[PATCH v3 0/3] Optimizations for khugepaged

[PATCH v3 0/3] Optimizations for khugepaged

[Dev Jain]

If the underlying folio mapped by the ptes is large, we can process those
ptes in a batch using folio_pte_batch().

For arm64 specifically, this results in a 16x reduction in the number of
ptep_get() calls, since on a contig block, ptep_get() on arm64 will iterate
through all 16 entries to collect a/d bits. Next, ptep_clear() will cause
a TLBI for every contig block in the range via contpte_try_unfold().
Instead, use clear_ptes() to only do the TLBI at the first and last
contig block of the range.

For split folios, there will be no pte batching; the batch size returned
by folio_pte_batch() will be 1. For pagetable split folios, the ptes will
still point to the same large folio; for arm64, this results in the
optimization described above, and for other arches, a minor improvement
is expected due to a reduction in the number of function calls and
batching atomic operations.

---
Rebased on today's mm-new.

v2->v3:
 - Drop patch 3 (was merged separately)
 - Add patch 1 (David)
 - Coding style change, drop mapped_folio (Lorenzo)

v1->v2:
 - Use for loop instead of do-while loop (Lorenzo)
 - Remove folio_test_large check since the subpage-check condition
   will imply that (Baolin)
 - Combine patch 1 and 2 into this series, add new patch 3

David Hildenbrand (1):
  mm: add get_and_clear_ptes() and clear_ptes()

Dev Jain (2):
  khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE
    batching
  khugepaged: Optimize collapse_pte_mapped_thp() by PTE batching

 arch/arm64/mm/mmu.c     |  2 +-
 include/linux/pgtable.h |  6 +++++
 mm/khugepaged.c         | 57 +++++++++++++++++++++++++++--------------
 mm/mremap.c             |  2 +-
 mm/rmap.c               |  2 +-
 5 files changed, 47 insertions(+), 22 deletions(-)

-- 
2.30.2

[PATCH v3 1/3] mm: add get_and_clear_ptes() and clear_ptes()

[Dev Jain]

From: David Hildenbrand <david@redhat.com>

From: David Hildenbrand <david@redhat.com>

Let's add variants to be used where "full" does not apply -- which will
be the majority of cases in the future. "full" really only applies if
we are about to tear down a full MM.

Use get_and_clear_ptes() in existing code, clear_ptes() users will
be added next.

Should we make these inline functions instead and add separate docs?
Probably not worth it for now.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 arch/arm64/mm/mmu.c     | 2 +-
 include/linux/pgtable.h | 6 ++++++
 mm/mremap.c             | 2 +-
 mm/rmap.c               | 2 +-
 4 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index abd9725796e9..20a89ab97dc5 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1528,7 +1528,7 @@ early_initcall(prevent_bootmem_remove_init);
 pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t *ptep, unsigned int nr)
 {
-	pte_t pte = get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, /* full = */ 0);
+	pte_t pte = get_and_clear_ptes(vma->vm_mm, addr, ptep, nr);
 
 	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
 		/*
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index e3b99920be05..e45986b54277 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -736,6 +736,9 @@ static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
 }
 #endif
 
+#define get_and_clear_ptes(_mm, _addr, _ptep, _nr) \
+	get_and_clear_full_ptes(_mm, _addr, _ptep, _nr, 0)
+
 #ifndef clear_full_ptes
 /**
  * clear_full_ptes - Clear present PTEs that map consecutive pages of the same
@@ -768,6 +771,9 @@ static inline void clear_full_ptes(struct mm_struct *mm, unsigned long addr,
 }
 #endif
 
+#define clear_ptes(_mm, _addr, _ptep, _nr) \
+	clear_full_ptes(_mm, _addr, _ptep, _nr, 0)
+
 /*
  * If two threads concurrently fault at the same page, the thread that
  * won the race updates the PTE and its local TLB/Cache. The other thread
diff --git a/mm/mremap.c b/mm/mremap.c
index ac39845e9718..677a4d744df9 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -280,7 +280,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
 							 old_pte, max_nr_ptes);
 			force_flush = true;
 		}
-		pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
+		pte = get_and_clear_ptes(mm, old_addr, old_ptep, nr_ptes);
 		pte = move_pte(pte, old_addr, new_addr);
 		pte = move_soft_dirty_pte(pte);
 
diff --git a/mm/rmap.c b/mm/rmap.c
index f93ce27132ab..568198e9efc2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2036,7 +2036,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			flush_cache_range(vma, address, end_addr);
 
