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

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Message-ID: <20250626092905.31305-1-ioworker0@gmail.com>
Date: Thu, 26 Jun 2025 17:29:05 +0800
From: Lance Yang <ioworker0@...il.com>
To: david@...hat.com
Cc: 21cnbao@...il.com,
	akpm@...ux-foundation.org,
	baolin.wang@...ux.alibaba.com,
	chrisl@...nel.org,
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	lance.yang@...ux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-riscv@...ts.infradead.org,
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	v-songbaohua@...o.com,
	x86@...nel.org,
	ying.huang@...el.com,
	zhengtangquan@...o.com
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

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:
--

Thanks,
Lance