[PATCH v2 0/2] promote mapped executable folios after first usage for MGLRU

[PATCH v2 0/2] promote mapped executable folios after first usage for MGLRU

[Baolin Wang]

Now MGLRU's protection of mapped executable file folios is less reliable.
Follow the classical LRU's logic, promoting mapped executable file folios
after their first usage to give executable code a better chance to stay in
memory and improve workload performance (See patch 2 for more details).

Changes from v1:
 - Add a new patch to clean up vma flags as preparation.
 - Add a new helper to check exec file folios (per Kaisui).
 - Promote exec file folios in lru_gen_set_refs (Per Sashiko). 

Baolin Wang (2):
  mm: vmscan: convert folio_referenced() to use vma_flags_t
  mm: mglru: promote mapped executable folios after first usage

 include/linux/mm_inline.h |  6 +++++
 include/linux/rmap.h      |  7 +++---
 mm/rmap.c                 | 19 +++++++++------
 mm/vmscan.c               | 51 +++++++++++++++++++++++----------------
 4 files changed, 50 insertions(+), 33 deletions(-)

-- 
2.47.3

[PATCH v2 1/2] mm: vmscan: convert folio_referenced() to use vma_flags_t

[Baolin Wang]

Replace use of the legacy vm_flags_t flags with vma_flags_t values for
folio_referenced() and related logic. This is also a preparation for the
following changes.

No functional changes.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 include/linux/rmap.h |  7 +++----
 mm/rmap.c            | 19 +++++++++++--------
 mm/vmscan.c          | 14 +++++++-------
 3 files changed, 21 insertions(+), 19 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 8dc0871e5f00..e090aa09825d 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -843,7 +843,7 @@ static inline int folio_try_share_anon_rmap_pmd(struct folio *folio,
  * Called from mm/vmscan.c to handle paging out
  */
 int folio_referenced(struct folio *, int is_locked,
-			struct mem_cgroup *memcg, vm_flags_t *vm_flags);
+		struct mem_cgroup *memcg, vma_flags_t *vma_flags);
 
 void try_to_migrate(struct folio *folio, enum ttu_flags flags);
 void try_to_unmap(struct folio *, enum ttu_flags flags);
@@ -975,10 +975,9 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio,
 #define anon_vma_prepare(vma)	(0)
 
 static inline int folio_referenced(struct folio *folio, int is_locked,
-				  struct mem_cgroup *memcg,
-				  vm_flags_t *vm_flags)
+		struct mem_cgroup *memcg, vma_flags_t *vma_flags)
 {
-	*vm_flags = 0;
+	vma_flags_clear_all(vma_flags);
 	return 0;
 }
 
diff --git a/mm/rmap.c b/mm/rmap.c
index e854679553b9..4085540b6a25 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -907,7 +907,7 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
 struct folio_referenced_arg {
 	int mapcount;
 	int referenced;
-	vm_flags_t vm_flags;
+	vma_flags_t vma_flags;
 	struct mem_cgroup *memcg;
 };
 
@@ -926,7 +926,7 @@ static bool folio_referenced_one(struct folio *folio,
 		address = pvmw.address;
 		nr = 1;
 
-		if (vma->vm_flags & VM_LOCKED) {
+		if (vma_test(vma, VMA_LOCKED_BIT)) {
 			ptes++;
 			pra->mapcount--;
 
@@ -947,7 +947,7 @@ static bool folio_referenced_one(struct folio *folio,
 			/* Restore the mlock which got missed */
 			mlock_vma_folio(folio, vma);
 			page_vma_mapped_walk_done(&pvmw);
-			pra->vm_flags |= VM_LOCKED;
+			vma_flags_set(&pra->vma_flags, VMA_LOCKED_BIT);
 			return false; /* To break the loop */
 		}
 
