Re: [PATCH 2/7] mm/mglru: introduce helpers for manipulating gen and refs flags
From: Ridong Chen
Date: Wed Aug 19 2026 - 22:05:17 EST
On 8/20/2026 9:43 AM, Ridong Chen wrote:
On 8/18/2026 1:38 PM, Kairui Song via B4 Relay wrote:
From: Kairui Song <kasong@xxxxxxxxxxx>
Instead of doing bit ops on folio->flags.f, introduce helpers for
adjusting folio's refs and gen info, make the code easier to debug and
understand.
No functional change is intended: some combined atomic operations are
split into two, which only creates harmless transient states.
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
include/linux/mm_inline.h | 79 +++++++++++++++++++++++++++++++++++++++++------
include/linux/mmzone.h | 2 ++
mm/folio.c | 19 +++++++-----
mm/vmscan.c | 61 ++++++++++++++++++++----------------
4 files changed, 117 insertions(+), 44 deletions(-)
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 621c8653d8f7..93bf3fa221f8 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -142,10 +142,43 @@ static inline int lru_tier_from_refs(int refs, bool workingset)
return workingset ? MAX_NR_TIERS - 1 : order_base_2(refs);
}
-static inline int folio_lru_refs(const struct folio *folio)
+/**
+ * lru_gen_from_flags - Return the LRU generation number from folio flags.
+ * @flags: folio flags
+ *
+ * Returns: A number between 0 and LRU_GEN_MAX, inclusive. Returns -1 if the
+ * flags indicate the folio is off the list (e.g., isolated).
+ */
+static inline int lru_gen_from_flags(unsigned long flags)
+{
+ int gen = ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF);
+
+ BUILD_BUG_ON(LRU_GEN_MASK & LRU_REFS_MASK);
+ gen -= 1;
+ VM_WARN_ON_ONCE(gen != -1 && gen > LRU_GEN_MAX);
Perhaps we could define a macro such as GEN_OFF = -1 to make the code more self- explanatory, I found this warning a bit confusing at first glance.
LRU_GEN_MAX already bears some resemblance to MAX_NR_GENS, so introducing yet another macro may add some clutter.
Ah, I just noticed Baolin already mentioned that, sorry for the noise.
Just my two cents.
+ return gen;
+}
+
+/**
+ * lru_gen_set_flags - Set the LRU generation number to specified folio flags.
+ * @flags: pointer to the folio flags
+ * @gen: generation number, between 0 and LRU_GEN_MAX, inclusive.
+ */
+static inline void lru_gen_set_flags(unsigned long *flags, int gen)
{
- unsigned long flags = READ_ONCE(folio->flags.f);
+ VM_WARN_ON_ONCE(gen > LRU_GEN_MAX || gen < 0);
+ BUILD_BUG_ON((LRU_GEN_MAX + 1) != MAX_NR_GENS);
+
+ *flags &= ~LRU_GEN_MASK;
+ *flags |= (gen + 1UL) << LRU_GEN_PGOFF;
+}
+/**
+ * lru_refs_from_flags - Return LRU referenced / access count from folio flags.
+ * @flags: folio flags
+ */
+static inline int lru_refs_from_flags(unsigned long flags)
+{
if (!(flags & BIT(PG_referenced)))
return 0;
/*
@@ -155,18 +188,47 @@ static inline int folio_lru_refs(const struct folio *folio)
return ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) + 1;
}
-static inline int folio_lru_gen(const struct folio *folio)
+/**
+ * lru_refs_set_flags - Set the LRU referenced / access count to specified folio flags.
+ * @flags: pointer to the folio flags
+ * @refs: referenced / access count number, between 0 and LRU_REFS_MAX, inclusive.
