linux-kernel - Re: [PATCH v6 0/5] support batch checking of references and unmapping for large folios

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Message-Id: <20260209175316.2ef64ee244599765a74a6975@linux-foundation.org>
Date: Mon, 9 Feb 2026 17:53:16 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: david@...nel.org, catalin.marinas@....com, will@...nel.org,
 lorenzo.stoakes@...cle.com, ryan.roberts@....com, Liam.Howlett@...cle.com,
 vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
 riel@...riel.com, harry.yoo@...cle.com, jannh@...gle.com,
 willy@...radead.org, baohua@...nel.org, dev.jain@....com,
 linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/5] support batch checking of references and
 unmapping for large folios

On Mon,  9 Feb 2026 22:07:23 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:

> Currently, folio_referenced_one() always checks the young flag for each PTE
> sequentially, which is inefficient for large folios. This inefficiency is
> especially noticeable when reclaiming clean file-backed large folios, where
> folio_referenced() is observed as a significant performance hotspot.
> 
> Moreover, on Arm architecture, which supports contiguous PTEs, there is already
> an optimization to clear the young flags for PTEs within a contiguous range.
> However, this is not sufficient. We can extend this to perform batched operations
> for the entire large folio (which might exceed the contiguous range: CONT_PTE_SIZE).
> 
> Similar to folio_referenced_one(), we can also apply batched unmapping for large
> file folios to optimize the performance of file folio reclamation. By supporting
> batched checking of the young flags, flushing TLB entries, and unmapping, I can
> observed a significant performance improvements in my performance tests for file
> folios reclamation. Please check the performance data in the commit message of
> each patch.
> 

Thanks, I updated mm.git to this version.  Below is how v6 altered
mm.git.

I notice that this fix:

https://lore.kernel.org/all/de141225-a0c1-41fd-b3e1-bcab09827ddd@linux.alibaba.com/T/#u

was not carried forward.  Was this deliberate?

Also, regarding the 80-column tricks in folio_referenced_one(): we're
allowed to do this ;)


				unsigned long end_addr;
				unsigned int max_nr;

				end_addr = pmd_addr_end(address, vma->vm_end);
				max_nr = (end_addr - address) >> PAGE_SHIFT;




 arch/arm64/include/asm/pgtable.h |    2 +-
 include/linux/pgtable.h          |   16 ++++++++++------
 mm/rmap.c                        |    9 +++------
 3 files changed, 14 insertions(+), 13 deletions(-)

--- a/arch/arm64/include/asm/pgtable.h~b
+++ a/arch/arm64/include/asm/pgtable.h
@@ -1843,7 +1843,7 @@ static inline int clear_flush_young_ptes
 					 unsigned long addr, pte_t *ptep,
 					 unsigned int nr)
 {
-	if (likely(nr == 1 && !pte_valid_cont(__ptep_get(ptep))))
+	if (likely(nr == 1 && !pte_cont(__ptep_get(ptep))))
 		return __ptep_clear_flush_young(vma, addr, ptep);
 
 	return contpte_clear_flush_young_ptes(vma, addr, ptep, nr);
--- a/include/linux/pgtable.h~b
+++ a/include/linux/pgtable.h
@@ -1070,8 +1070,8 @@ static inline void wrprotect_ptes(struct
 
 #ifndef clear_flush_young_ptes
 /**
- * clear_flush_young_ptes - Clear the access bit and perform a TLB flush for PTEs
- *			    that map consecutive pages of the same folio.
+ * clear_flush_young_ptes - Mark PTEs that map consecutive pages of the same
+ *			    folio as old and flush the TLB.
  * @vma: The virtual memory area the pages are mapped into.
  * @addr: Address the first page is mapped at.
  * @ptep: Page table pointer for the first entry.
@@ -1087,13 +1087,17 @@ static inline void wrprotect_ptes(struct
  * pages that belong to the same folio.  The PTEs are all in the same PMD.
  */
 static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
-					 unsigned long addr, pte_t *ptep,
-					 unsigned int nr)
+		unsigned long addr, pte_t *ptep, unsigned int nr)
 {
-	int i, young = 0;
+	int young = 0;
 
-	for (i = 0; i < nr; ++i, ++ptep, addr += PAGE_SIZE)
+	for (;;) {
 		young |= ptep_clear_flush_young(vma, addr, ptep);
+		if (--nr == 0)
+			break;
+		ptep++;
+		addr += PAGE_SIZE;
+	}
 
 	return young;
 }
--- a/mm/rmap.c~b
+++ a/mm/rmap.c
@@ -963,10 +963,8 @@ static bool folio_referenced_one(struct
 				referenced++;
 		} else if (pvmw.pte) {
 			if (folio_test_large(folio)) {
-				unsigned long end_addr =
-					pmd_addr_end(address, vma->vm_end);
-				unsigned int max_nr =
-					(end_addr - address) >> PAGE_SHIFT;
+				unsigned long end_addr = pmd_addr_end(address, vma->vm_end);
+				unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
 				pte_t pteval = ptep_get(pvmw.pte);
 
 				nr = folio_pte_batch(folio, pvmw.pte,
@@ -974,8 +972,7 @@ static bool folio_referenced_one(struct
 			}
 
 			ptes += nr;
-			if (clear_flush_young_ptes_notify(vma, address,
-						pvmw.pte, nr))
+			if (clear_flush_young_ptes_notify(vma, address, pvmw.pte, nr))
 				referenced++;
 			/* Skip the batched PTEs */
 			pvmw.pte += nr - 1;
_