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

[Lance Yang <ioworker0@gmail.com>]

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. */
-	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;
 }
 
 /*
--

... then things work as expected for the lazyfree case, without any
splitting.