 			/* Nuke the page table entry. */
-			pteval = get_and_clear_full_ptes(mm, address, pvmw.pte, nr_pages, 0);
+			pteval = get_and_clear_ptes(mm, address, pvmw.pte, nr_pages);
 			/*
 			 * We clear the PTE but do not flush so potentially
 			 * a remote CPU could still be writing to the folio.
-- 
2.30.2

[PATCH v3 2/3] khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE batching

[Dev Jain]

Use PTE batching to optimize __collapse_huge_page_copy_succeeded().

On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
Then, calling ptep_clear() for every pte will cause a TLB flush for every
contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
which will flush the TLB only for the (if any) starting and ending contpte
block, if they partially overlap with the range khugepaged is looking at.

For all arches, there should be a benefit due to batching atomic operations
on mapcounts due to folio_remove_rmap_ptes() and saving some calls.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/khugepaged.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a55fb1dcd224..63517ef7eafb 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -700,12 +700,15 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 						spinlock_t *ptl,
 						struct list_head *compound_pagelist)
 {
+	unsigned long end = address + HPAGE_PMD_SIZE;
 	struct folio *src, *tmp;
-	pte_t *_pte;
 	pte_t pteval;
+	pte_t *_pte;
+	int nr_ptes;
 
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
-	     _pte++, address += PAGE_SIZE) {
+	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
+	     address += nr_ptes * PAGE_SIZE) {
+		nr_ptes = 1;
 		pteval = ptep_get(_pte);
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
 			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
@@ -722,18 +725,26 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 			struct page *src_page = pte_page(pteval);
 
 			src = page_folio(src_page);
-			if (!folio_test_large(src))
+
+			if (folio_test_large(src)) {
+				int max_nr_ptes = (end - address) >> PAGE_SHIFT;
+
+				nr_ptes = folio_pte_batch(src, _pte, pteval, max_nr_ptes);
+			} else {
 				release_pte_folio(src);
+			}
+
 			/*
 			 * ptl mostly unnecessary, but preempt has to
 			 * be disabled to update the per-cpu stats
 			 * inside folio_remove_rmap_pte().
 			 */
 			spin_lock(ptl);
-			ptep_clear(vma->vm_mm, address, _pte);
-			folio_remove_rmap_pte(src, src_page, vma);
+			clear_ptes(vma->vm_mm, address, _pte, nr_ptes);
+			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
 			spin_unlock(ptl);
-			free_folio_and_swap_cache(src);
+			free_swap_cache(src);
+			folio_put_refs(src, nr_ptes);
 		}
 	}
 
-- 
2.30.2

[PATCH v3 3/3] khugepaged: Optimize collapse_pte_mapped_thp() by PTE batching

[Dev Jain]

Use PTE batching to optimize collapse_pte_mapped_thp().

On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
Then, calling ptep_clear() for every pte will cause a TLB flush for every
contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
which will flush the TLB only for the (if any) starting and ending contpte
block, if they partially overlap with the range khugepaged is looking at.

For all arches, there should be a benefit due to batching atomic operations
on mapcounts due to folio_remove_rmap_ptes() and saving some calls.

Note that we do not need to make a change to the check
"if (folio_page(folio, i) != page)"; if i'th page of the folio is equal
to the first page of our batch, then i + 1, .... i + nr_batch_ptes - 1
pages of the folio will be equal to the corresponding pages of our
batch mapping consecutive pages.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/khugepaged.c | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 63517ef7eafb..1ff0c7dd2be4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1503,15 +1503,16 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
 int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 			    bool install_pmd)
 {
+	int nr_mapped_ptes = 0, nr_batch_ptes, result = SCAN_FAIL;
 	struct mmu_notifier_range range;
 	bool notified = false;
 	unsigned long haddr = addr & HPAGE_PMD_MASK;
+	unsigned long end = haddr + HPAGE_PMD_SIZE;
 	struct vm_area_struct *vma = vma_lookup(mm, haddr);
 	struct folio *folio;
 	pte_t *start_pte, *pte;
 	pmd_t *pmd, pgt_pmd;
 	spinlock_t *pml = NULL, *ptl;
-	int nr_ptes = 0, result = SCAN_FAIL;
 	int i;
 
 	mmap_assert_locked(mm);
@@ -1625,11 +1626,15 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 		goto abort;
 