+ */
+static inline void lru_refs_set_flags(unsigned long *flags, unsigned int refs)
+{
+ VM_WARN_ON_ONCE(refs > LRU_REFS_MAX);
+ BUILD_BUG_ON(LRU_REFS_MAX != (LRU_REFS_MASK >> LRU_REFS_PGOFF) + 1);
+
+ *flags &= ~LRU_REFS_FLAGS;
+ if (!refs)
+ return;
+ *flags |= (BIT(PG_referenced) | ((refs - 1UL) << LRU_REFS_PGOFF));
+}
+
+static inline int folio_lru_refs(const struct folio *folio)
{
- unsigned long flags = READ_ONCE(folio->flags.f);
+ return lru_refs_from_flags(READ_ONCE(*const_folio_flags(folio, 0)));
+}
+
+static inline void folio_set_lru_refs(struct folio *folio, unsigned int refs)
+{
+ unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
+
+ do {
+ new_flags = old_flags;
+ lru_refs_set_flags(&new_flags, refs);
+ } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
+}
- return ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
+static inline int folio_lru_gen(const struct folio *folio)
+{
+ return lru_gen_from_flags(READ_ONCE(*const_folio_flags(folio, 0)));
}
static inline bool lru_gen_is_active(const struct lruvec *lruvec, int gen)
{
unsigned long max_seq = lruvec->lrugen.max_seq;
- VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
+ VM_WARN_ON_ONCE(gen > LRU_GEN_MAX);
/* see the comment on MIN_NR_GENS */
return gen == lru_gen_from_seq(max_seq) || gen == lru_gen_from_seq(max_seq - 1);
@@ -270,7 +332,7 @@ static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
gen = lru_gen_from_seq(seq);
flags = (gen + 1UL) << LRU_GEN_PGOFF;
/* see the comment on MIN_NR_GENS about PG_active */
- set_mask_bits(&folio->flags.f, LRU_GEN_MASK | BIT(PG_active), flags);
+ set_mask_bits(folio_flags(folio, 0), LRU_GEN_MASK | BIT(PG_active), flags);
lru_gen_update_size(lruvec, folio, -1, gen);
/* for folio_rotate_reclaimable() */
@@ -295,7 +357,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
/* for folio_migrate_flags() */
flags = !reclaiming && lru_gen_is_active(lruvec, gen) ? BIT(PG_active) : 0;
- flags = set_mask_bits(&folio->flags.f, LRU_GEN_MASK, flags);
+ flags = set_mask_bits(folio_flags(folio, 0), LRU_GEN_MASK, flags);
gen = ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
lru_gen_update_size(lruvec, folio, gen, -1);
@@ -339,7 +401,6 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
static inline void folio_migrate_refs(struct folio *new, const struct folio *old)
{
-
}
#endif /* CONFIG_LRU_GEN */
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 94f9c3ff5416..32d9354a754f 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -496,7 +496,9 @@ enum lruvec_flags {
#ifndef __GENERATING_BOUNDS_H
#define LRU_GEN_MASK ((BIT(LRU_GEN_WIDTH) - 1) << LRU_GEN_PGOFF)
+#define LRU_GEN_MAX (BIT(LRU_GEN_WIDTH - 1) - 1)
#define LRU_REFS_MASK ((BIT(LRU_REFS_WIDTH) - 1) << LRU_REFS_PGOFF)
+#define LRU_REFS_MAX BIT(LRU_REFS_WIDTH)
/*
* For folios accessed multiple times through file descriptors,
diff --git a/mm/folio.c b/mm/folio.c
index 59c477120b9a..0adfe4f5ef72 100644
--- a/mm/folio.c
+++ b/mm/folio.c
@@ -353,26 +353,28 @@ static void __lru_cache_activate_folio(struct folio *folio)
static void lru_gen_inc_refs(struct folio *folio)
{
- unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
+ unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
+ int refs;
if (folio_test_unevictable(folio))
return;
/* see the comment on LRU_REFS_FLAGS */
- if (!folio_test_referenced(folio)) {
- set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
+ if (!folio_lru_refs(folio)) {
+ folio_set_lru_refs(folio, 1);
return;
}
do {
- if ((old_flags & LRU_REFS_MASK) == LRU_REFS_MASK) {
+ new_flags = old_flags;
+ refs = lru_refs_from_flags(old_flags);
+ if (refs == LRU_REFS_MAX) {
if (!folio_test_workingset(folio))
folio_set_workingset(folio);
return;
}
-
- new_flags = old_flags + BIT(LRU_REFS_PGOFF);
- } while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
+ lru_refs_set_flags(&new_flags, refs + 1);
+ } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
}
static bool lru_gen_clear_refs(struct folio *folio)
@@ -384,7 +386,8 @@ static bool lru_gen_clear_refs(struct folio *folio)
if (gen < 0)
return true;
- set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS | BIT(PG_workingset), 0);
+ folio_set_lru_refs(folio, 0);
+ folio_clear_workingset(folio);
rcu_read_lock();
