Re: [PATCH v4 3/5] arm64: mm: support batch clearing of the young flag for large folios
From: Baolin Wang
Date: Sat Jan 03 2026 - 20:55:39 EST
On 1/2/26 8:12 PM, Ryan Roberts wrote:
On 25/12/2025 02:48, Baolin Wang wrote:
On 2025/12/24 22:07, Ryan Roberts wrote:
On 23/12/2025 05:48, Baolin Wang wrote:
Currently, contpte_ptep_test_and_clear_young() and
contpte_ptep_clear_flush_young()
only clear the young flag and flush TLBs for PTEs within the contiguous range.
To support batch PTE operations for other sized large folios in the following
patches, adding a new parameter to specify the number of PTEs that map
consecutive
pages of the same large folio in a single VMA and a single page table.
While we are at it, rename the functions to maintain consistency with other
contpte_*() functions.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
arch/arm64/include/asm/pgtable.h | 12 ++++++------
arch/arm64/mm/contpte.c | 33 ++++++++++++++++++--------------
2 files changed, 25 insertions(+), 20 deletions(-)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 445e18e92221..d5fbe72e820a 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1648,10 +1648,10 @@ extern void contpte_clear_full_ptes(struct mm_struct
*mm, unsigned long addr,
extern pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
unsigned long addr, pte_t *ptep,
unsigned int nr, int full);
-extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep);
-extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep);
+int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep, unsigned int nr);
+int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep, unsigned int nr);
extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, unsigned int nr);
extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
@@ -1823,7 +1823,7 @@ static inline int ptep_test_and_clear_young(struct
vm_area_struct *vma,
if (likely(!pte_valid_cont(orig_pte)))
return __ptep_test_and_clear_young(vma, addr, ptep);
- return contpte_ptep_test_and_clear_young(vma, addr, ptep);
+ return contpte_test_and_clear_young_ptes(vma, addr, ptep, CONT_PTES);
As per your fixup patch, I agree that nr should be 1 here, not CONT_PTES.
Yes.
}
#define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
@@ -1835,7 +1835,7 @@ static inline int ptep_clear_flush_young(struct
vm_area_struct *vma,
if (likely(!pte_valid_cont(orig_pte)))
return __ptep_clear_flush_young(vma, addr, ptep);
- return contpte_ptep_clear_flush_young(vma, addr, ptep);
+ return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
And same here.
}
#define wrprotect_ptes wrprotect_ptes
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index e4ddeb46f25d..b929a455103f 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -508,8 +508,9 @@ pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
}
EXPORT_SYMBOL_GPL(contpte_get_and_clear_full_ptes);
-int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep)
+int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep,
+ unsigned int nr)
{
/*
* ptep_clear_flush_young() technically requires us to clear the access
@@ -518,41 +519,45 @@ int contpte_ptep_test_and_clear_young(struct
vm_area_struct *vma,
* contig range when the range is covered by a single folio, we can get
* away with clearing young for the whole contig range here, so we avoid
* having to unfold.
+ *
+ * The 'nr' means consecutive (present) PTEs that map consecutive pages
+ * of the same large folio in a single VMA and a single page table.
*/
+ unsigned long end = addr + nr * PAGE_SIZE;
int young = 0;
- int i;
- ptep = contpte_align_down(ptep);
- addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
-
- for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
+ ptep = contpte_align_addr_ptep(&addr, &end, ptep, nr);
+ for (; addr != end; ptep++, addr += PAGE_SIZE)
young |= __ptep_test_and_clear_young(vma, addr, ptep);
return young;
}
-EXPORT_SYMBOL_GPL(contpte_ptep_test_and_clear_young);
+EXPORT_SYMBOL_GPL(contpte_test_and_clear_young_ptes);
-int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep)
+int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep,
+ unsigned int nr)
{
int young;
- young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
+ young = contpte_test_and_clear_young_ptes(vma, addr, ptep, nr);
if (young) {
+ unsigned long end = addr + nr * PAGE_SIZE;
+
+ contpte_align_addr_ptep(&addr, &end, ptep, nr);
/*
* See comment in __ptep_clear_flush_young(); same rationale for
* eliding the trailing DSB applies here.
*/
- addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
- __flush_tlb_range_nosync(vma->vm_mm, addr, addr + CONT_PTE_SIZE,
+ __flush_tlb_range_nosync(vma->vm_mm, addr, end,
PAGE_SIZE, true, 3);
Hmm... The requirement is that we must flush the _page_ if clearing access for a
pte that does not have the contiguous bit set, or we must flush the _contpte
block_ if clearing access for a pte that does have the contiguous bit set.
With your changes, you may call for a large range that covers multiple contpte
blocks but only has a single pte in a single contpte block for which the access
bit was previously set. But that will cause flushing the TLB for the full range.
Could this cause a performance issue? Yes, no, maybe... I think it's unlikely
but I wouldn't rule it out in some edge case.
I wonder if it's better to track the sub-ranges where access was cleared and
only issue tlbi for those sub-ranges? Probably just keep it simple (the way you
have done it) until/unless we see an actual problem?
Good question. Indeed, as you said, we flush the TLB per folio now, which might
increase the flush range. However, I think this approach is relatively
reasonable for now.
First, the mm-core also tracks the access status per folio, and it's really
unnecessary to add excessive complexity to track the access status of sub-pages
(or sub-ranges). I can already imagine that tracking the access status for each
cont-block range as well as for non-cont pages across the entire large folio
range, which can be too complicated.
Second, __flush_tlb_range_nosync() is a lightweight flush. I quickly ran a
measurement on my machine and found that the overhead of
__flush_tlb_range_nosync() barely changes between nr=16 and nr=256 (both are
around 40 ns).
I'm not concerned about the direct cost of the flush; I agree it should be
lightweight given we elide the trailing DSB. (although there is a possible case
on older HW that doesn't support TLBI-by-range where this will be converted to
multiple TLBI-by-page instructions and that can cause stalls if there are too
many of them).
My concern was the opportunity cost of evicting the entries for all the
non-accessed parts of the folio from the TLB. But of course, I'm talking
nonsense because the architecture does not allow caching non-accessed entries in
the TLB.
Ah, now I understand your concern:). Yes, agree.
So doesn't sound like a problem; I think we can ignore this. Sorry for the noise.
OK. No worries.
Therefore, I would still prefer to keep the logic here simple.
Agreed.
Thanks for your reviewing and valuable input.