[PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging

James Houghton
2024-05-29T19:16:10Z

Secondary MMUs are currently consulted for access/age information at
eviction time, but before then, we don't get accurate age information.
That is, pages that are mostly accessed through a secondary MMU (like
guest memory, used by KVM) will always just proceed down to the oldest
generation, and then at eviction time, if KVM reports the page to be
young, the page will be activated/promoted back to the youngest
generation.

Do not do look around if there is a secondary MMU we have to interact
with.

The added feature bit (0x8), if disabled, will make MGLRU behave as if
there are no secondary MMUs subscribed to MMU notifiers except at
eviction time.

Suggested-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: James Houghton <jthoughton@google.com>
---
Documentation/admin-guide/mm/multigen_lru.rst | 6 +-
include/linux/mmzone.h | 6 +-
mm/rmap.c | 9 +-
mm/vmscan.c | 144 ++++++++++++++----
4 files changed, 123 insertions(+), 42 deletions(-)

diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst
index 33e068830497..1e578e0c4c0c 100644
--- a/Documentation/admin-guide/mm/multigen_lru.rst
+++ b/Documentation/admin-guide/mm/multigen_lru.rst
@@ -48,6 +48,10 @@ Values Components
verified on x86 varieties other than Intel and AMD. If it is
disabled, the multi-gen LRU will suffer a negligible
performance degradation.
+0x0008 Continuously clear the accessed bit in secondary MMU page
+ tables instead of waiting until eviction time. This results in
+ accurate page age information for pages that are mainly used by
+ a secondary MMU.
[yYnN] Apply to all the components above.
====== ===============================================================

@@ -56,7 +60,7 @@ E.g.,

echo y >/sys/kernel/mm/lru_gen/enabled
cat /sys/kernel/mm/lru_gen/enabled
- 0x0007
+ 0x000f
echo 5 >/sys/kernel/mm/lru_gen/enabled
cat /sys/kernel/mm/lru_gen/enabled
0x0005
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8f9c9590a42c..869824ef5f3b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -400,6 +400,7 @@ enum {
LRU_GEN_CORE,
LRU_GEN_MM_WALK,
LRU_GEN_NONLEAF_YOUNG,
+ LRU_GEN_SECONDARY_MMU_WALK,
NR_LRU_GEN_CAPS
};

@@ -557,7 +558,7 @@ struct lru_gen_memcg {

void lru_gen_init_pgdat(struct pglist_data *pgdat);
void lru_gen_init_lruvec(struct lruvec *lruvec);
-void lru_gen_look_around(struct page_vma_mapped_walk *pvmw);
+bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw);

void lru_gen_init_memcg(struct mem_cgroup *memcg);
void lru_gen_exit_memcg(struct mem_cgroup *memcg);
@@ -576,8 +577,9 @@ static inline void lru_gen_init_lruvec(struct lruvec *lruvec)
{
}

-static inline void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
+static inline bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
{
+ return false;
}

static inline void lru_gen_init_memcg(struct mem_cgroup *memcg)
diff --git a/mm/rmap.c b/mm/rmap.c
index e8fc5ecb59b2..24a3ff639919 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -870,13 +870,10 @@ static bool folio_referenced_one(struct folio *folio,
continue;
}

- if (pvmw.pte) {
- if (lru_gen_enabled() &&
- pte_young(ptep_get(pvmw.pte))) {
- lru_gen_look_around(&pvmw);
+ if (lru_gen_enabled() && pvmw.pte) {
+ if (lru_gen_look_around(&pvmw))
referenced++;
- }
-
+ } else if (pvmw.pte) {
if (ptep_clear_flush_young_notify(vma, address,
pvmw.pte))
referenced++;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d55e8d07ffc4..0d89f712f45c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -56,6 +56,7 @@
#include <linux/khugepaged.h>
#include <linux/rculist_nulls.h>
#include <linux/random.h>
+#include <linux/mmu_notifier.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>
@@ -2579,6 +2580,12 @@ static bool should_clear_pmd_young(void)
return arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG);
}