seq = READ_ONCE(folio_lruvec(folio)->lrugen.min_seq[type]);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c1404a59523d..080132997d87 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -843,19 +843,22 @@ static bool lru_gen_set_refs(struct folio *folio, const vma_flags_t *vma_flags)
if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
/* Activate file-backed executable folios after first usage. */
if (is_exec_file_folio(folio, vma_flags)) {
- set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_workingset));
+ folio_set_lru_refs(folio, 0);
+ folio_set_workingset(folio);
return true;
}
- set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
+ folio_set_lru_refs(folio, 1);
return false;
}
/* Promote on second access */
- if (folio_lru_refs(folio) > 1)
- set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_workingset));
- else
+ if (folio_lru_refs(folio) > 1) {
+ folio_set_lru_refs(folio, 0);
+ folio_set_workingset(folio);
+ } else {
folio_mark_accessed(folio);
+ }
return true;
}
#else
@@ -3266,11 +3269,10 @@ 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, const vma_flags_t *vma_flags)
+static int folio_update_gen(struct folio *folio, int new_gen, const vma_flags_t *vma_flags)
{
- unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
-
- VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
+ unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
+ int old_gen;
/*
* See the comment on LRU_REFS_FLAGS, and activate file-backed
@@ -3279,20 +3281,24 @@ static int folio_update_gen(struct folio *folio, int gen, const vma_flags_t *vma
*/
if (!folio_test_referenced(folio) && !folio_test_workingset(folio) &&
!is_exec_file_folio(folio, vma_flags)) {
- set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
+ folio_set_lru_refs(folio, 1);
return -1;
}
do {
+ old_gen = lru_gen_from_flags(old_flags);
+ new_flags = old_flags;
+
/* lru_gen_del_folio() has isolated this page? */
- if (!(old_flags & LRU_GEN_MASK))
- return -1;
+ if (old_gen < 0)
+ break;
- new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
- new_flags |= ((gen + 1UL) << LRU_GEN_PGOFF) | BIT(PG_workingset);
- } while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
+ lru_gen_set_flags(&new_flags, new_gen);
+ lru_refs_set_flags(&new_flags, 0);
+ new_flags |= BIT(PG_workingset);
+ } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
- return ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
+ return old_gen;
}
/* protect pages accessed multiple times through file descriptors */
@@ -3301,21 +3307,20 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
int type = folio_is_file_lru(folio);
struct lru_gen_folio *lrugen = &lruvec->lrugen;
int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
- unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
-
- VM_WARN_ON_ONCE_FOLIO(!(old_flags & LRU_GEN_MASK), folio);
+ unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
do {
- new_gen = ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
+ new_gen = lru_gen_from_flags(old_flags);
+
/* folio_update_gen() has promoted this page? */
if (new_gen >= 0 && new_gen != old_gen)
return new_gen;
+ new_flags = old_flags;
new_gen = (old_gen + 1) % MAX_NR_GENS;
-
- new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
- new_flags |= (new_gen + 1UL) << LRU_GEN_PGOFF;
- } while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
+ lru_gen_set_flags(&new_flags, new_gen);
+ lru_refs_set_flags(&new_flags, 0);
+ } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
lru_gen_update_size(lruvec, folio, old_gen, new_gen);
@@ -4711,7 +4716,7 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
/* see the comment on LRU_REFS_FLAGS */
if (!folio_test_referenced(folio))
- set_mask_bits(&folio->flags.f, LRU_REFS_MASK, 0);
+ folio_set_lru_refs(folio, 0);
success = lru_gen_del_folio(lruvec, folio, true);
VM_WARN_ON_ONCE_FOLIO(!success, folio);
@@ -4927,8 +4932,10 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
}
/* don't add rejected folios to the oldest generation */
- if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
- set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_active));
+ if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) {
+ folio_set_lru_refs(folio, 0);
+ folio_set_active(folio);
+ }
}
move_folios_to_lru(&list);
--
Best regards
Ridong