@@ -1015,8 +1015,11 @@ static bool folio_referenced_one(struct folio *folio,
 		referenced++;
 
 	if (referenced) {
+		vma_flags_t vma_flags = vma->flags;
+
 		pra->referenced++;
-		pra->vm_flags |= vma->vm_flags & ~VM_LOCKED;
+		vma_flags_clear(&vma_flags, VMA_LOCKED_BIT);
+		vma_flags_set_mask(&pra->vma_flags, vma_flags);
 	}
 
 	if (!pra->mapcount)
@@ -1054,7 +1057,7 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
  * @folio: The folio to test.
  * @is_locked: Caller holds lock on the folio.
  * @memcg: target memory cgroup
- * @vm_flags: A combination of all the vma->vm_flags which referenced the folio.
+ * @vma_flags: A combination of all the vma->flags which referenced the folio.
  *
  * Quick test_and_clear_referenced for all mappings of a folio,
  *
@@ -1062,7 +1065,7 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
  * the function bailed out due to rmap lock contention.
  */
 int folio_referenced(struct folio *folio, int is_locked,
-		     struct mem_cgroup *memcg, vm_flags_t *vm_flags)
+		struct mem_cgroup *memcg, vma_flags_t *vma_flags)
 {
 	bool we_locked = false;
 	struct folio_referenced_arg pra = {
@@ -1078,7 +1081,7 @@ int folio_referenced(struct folio *folio, int is_locked,
 	};
 
 	VM_WARN_ON_ONCE_FOLIO(folio_is_zone_device(folio), folio);
-	*vm_flags = 0;
+	vma_flags_clear_all(vma_flags);
 	if (!pra.mapcount)
 		return 0;
 
@@ -1092,7 +1095,7 @@ int folio_referenced(struct folio *folio, int is_locked,
 	}
 
 	rmap_walk(folio, &rwc);
-	*vm_flags = pra.vm_flags;
+	vma_flags_set_mask(vma_flags, pra.vma_flags);
 
 	if (we_locked)
 		folio_unlock(folio);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 986dde8e7429..de62899c108d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -861,16 +861,16 @@ static enum folio_references folio_check_references(struct folio *folio,
 						  struct scan_control *sc)
 {
 	int referenced_ptes, referenced_folio;
-	vm_flags_t vm_flags;
+	vma_flags_t vma_flags;
 
 	referenced_ptes = folio_referenced(folio, 1, sc->target_mem_cgroup,
-					   &vm_flags);
+					   &vma_flags);
 
 	/*
 	 * The supposedly reclaimable folio was found to be in a VM_LOCKED vma.
 	 * Let the folio, now marked Mlocked, be moved to the unevictable list.
 	 */
-	if (vm_flags & VM_LOCKED)
+	if (vma_flags_test(&vma_flags, VMA_LOCKED_BIT))
 		return FOLIOREF_ACTIVATE;
 
 	/*
@@ -914,7 +914,7 @@ static enum folio_references folio_check_references(struct folio *folio,
 		/*
 		 * Activate file-backed executable folios after first usage.
 		 */
-		if ((vm_flags & VM_EXEC) && folio_is_file_lru(folio))
+		if (vma_flags_test(&vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio))
 			return FOLIOREF_ACTIVATE;
 
 		return FOLIOREF_KEEP;
@@ -2065,7 +2065,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
 {
 	unsigned long nr_taken;
 	unsigned long nr_scanned;
-	vm_flags_t vm_flags;
+	vma_flags_t vma_flags;
 	LIST_HEAD(l_hold);	/* The folios which were snipped off */
 	LIST_HEAD(l_active);
 	LIST_HEAD(l_inactive);
@@ -2109,7 +2109,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
 
 		/* Referenced or rmap lock contention: rotate */
 		if (folio_referenced(folio, 0, sc->target_mem_cgroup,
-				     &vm_flags) != 0) {
+				     &vma_flags) != 0) {
 			/*
 			 * Identify referenced, file-backed active folios and
 			 * give them one more trip around the active list. So
@@ -2119,7 +2119,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
 			 * IO, plus JVM can create lots of anon VM_EXEC folios,
 			 * so we ignore them here.
 			 */
-			if ((vm_flags & VM_EXEC) && folio_is_file_lru(folio)) {
+			if (vma_flags_test(&vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio)) {
 				nr_rotated += folio_nr_pages(folio);
 				list_add(&folio->lru, &l_active);
 				continue;
-- 
2.47.3

[PATCH v2 2/2] mm: mglru: promote mapped executable folios after first usage

[Baolin Wang]

Classical LRU protects mapped executable file folios through commit
8cab4754d24a0 ("vmscan: make mapped executable pages the first class
citizen") and commit c909e99364c8 ("vmscan: activate executable pages
after first usage"), giving executable code a better chance to stay in
memory, avoiding IO thrashing and improving workload performance.