+static bool should_walk_secondary_mmu(void)
+{
+ return IS_ENABLED(CONFIG_LRU_GEN_WALKS_SECONDARY_MMU) &&
+ get_cap(LRU_GEN_SECONDARY_MMU_WALK);
+}
+
/******************************************************************************
* shorthand helpers
******************************************************************************/
@@ -3276,7 +3283,8 @@ static bool get_next_vma(unsigned long mask, unsigned long size, struct mm_walk
return false;
}

-static unsigned long get_pte_pfn(pte_t pte, struct vm_area_struct *vma, unsigned long addr)
+static unsigned long get_pte_pfn(pte_t pte, struct vm_area_struct *vma, unsigned long addr,
+ struct pglist_data *pgdat)
{
unsigned long pfn = pte_pfn(pte);

@@ -3291,10 +3299,15 @@ static unsigned long get_pte_pfn(pte_t pte, struct vm_area_struct *vma, unsigned
if (WARN_ON_ONCE(!pfn_valid(pfn)))
return -1;

+ /* try to avoid unnecessary memory loads */
+ if (pfn < pgdat->node_start_pfn || pfn >= pgdat_end_pfn(pgdat))
+ return -1;
+
return pfn;
}

-static unsigned long get_pmd_pfn(pmd_t pmd, struct vm_area_struct *vma, unsigned long addr)
+static unsigned long get_pmd_pfn(pmd_t pmd, struct vm_area_struct *vma, unsigned long addr,
+ struct pglist_data *pgdat)
{
unsigned long pfn = pmd_pfn(pmd);

@@ -3309,6 +3322,10 @@ static unsigned long get_pmd_pfn(pmd_t pmd, struct vm_area_struct *vma, unsigned
if (WARN_ON_ONCE(!pfn_valid(pfn)))
return -1;

+ /* try to avoid unnecessary memory loads */
+ if (pfn < pgdat->node_start_pfn || pfn >= pgdat_end_pfn(pgdat))
+ return -1;
+
return pfn;
}

@@ -3317,10 +3334,6 @@ static struct folio *get_pfn_folio(unsigned long pfn, struct mem_cgroup *memcg,
{
struct folio *folio;

- /* try to avoid unnecessary memory loads */
- if (pfn < pgdat->node_start_pfn || pfn >= pgdat_end_pfn(pgdat))
- return NULL;
-
folio = pfn_folio(pfn);
if (folio_nid(folio) != pgdat->node_id)
return NULL;
@@ -3343,6 +3356,32 @@ static bool suitable_to_scan(int total, int young)
return young * n >= total;
}

+static bool lru_gen_notifier_test_young(struct mm_struct *mm,
+ unsigned long addr)
+{
+ return should_walk_secondary_mmu() && mmu_notifier_test_young(mm, addr);
+}
+
+static bool lru_gen_notifier_clear_young(struct mm_struct *mm,
+ unsigned long start,
+ unsigned long end)
+{
+ return should_walk_secondary_mmu() &&
+ mmu_notifier_clear_young(mm, start, end);
+}
+
+static bool lru_gen_pmdp_test_and_clear_young(struct vm_area_struct *vma,
+ unsigned long addr,
+ pmd_t *pmd)
+{
+ bool young = pmdp_test_and_clear_young(vma, addr, pmd);
+
+ if (lru_gen_notifier_clear_young(vma->vm_mm, addr, addr + PMD_SIZE))
+ young = true;
+
+ return young;
+}
+
static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
struct mm_walk *args)
{
@@ -3357,8 +3396,9 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
DEFINE_MAX_SEQ(walk->lruvec);
int old_gen, new_gen = lru_gen_from_seq(max_seq);
+ struct mm_struct *mm = args->mm;