 	/* step 2: clear page table and adjust rmap */
-	for (i = 0, addr = haddr, pte = start_pte;
-	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
+	for (i = 0, addr = haddr, pte = start_pte; i < HPAGE_PMD_NR;
+	     i += nr_batch_ptes, addr += nr_batch_ptes * PAGE_SIZE,
+	     pte += nr_batch_ptes) {
+		int max_nr_batch_ptes = (end - addr) >> PAGE_SHIFT;
 		struct page *page;
 		pte_t ptent = ptep_get(pte);
 
+		nr_batch_ptes = 1;
+
 		if (pte_none(ptent))
 			continue;
 		/*
@@ -1643,26 +1648,29 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 			goto abort;
 		}
 		page = vm_normal_page(vma, addr, ptent);
+
 		if (folio_page(folio, i) != page)
 			goto abort;
 
+		nr_batch_ptes = folio_pte_batch(folio, pte, ptent, max_nr_batch_ptes);
+
 		/*
 		 * Must clear entry, or a racing truncate may re-remove it.
 		 * TLB flush can be left until pmdp_collapse_flush() does it.
 		 * PTE dirty? Shmem page is already dirty; file is read-only.
 		 */
-		ptep_clear(mm, addr, pte);
-		folio_remove_rmap_pte(folio, page, vma);
-		nr_ptes++;
+		clear_ptes(mm, addr, pte, nr_batch_ptes);
+		folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma);
+		nr_mapped_ptes += nr_batch_ptes;
 	}
 
 	if (!pml)
 		spin_unlock(ptl);
 
 	/* step 3: set proper refcount and mm_counters. */
-	if (nr_ptes) {
-		folio_ref_sub(folio, nr_ptes);
-		add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
+	if (nr_mapped_ptes) {
+		folio_ref_sub(folio, nr_mapped_ptes);
+		add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);
 	}
 
 	/* step 4: remove empty page table */
@@ -1695,10 +1703,10 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 			: SCAN_SUCCEED;
 	goto drop_folio;
 abort:
-	if (nr_ptes) {
+	if (nr_mapped_ptes) {
 		flush_tlb_mm(mm);
-		folio_ref_sub(folio, nr_ptes);
-		add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
+		folio_ref_sub(folio, nr_mapped_ptes);
+		add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);
 	}
 unlock:
 	if (start_pte)
-- 
2.30.2

Re: [PATCH v3 2/3] khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE batching

[David Hildenbrand]

On 22.07.25 17:05, Dev Jain wrote:
> Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.

I suggest not talking so much about arm specifics.

Simply say that batching reduced the number of TLB flushes, especially 
on architectures that support cont-pte optimizations.

> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.

I would rephrase that to "Independent of that, batching PTE unmapping 
has known performance benfits (i.e., less refcount and mapcount updates)".

> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   mm/khugepaged.c | 25 ++++++++++++++++++-------
>   1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a55fb1dcd224..63517ef7eafb 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -700,12 +700,15 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>   						spinlock_t *ptl,
>   						struct list_head *compound_pagelist)
>   {
> +	unsigned long end = address + HPAGE_PMD_SIZE;
>   	struct folio *src, *tmp;
> -	pte_t *_pte;
>   	pte_t pteval;
> +	pte_t *_pte;
> +	int nr_ptes;

Nit: I guess we should switch to "unsigned int" here now for consistency 
with folio_pte_batch().

>   
> -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> -	     _pte++, address += PAGE_SIZE) {
> +	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> +	     address += nr_ptes * PAGE_SIZE) {
> +		nr_ptes = 1;
>   		pteval = ptep_get(_pte);
>   		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>   			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> @@ -722,18 +725,26 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>   			struct page *src_page = pte_page(pteval);
>   
>   			src = page_folio(src_page);
> -			if (!folio_test_large(src))
> +
> +			if (folio_test_large(src)) {
> +				int max_nr_ptes = (end - address) >> PAGE_SHIFT;

Dito.