However, MGLRU's protection of mapped executable file folios is less
reliable. Although shrink_folio_list() checks references, the access flag
of mapped executable file folios may have already been checked and
cleared by lru_gen_look_around() or walk_mm(). Additionally,
folio_update_gen() or lru_gen_set_refs() only sets the 'PG_referenced'
flag for mapped executable file folios, which causes shrink_folio_list()
to ignore the first usage of these mapped executable file folios and
reclaim them easily.

Follow the classical LRU's logic, promoting mapped executable file folios
after their first usage in folio_update_gen() and lru_gen_set_refs(),
giving executable code a better chance to stay in memory.

On my 32-core Arm machine, with the memcg limit set to 2G, running
'make -j32' to build kernel showed some improvement in sys time.

base			patched
9248.543s		7861.579s

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 include/linux/mm_inline.h |  6 ++++++
 mm/vmscan.c               | 41 ++++++++++++++++++++++++---------------
 2 files changed, 31 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index b5c4dc0f3fe3..30fdf1056312 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -30,6 +30,12 @@ static inline int folio_is_file_lru(const struct folio *folio)
 	return !folio_test_swapbacked(folio);
 }
 
+static inline bool folio_is_exec_file(const struct folio *folio,
+		const vma_flags_t *vma_flags)
+{
+	return vma_flags_test(vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio);
+}
+
 static __always_inline void __update_lru_size(struct lruvec *lruvec,
 				enum lru_list lru, enum zone_type zid,
 				long nr_pages)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index de62899c108d..fa808721b64d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -835,11 +835,15 @@ enum folio_references {
  * with PG_active set. In contrast, the aging (page table walk) path uses
  * folio_update_gen().
  */
-static bool lru_gen_set_refs(struct folio *folio)
+static bool lru_gen_set_refs(struct folio *folio, const vma_flags_t *vma_flags)
 {
 	/* see the comment on LRU_REFS_FLAGS */
 	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
 		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
+		/* Activate file-backed executable folios after first usage. */
+		if (folio_is_exec_file(folio, vma_flags))
+			return true;
+
 		return false;
 	}
 
@@ -851,7 +855,7 @@ static bool lru_gen_set_refs(struct folio *folio)
 	return true;
 }
 #else
-static bool lru_gen_set_refs(struct folio *folio)
+static bool lru_gen_set_refs(struct folio *folio, const vma_flags_t *vma_flags)
 {
 	return false;
 }
@@ -886,7 +890,7 @@ static enum folio_references folio_check_references(struct folio *folio,
 		if (!referenced_ptes)
 			return FOLIOREF_RECLAIM;
 
-		return lru_gen_set_refs(folio) ? FOLIOREF_ACTIVATE : FOLIOREF_KEEP;
+		return lru_gen_set_refs(folio, &vma_flags) ? FOLIOREF_ACTIVATE : FOLIOREF_KEEP;
 	}
 
 	referenced_folio = folio_test_clear_referenced(folio);
@@ -914,7 +918,7 @@ static enum folio_references folio_check_references(struct folio *folio,
 		/*
 		 * Activate file-backed executable folios after first usage.
 		 */
-		if (vma_flags_test(&vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio))
+		if (folio_is_exec_file(folio, &vma_flags))
 			return FOLIOREF_ACTIVATE;
 
 		return FOLIOREF_KEEP;
@@ -2119,7 +2123,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
 			 * IO, plus JVM can create lots of anon VM_EXEC folios,
 			 * so we ignore them here.
 			 */
-			if (vma_flags_test(&vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio)) {
+			if (folio_is_exec_file(folio, &vma_flags)) {
 				nr_rotated += folio_nr_pages(folio);
 				list_add(&folio->lru, &l_active);
 				continue;
@@ -3188,7 +3192,7 @@ static bool positive_ctrl_err(struct ctrl_pos *sp, struct ctrl_pos *pv)
  ******************************************************************************/
 
 /* promote pages accessed through page tables */
-static int folio_update_gen(struct folio *folio, int gen)
+static int folio_update_gen(struct folio *folio, int gen, const vma_flags_t *vma_flags)
 {
 	unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
 
@@ -3196,10 +3200,15 @@ static int folio_update_gen(struct folio *folio, int gen)
 
 	/* see the comment on LRU_REFS_FLAGS */
 	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
+		/* Activate file-backed executable folios after first usage. */
+		if (folio_is_exec_file(folio, vma_flags))
+			goto promote;
+
 		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
 		return -1;
 	}
 
