,

David / dhildenb