> +
> +				nr_ptes = folio_pte_batch(src, _pte, pteval, max_nr_ptes);
> +			} else {
>   				release_pte_folio(src);
> +			}
> +

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 3/3] khugepaged: Optimize collapse_pte_mapped_thp() by PTE batching

[David Hildenbrand]

On 22.07.25 17:05, Dev Jain wrote:
> Use PTE batching to optimize collapse_pte_mapped_thp().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.
> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.

Please simplify that (and make it less arm specific) like suggested for 
patch #2.

PTE batching has known benefits on all architectures :)

> 
> Note that we do not need to make a change to the check
> "if (folio_page(folio, i) != page)"; if i'th page of the folio is equal
> to the first page of our batch, then i + 1, .... i + nr_batch_ptes - 1
> pages of the folio will be equal to the corresponding pages of our
> batch mapping consecutive pages.

Yeah, that must stay to make the comment from "step 1" happy.

> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   mm/khugepaged.c | 32 ++++++++++++++++++++------------
>   1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 63517ef7eafb..1ff0c7dd2be4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1503,15 +1503,16 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
>   int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   			    bool install_pmd)
>   {
> +	int nr_mapped_ptes = 0, nr_batch_ptes, result = SCAN_FAIL;
>   	struct mmu_notifier_range range;
>   	bool notified = false;
>   	unsigned long haddr = addr & HPAGE_PMD_MASK;
> +	unsigned long end = haddr + HPAGE_PMD_SIZE;
>   	struct vm_area_struct *vma = vma_lookup(mm, haddr);
>   	struct folio *folio;
>   	pte_t *start_pte, *pte;
>   	pmd_t *pmd, pgt_pmd;
>   	spinlock_t *pml = NULL, *ptl;
> -	int nr_ptes = 0, result = SCAN_FAIL;
>   	int i;
>   
>   	mmap_assert_locked(mm);
> @@ -1625,11 +1626,15 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   		goto abort;
>   
>   	/* step 2: clear page table and adjust rmap */
> -	for (i = 0, addr = haddr, pte = start_pte;
> -	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
> +	for (i = 0, addr = haddr, pte = start_pte; i < HPAGE_PMD_NR;
> +	     i += nr_batch_ptes, addr += nr_batch_ptes * PAGE_SIZE,
> +	     pte += nr_batch_ptes) {
 > +		int max_nr_batch_ptes = (end - addr) >> PAGE_SHIFT;>   		struct 
page *page;
>   		pte_t ptent = ptep_get(pte);
>   
> +		nr_batch_ptes = 1;
> +
>   		if (pte_none(ptent))
>   			continue;
>   		/*
> @@ -1643,26 +1648,29 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   			goto abort;
>   		}
>   		page = vm_normal_page(vma, addr, ptent);
> +
>   		if (folio_page(folio, i) != page)
>   			goto abort;
>   
> +		nr_batch_ptes = folio_pte_batch(folio, pte, ptent, max_nr_batch_ptes);

Same comment regarding matching types for folio_pte_batch(), now that we 
changed that -- unsigned int.

> +
>   		/*
>   		 * Must clear entry, or a racing truncate may re-remove it.
>   		 * TLB flush can be left until pmdp_collapse_flush() does it.
>   		 * PTE dirty? Shmem page is already dirty; file is read-only.
>   		 */
> -		ptep_clear(mm, addr, pte);
> -		folio_remove_rmap_pte(folio, page, vma);
> -		nr_ptes++;
> +		clear_ptes(mm, addr, pte, nr_batch_ptes);
> +		folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma);
> +		nr_mapped_ptes += nr_batch_ptes;
>   	}
>   
>   	if (!pml)
>   		spin_unlock(ptl);
>   
>   	/* step 3: set proper refcount and mm_counters. */
> -	if (nr_ptes) {
> -		folio_ref_sub(folio, nr_ptes);
> -		add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
> +	if (nr_mapped_ptes) {
> +		folio_ref_sub(folio, nr_mapped_ptes);
> +		add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);
>   	}
>   
>   	/* step 4: remove empty page table */
> @@ -1695,10 +1703,10 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   			: SCAN_SUCCEED;
>   	goto drop_folio;
>   abort:
> -	if (nr_ptes) {
> +	if (nr_mapped_ptes) {
>   		flush_tlb_mm(mm);
> -		folio_ref_sub(folio, nr_ptes);
> -		add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
> +		folio_ref_sub(folio, nr_mapped_ptes);
> +		add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);