+promote:
 	do {
 		/* lru_gen_del_folio() has isolated this page? */
 		if (!(old_flags & LRU_GEN_MASK))
@@ -3428,8 +3437,8 @@ static bool suitable_to_scan(int total, int young)
 	return young * n >= total;
 }
 
-static void walk_update_folio(struct lru_gen_mm_walk *walk, struct folio *folio,
-			      int new_gen, bool dirty)
+static void walk_update_folio(struct lru_gen_mm_walk *walk, struct vm_area_struct *vma,
+		struct folio *folio, int new_gen, bool dirty)
 {
 	int old_gen;
 
@@ -3442,10 +3451,10 @@ static void walk_update_folio(struct lru_gen_mm_walk *walk, struct folio *folio,
 		folio_mark_dirty(folio);
 
 	if (walk) {
-		old_gen = folio_update_gen(folio, new_gen);
+		old_gen = folio_update_gen(folio, new_gen, &vma->flags);
 		if (old_gen >= 0 && old_gen != new_gen)
 			update_batch_size(walk, folio, old_gen, new_gen);
-	} else if (lru_gen_set_refs(folio)) {
+	} else if (lru_gen_set_refs(folio, &vma->flags)) {
 		old_gen = folio_lru_gen(folio);
 		if (old_gen >= 0 && old_gen != new_gen)
 			folio_activate(folio);
@@ -3518,7 +3527,7 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 			continue;
 
 		if (last != folio) {
-			walk_update_folio(walk, last, gen, dirty);
+			walk_update_folio(walk, args->vma, last, gen, dirty);
 
 			last = folio;
 			dirty = false;
@@ -3531,7 +3540,7 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 		walk->mm_stats[MM_LEAF_YOUNG] += nr;
 	}
 
-	walk_update_folio(walk, last, gen, dirty);
+	walk_update_folio(walk, args->vma, last, gen, dirty);
 	last = NULL;
 
 	if (i < PTRS_PER_PTE && get_next_vma(PMD_MASK, PAGE_SIZE, args, &start, &end))
@@ -3609,7 +3618,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
 			goto next;
 
 		if (last != folio) {
-			walk_update_folio(walk, last, gen, dirty);
+			walk_update_folio(walk, vma, last, gen, dirty);
 
 			last = folio;
 			dirty = false;
@@ -3623,7 +3632,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
 		i = i > MIN_LRU_BATCH ? 0 : find_next_bit(bitmap, MIN_LRU_BATCH, i) + 1;
 	} while (i <= MIN_LRU_BATCH);
 
-	walk_update_folio(walk, last, gen, dirty);
+	walk_update_folio(walk, vma, last, gen, dirty);
 
 	lazy_mmu_mode_disable();
 	spin_unlock(ptl);
@@ -4258,7 +4267,7 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
 			continue;
 
 		if (last != folio) {
-			walk_update_folio(walk, last, gen, dirty);
+			walk_update_folio(walk, vma, last, gen, dirty);
 
 			last = folio;
 			dirty = false;
@@ -4270,7 +4279,7 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
 		young += nr;
 	}
 
-	walk_update_folio(walk, last, gen, dirty);
+	walk_update_folio(walk, vma, last, gen, dirty);
 
 	lazy_mmu_mode_disable();
 
-- 
2.47.3

Re: [PATCH v2 1/2] mm: vmscan: convert folio_referenced() to use vma_flags_t

[David Hildenbrand (Arm)]

On 7/16/26 12:46, Baolin Wang wrote:
> Replace use of the legacy vm_flags_t flags with vma_flags_t values for
> folio_referenced() and related logic. This is also a preparation for the
> following changes.
> 
> No functional changes.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH v2 2/2] mm: mglru: promote mapped executable folios after first usage

[David Hildenbrand (Arm)]

