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

Andrew Morton
2026-02-10T01:53:21Z

On Mon, 9 Feb 2026 22:07:23 +0800 Baolin Wang <baolin.wang@linux.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;
_