Doing the TLB flush and adjusting the refcount after dropping the PTL 
... interesting. Well, nothing surprises me in khugpaged code anymore.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 1/3] mm: add get_and_clear_ptes() and clear_ptes()

[Andrew Morton]

On Tue, 22 Jul 2025 20:35:57 +0530 Dev Jain <dev.jain@arm.com> wrote:

> Let's add variants to be used where "full" does not apply -- which will
> be the majority of cases in the future. "full" really only applies if
> we are about to tear down a full MM.
> 
> Use get_and_clear_ptes() in existing code, clear_ptes() users will
> be added next.
> 
> Should we make these inline functions instead and add separate docs?

inlined functions would be preferable.

> Probably not worth it for now.

Well, as David has called for a v4 I suggest you make that change.

Thanks, I'll queue the series in mm-new for a bit of testing.

Re: [PATCH v3 1/3] mm: add get_and_clear_ptes() and clear_ptes()

[Baolin Wang]

On 2025/7/22 23:05, Dev Jain wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> From: David Hildenbrand <david@redhat.com>
> 
> Let's add variants to be used where "full" does not apply -- which will
> be the majority of cases in the future. "full" really only applies if
> we are about to tear down a full MM.
> 
> Use get_and_clear_ptes() in existing code, clear_ptes() users will
> be added next.
> 
> Should we make these inline functions instead and add separate docs?
> Probably not worth it for now.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> ---
>   arch/arm64/mm/mmu.c     | 2 +-
>   include/linux/pgtable.h | 6 ++++++
>   mm/mremap.c             | 2 +-
>   mm/rmap.c               | 2 +-
>   4 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index abd9725796e9..20a89ab97dc5 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1528,7 +1528,7 @@ early_initcall(prevent_bootmem_remove_init);
>   pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
>   			     pte_t *ptep, unsigned int nr)
>   {
> -	pte_t pte = get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, /* full = */ 0);
> +	pte_t pte = get_and_clear_ptes(vma->vm_mm, addr, ptep, nr);
>   
>   	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
>   		/*
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index e3b99920be05..e45986b54277 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -736,6 +736,9 @@ static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
>   }
>   #endif
>   
> +#define get_and_clear_ptes(_mm, _addr, _ptep, _nr) \
> +	get_and_clear_full_ptes(_mm, _addr, _ptep, _nr, 0)
> +
>   #ifndef clear_full_ptes
>   /**
>    * clear_full_ptes - Clear present PTEs that map consecutive pages of the same
> @@ -768,6 +771,9 @@ static inline void clear_full_ptes(struct mm_struct *mm, unsigned long addr,
>   }
>   #endif
>   
> +#define clear_ptes(_mm, _addr, _ptep, _nr) \
> +	clear_full_ptes(_mm, _addr, _ptep, _nr, 0)
> +
>   /*
>    * If two threads concurrently fault at the same page, the thread that
>    * won the race updates the PTE and its local TLB/Cache. The other thread
> diff --git a/mm/mremap.c b/mm/mremap.c
> index ac39845e9718..677a4d744df9 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -280,7 +280,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>   							 old_pte, max_nr_ptes);
>   			force_flush = true;
>   		}
> -		pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
> +		pte = get_and_clear_ptes(mm, old_addr, old_ptep, nr_ptes);
>   		pte = move_pte(pte, old_addr, new_addr);
>   		pte = move_soft_dirty_pte(pte);
>   
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f93ce27132ab..568198e9efc2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2036,7 +2036,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>   			flush_cache_range(vma, address, end_addr);
>   
>   			/* Nuke the page table entry. */
> -			pteval = get_and_clear_full_ptes(mm, address, pvmw.pte, nr_pages, 0);
> +			pteval = get_and_clear_ptes(mm, address, pvmw.pte, nr_pages);
>   			/*
>   			 * We clear the PTE but do not flush so potentially
>   			 * a remote CPU could still be writing to the folio.