On 7/16/26 12:46, Baolin Wang wrote:
> Classical LRU protects mapped executable file folios through commit
> 8cab4754d24a0 ("vmscan: make mapped executable pages the first class
> citizen") and commit c909e99364c8 ("vmscan: activate executable pages
> after first usage"), giving executable code a better chance to stay in
> memory, avoiding IO thrashing and improving workload performance.
> 
> However, MGLRU's protection of mapped executable file folios is less
> reliable. Although shrink_folio_list() checks references, the access flag
> of mapped executable file folios may have already been checked and
> cleared by lru_gen_look_around() or walk_mm(). Additionally,
> folio_update_gen() or lru_gen_set_refs() only sets the 'PG_referenced'
> flag for mapped executable file folios, which causes shrink_folio_list()
> to ignore the first usage of these mapped executable file folios and
> reclaim them easily.
> 
> Follow the classical LRU's logic, promoting mapped executable file folios
> after their first usage in folio_update_gen() and lru_gen_set_refs(),
> giving executable code a better chance to stay in memory.
> 
> On my 32-core Arm machine, with the memcg limit set to 2G, running
> 'make -j32' to build kernel showed some improvement in sys time.
> 
> base			patched
> 9248.543s		7861.579s
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  include/linux/mm_inline.h |  6 ++++++
>  mm/vmscan.c               | 41 ++++++++++++++++++++++++---------------
>  2 files changed, 31 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index b5c4dc0f3fe3..30fdf1056312 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -30,6 +30,12 @@ static inline int folio_is_file_lru(const struct folio *folio)
>  	return !folio_test_swapbacked(folio);
>  }
>  
> +static inline bool folio_is_exec_file(const struct folio *folio,
> +		const vma_flags_t *vma_flags)
> +{
> +	return vma_flags_test(vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio);
> +}

I don't quite like a generic folio_is* helper to consume vma flags. Whether it
is executable depends on the VMA. When called from a non-exec VMA it would just
give a wrong answer.

Not sure if something like "is_executable_file_folio()"/"is_exec_file_folio"
would be better, to distinguish from the actualy folio_is* helpers that are
independent of context.

Also, can this helper just go to vmscan.c?

-- 
Cheers,

David

Re: [PATCH v2 2/2] mm: mglru: promote mapped executable folios after first usage

[Baolin Wang]

On 7/16/26 7:10 PM, David Hildenbrand (Arm) wrote:
> On 7/16/26 12:46, Baolin Wang wrote:
>> Classical LRU protects mapped executable file folios through commit
>> 8cab4754d24a0 ("vmscan: make mapped executable pages the first class
>> citizen") and commit c909e99364c8 ("vmscan: activate executable pages
>> after first usage"), giving executable code a better chance to stay in
>> memory, avoiding IO thrashing and improving workload performance.
>>
>> However, MGLRU's protection of mapped executable file folios is less
>> reliable. Although shrink_folio_list() checks references, the access flag
>> of mapped executable file folios may have already been checked and
>> cleared by lru_gen_look_around() or walk_mm(). Additionally,
>> folio_update_gen() or lru_gen_set_refs() only sets the 'PG_referenced'
>> flag for mapped executable file folios, which causes shrink_folio_list()
>> to ignore the first usage of these mapped executable file folios and
>> reclaim them easily.
>>
>> Follow the classical LRU's logic, promoting mapped executable file folios
>> after their first usage in folio_update_gen() and lru_gen_set_refs(),
>> giving executable code a better chance to stay in memory.
>>
>> On my 32-core Arm machine, with the memcg limit set to 2G, running
>> 'make -j32' to build kernel showed some improvement in sys time.
>>
>> base			patched
>> 9248.543s		7861.579s
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   include/linux/mm_inline.h |  6 ++++++
>>   mm/vmscan.c               | 41 ++++++++++++++++++++++++---------------
>>   2 files changed, 31 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
>> index b5c4dc0f3fe3..30fdf1056312 100644
>> --- a/include/linux/mm_inline.h
>> +++ b/include/linux/mm_inline.h
>> @@ -30,6 +30,12 @@ static inline int folio_is_file_lru(const struct folio *folio)
>>   	return !folio_test_swapbacked(folio);
>>   }
>>   
>> +static inline bool folio_is_exec_file(const struct folio *folio,
>> +		const vma_flags_t *vma_flags)
>> +{
>> +	return vma_flags_test(vma_flags, VMA_EXEC_BIT) && folio_is_file_lru(folio);
>> +}
> 
> I don't quite like a generic folio_is* helper to consume vma flags. Whether it
> is executable depends on the VMA. When called from a non-exec VMA it would just
> give a wrong answer.
> 
> Not sure if something like "is_executable_file_folio()"/"is_exec_file_folio"
> would be better, to distinguish from the actualy folio_is* helpers that are
> independent of context.

OK. 'is_exec_file_folio' sounds better to me.

> Also, can this helper just go to vmscan.c?

Yes. I originally expected this helper might be used by other callers, 
but for now it's only used in vmscan.c, so will do.

Thanks for taking a look.