- pte = pte_offset_map_nolock(args->mm, pmd, start & PMD_MASK, &ptl);
+ pte = pte_offset_map_nolock(mm, pmd, start & PMD_MASK, &ptl);
if (!pte)
return false;
if (!spin_trylock(ptl)) {
@@ -3376,11 +3416,12 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
total++;
walk->mm_stats[MM_LEAF_TOTAL]++;

- pfn = get_pte_pfn(ptent, args->vma, addr);
+ pfn = get_pte_pfn(ptent, args->vma, addr, pgdat);
if (pfn == -1)
continue;

- if (!pte_young(ptent)) {
+ if (!pte_young(ptent) &&
+ !lru_gen_notifier_test_young(mm, addr)) {
walk->mm_stats[MM_LEAF_OLD]++;
continue;
}
@@ -3389,8 +3430,9 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
if (!folio)
continue;

- if (!ptep_test_and_clear_young(args->vma, addr, pte + i))
- VM_WARN_ON_ONCE(true);
+ lru_gen_notifier_clear_young(mm, addr, addr + PAGE_SIZE);
+ if (pte_young(ptent))
+ ptep_test_and_clear_young(args->vma, addr, pte + i);

young++;
walk->mm_stats[MM_LEAF_YOUNG]++;
@@ -3456,22 +3498,25 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
/* don't round down the first address */
addr = i ? (*first & PMD_MASK) + i * PMD_SIZE : *first;

- pfn = get_pmd_pfn(pmd[i], vma, addr);
- if (pfn == -1)
- goto next;
-
- if (!pmd_trans_huge(pmd[i])) {
- if (should_clear_pmd_young())
+ if (pmd_present(pmd[i]) && !pmd_trans_huge(pmd[i])) {
+ if (should_clear_pmd_young() &&
+ !should_walk_secondary_mmu())
pmdp_test_and_clear_young(vma, addr, pmd + i);
goto next;
}

+ pfn = get_pmd_pfn(pmd[i], vma, addr, pgdat);
+ if (pfn == -1)
+ goto next;
+
folio = get_pfn_folio(pfn, memcg, pgdat, walk->can_swap);
if (!folio)
goto next;

- if (!pmdp_test_and_clear_young(vma, addr, pmd + i))
+ if (!lru_gen_pmdp_test_and_clear_young(vma, addr, pmd + i)) {
+ walk->mm_stats[MM_LEAF_OLD]++;
goto next;
+ }

walk->mm_stats[MM_LEAF_YOUNG]++;

@@ -3528,19 +3573,18 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
}

if (pmd_trans_huge(val)) {
- unsigned long pfn = pmd_pfn(val);
struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
+ unsigned long pfn = get_pmd_pfn(val, vma, addr, pgdat);

walk->mm_stats[MM_LEAF_TOTAL]++;

- if (!pmd_young(val)) {
- walk->mm_stats[MM_LEAF_OLD]++;
+ if (pfn == -1)
continue;
- }

- /* try to avoid unnecessary memory loads */
- if (pfn < pgdat->node_start_pfn || pfn >= pgdat_end_pfn(pgdat))
+ if (!pmd_young(val) && !mm_has_notifiers(args->mm)) {
+ walk->mm_stats[MM_LEAF_OLD]++;
continue;
+ }

walk_pmd_range_locked(pud, addr, vma, args, bitmap, &first);
continue;
@@ -3548,7 +3592,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,

walk->mm_stats[MM_NONLEAF_TOTAL]++;

- if (should_clear_pmd_young()) {
+ if (should_clear_pmd_young() && !should_walk_secondary_mmu()) {
if (!pmd_young(val))
continue;

@@ -3994,6 +4038,26 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
* rmap/PT walk feedback
******************************************************************************/