Re: [PATCH v3 2/3] khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE batching

[Baolin Wang]

On 2025/7/22 23:05, Dev Jain wrote:
> Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.
> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>

With David's comments addressed, LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Re: [PATCH v3 3/3] khugepaged: Optimize collapse_pte_mapped_thp() by PTE batching

[Baolin Wang]

On 2025/7/22 23:05, Dev Jain wrote:
> Use PTE batching to optimize collapse_pte_mapped_thp().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.
> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.
> 
> Note that we do not need to make a change to the check
> "if (folio_page(folio, i) != page)"; if i'th page of the folio is equal
> to the first page of our batch, then i + 1, .... i + nr_batch_ptes - 1
> pages of the folio will be equal to the corresponding pages of our
> batch mapping consecutive pages.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>

With David's comments addressed, LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Re: [PATCH v3 1/3] mm: add get_and_clear_ptes() and clear_ptes()

[Dev Jain]

On 23/07/25 2:17 am, Andrew Morton wrote:
> On Tue, 22 Jul 2025 20:35:57 +0530 Dev Jain <dev.jain@arm.com> wrote:
>
>> Let's add variants to be used where "full" does not apply -- which will
>> be the majority of cases in the future. "full" really only applies if
>> we are about to tear down a full MM.
>>
>> Use get_and_clear_ptes() in existing code, clear_ptes() users will
>> be added next.
>>
>> Should we make these inline functions instead and add separate docs?
> inlined functions would be preferable.
>
>> Probably not worth it for now.
> Well, as David has called for a v4 I suggest you make that change.
>
> Thanks, I'll queue the series in mm-new for a bit of testing.

Thanks. I'll post v4 later in the day.

Re: [PATCH v3 2/3] khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE batching

[Dev Jain]

On 22/07/25 9:33 pm, David Hildenbrand wrote:
> On 22.07.25 17:05, Dev Jain wrote:
>> Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
>>
>> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for 
>> collapse.
>> Then, calling ptep_clear() for every pte will cause a TLB flush for 
>> every
>> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
>> which will flush the TLB only for the (if any) starting and ending 
>> contpte
>> block, if they partially overlap with the range khugepaged is looking 
>> at.
>
> I suggest not talking so much about arm specifics.
>
> Simply say that batching reduced the number of TLB flushes, especially 
> on architectures that support cont-pte optimizations.
Makes sense.
>
>>
>> For all arches, there should be a benefit due to batching atomic 
>> operations
>> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.
>
> I would rephrase that to "Independent of that, batching PTE unmapping 
> has known performance benfits (i.e., less refcount and mapcount 
> updates)".
Thanks.
>
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/khugepaged.c | 25 ++++++++++++++++++-------
>>   1 file changed, 18 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index a55fb1dcd224..63517ef7eafb 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -700,12 +700,15 @@ static void 
>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>                           spinlock_t *ptl,
>>                           struct list_head *compound_pagelist)
>>   {
>> +    unsigned long end = address + HPAGE_PMD_SIZE;
>>       struct folio *src, *tmp;
>> -    pte_t *_pte;
>>       pte_t pteval;
>> +    pte_t *_pte;
>> +    int nr_ptes;
>
> Nit: I guess we should switch to "unsigned int" here now for 
> consistency with folio_pte_batch().
Okay.
>
>>   -    for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>> -         _pte++, address += PAGE_SIZE) {
>> +    for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
>> +         address += nr_ptes * PAGE_SIZE) {
>> +        nr_ptes = 1;
>>           pteval = ptep_get(_pte);
>>           if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>>               add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>> @@ -722,18 +725,26 @@ static void 
>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>               struct page *src_page = pte_page(pteval);
>>                 src = page_folio(src_page);
>> -            if (!folio_test_large(src))
>> +
>> +            if (folio_test_large(src)) {
>> +                int max_nr_ptes = (end - address) >> PAGE_SHIFT;
>
> Dito.
>
>> +
>> +                nr_ptes = folio_pte_batch(src, _pte, pteval, 
>> max_nr_ptes);
>> +            } else {
>>                   release_pte_folio(src);
>> +            }
>> +
>
> Acked-by: David Hildenbrand <david@redhat.com>
Thanks.