Re: [PATCH v2 4/6] mm: add a batched helper to clear the young flag for large folios

[Baolin Wang]

On 3/2/26 5:07 PM, David Hildenbrand (Arm) wrote:
> On 2/27/26 10:44, Baolin Wang wrote:
> > Currently, MGLRU will call ptep_test_and_clear_young_notify() to check and
> > clear the young flag for each PTE sequentially, which is inefficient for
> > large folios reclamation.
> >
> > Moreover, on Arm64 architecture, which supports contiguous PTEs, the Arm64-
> > specific ptep_test_and_clear_young() already implements an optimization to
> > clear the young flags for PTEs within a contiguous range. However, this is not
> > sufficient. Similar to the Arm64 specific clear_flush_young_ptes(), we can
> > extend this to perform batched operations for the entire large folio (which
> > might exceed the contiguous range: CONT_PTE_SIZE).
> >
> > Thus, we can introduce a new batched helper: test_and_clear_young_ptes() and
> > its wrapper test_and_clear_young_ptes_notify() which are consistent with the
> > existing functions, to perform batched checking of the young flags for large
> > folios, which can help improve performance during large folio reclamation when
> > MGLRU is enabled. And it will be overridden by the architecture that implements
> > a more efficient batch operation in the following patches.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   include/linux/pgtable.h | 38 ++++++++++++++++++++++++++++++++++++++
> >   mm/internal.h           | 16 +++++++++++-----
> >   2 files changed, 49 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 776993d4567b..29bd9fd04e1e 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1103,6 +1103,44 @@ static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
> >   }
> >   #endif
> >   
> > +#ifndef test_and_clear_young_ptes
> > +/**
> > + * test_and_clear_young_ptes - Mark PTEs that map consecutive pages of the same
> > + *			       folio as old
> > + * @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.
> > + * @nr: Number of entries to clear access bit.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented as a simple
> > + * loop over ptep_test_and_clear_young().
> > + *
> > + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> > + * some PTEs might be write-protected.
> > + *
> > + * Context: The caller holds the page table lock.  The PTEs map consecutive
> > + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> > + *
> > + * Returns: whether any PTE was young.
> > + */
> > +static inline int test_and_clear_young_ptes(struct vm_area_struct *vma,
> > +					    unsigned long addr, pte_t *ptep,
> > +					    unsigned int nr)
>
> Two tabs ...

Ah, yes, not sure why I missed this one :(

> What happened to using a boolen as return type and for "int young"?
As I replied to you previously [1], I’d like to do this in a follow-up 
patchset that converts all functions that check the young flag. Does 
that sound OK to you?

[1] 
https://lore.kernel.org/all/32c538ce-6af8-48a8-86fc-d26ee253af54@xxxxxxxxxxxxxxxxx/