+static bool should_look_around(struct vm_area_struct *vma, unsigned long addr,
+ pte_t *pte, int *young)
+{
+ bool secondary_was_young =
+ mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE);
+
+ /*
+ * Look around if (1) the PTE is young and (2) we do not need to
+ * consult any secondary MMUs.
+ */
+ if (pte_young(ptep_get(pte))) {
+ ptep_test_and_clear_young(vma, addr, pte);
+ *young = true;
+ return !mm_has_notifiers(vma->vm_mm);
+ } else if (secondary_was_young)
+ *young = true;
+
+ return false;
+}
+
/*
* This function exploits spatial locality when shrink_folio_list() walks the
* rmap. It scans the adjacent PTEs of a young PTE and promotes hot pages. If
@@ -4001,7 +4065,7 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
* the PTE table to the Bloom filter. This forms a feedback loop between the
* eviction and the aging.
*/
-void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
+bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
{
int i;
unsigned long start;
@@ -4019,16 +4083,20 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
struct lru_gen_mm_state *mm_state = get_mm_state(lruvec);
DEFINE_MAX_SEQ(lruvec);
int old_gen, new_gen = lru_gen_from_seq(max_seq);
+ struct mm_struct *mm = pvmw->vma->vm_mm;

lockdep_assert_held(pvmw->ptl);
VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);

+ if (!should_look_around(vma, addr, pte, &young))
+ return young;
+
if (spin_is_contended(pvmw->ptl))
- return;
+ return young;

/* exclude special VMAs containing anon pages from COW */
if (vma->vm_flags & VM_SPECIAL)
- return;
+ return young;

/* avoid taking the LRU lock under the PTL when possible */
walk = current->reclaim_state ? current->reclaim_state->mm_walk : NULL;
@@ -4036,6 +4104,9 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
start = max(addr & PMD_MASK, vma->vm_start);
end = min(addr | ~PMD_MASK, vma->vm_end - 1) + 1;

+ if (end - start == PAGE_SIZE)
+ return young;
+
if (end - start > MIN_LRU_BATCH * PAGE_SIZE) {
if (addr - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
end = start + MIN_LRU_BATCH * PAGE_SIZE;
@@ -4049,7 +4120,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)

/* folio_update_gen() requires stable folio_memcg() */
if (!mem_cgroup_trylock_pages(memcg))
- return;
+ return young;

arch_enter_lazy_mmu_mode();

@@ -4059,19 +4130,21 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
unsigned long pfn;
pte_t ptent = ptep_get(pte + i);

- pfn = get_pte_pfn(ptent, vma, addr);
+ pfn = get_pte_pfn(ptent, vma, addr, pgdat);
if (pfn == -1)
continue;

- if (!pte_young(ptent))
+ if (!pte_young(ptent) &&
+ !lru_gen_notifier_test_young(mm, addr))
continue;

folio = get_pfn_folio(pfn, memcg, pgdat, can_swap);
if (!folio)
continue;

- if (!ptep_test_and_clear_young(vma, addr, pte + i))
- VM_WARN_ON_ONCE(true);
+ lru_gen_notifier_clear_young(mm, addr, addr + PAGE_SIZE);
+ if (pte_young(ptent))
+ ptep_test_and_clear_young(vma, addr, pte + i);

young++;

@@ -4101,6 +4174,8 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
/* feedback from rmap walkers to page table walkers */
if (mm_state && suitable_to_scan(i, young))
update_bloom_filter(mm_state, max_seq, pvmw->pmd);
+
+ return young;
}

/******************************************************************************
@@ -5137,6 +5212,9 @@ static ssize_t enabled_show(struct kobject *kobj, struct kobj_attribute *attr, c
if (should_clear_pmd_young())
caps |= BIT(LRU_GEN_NONLEAF_YOUNG);

+ if (should_walk_secondary_mmu())
+ caps |= BIT(LRU_GEN_SECONDARY_MMU_WALK);
+
return sysfs_emit(buf, "0x%04x\n", caps);
}

--
2.45.1.288.g0e0cd299f1-goog