Re: [RFC PATCH v2] mm: vmscan: avoid anon scanning for GFP_NOIO with low swapcache in MGLRU
From: Baolin Wang
Date: Thu Oct 22 2026 - 07:26:46 EST
On 10/22/26 6:27 PM, Barry Song (Xiaomi) wrote:
On Thu, Oct 22, 2026 at 4:49 PM Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 10/22/26 5:55 AM, Barry Song wrote:
On Wed, Oct 21, 2026 at 3:28 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
[...]
static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
{
unsigned long total;
int swappiness = get_swappiness(lruvec, sc);
struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+ swappiness = get_scan_swappiness(lruvec, sc, swappiness);
+
total = lruvec_evictable_size(lruvec, swappiness);
/* whether the size is big enough to be helpful */
@@ -5249,9 +5265,10 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
bool need_rotate = false, should_age = false;
long nr_batch, nr_to_scan;
int swappiness = get_swappiness(lruvec, sc);
+ int scan_swappiness = get_scan_swappiness(lruvec, sc, swappiness);
IMO, 'swappiness' and 'scan_swappiness' are really confusing. Why move
this logic into get_swappiness()? In get_swappiness(), we already have
some similar logic to adjust swappiness when there is no available swap
space.
Yes. It is very confusing. we can't do that in get_swappiness() as otherwise,
we will get cold/hot inversion due to GFP_NOIO. for a real swappiness=0,
we allow cold/hot inversion for anon, but for GFP_NOIO, this is not the
case.
But can_reclaim_anon_pages() doesn't only handle the GFP_NOIO case, it
also covers the swap space limitation scenario. The latter is what
concerns me more.
If we're only talking about the GFP_NOIO case, I'd agree with your
concern. However, we could introduce a helper to force anon aging and
add comments explaining why we need this. That would look more
maintainable than 2 confusing swappiness variables, no?
Just my 2 cents.
Something like below (untested):
diff --git a/mm/vmscan.c b/mm/vmscan.c
index daa7e37c2236..7c1d26e6b9d7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2890,7 +2890,7 @@ static int get_swappiness(struct lruvec *lruvec,
struct scan_control *sc)
if (swappiness == SWAPPINESS_ANON_ONLY)
return swappiness;
- if (!sc->may_swap)
+ if (!sc->may_swap || !(sc->gfp_mask & __GFP_IO))
return 0;
For GFP_NOIO, we should also check whether swapcache is low, since
GFP_NOIO can reclaim swapcache. We already have
reclaimable_anon_is_low() for this.
Ah, yes, just a quick sample to show what I had in mind. :). Sould also consider reclaimable_anon_is_low() for the GFP_NOIO case.
if (!can_demote(pgdat->node_id, sc, memcg) &&
@@ -4049,7 +4049,7 @@ static inline void flush_lru_batch(struct
list_head *head, struct list_head **ba
*batch_end = NULL;
}
-static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
+static bool inc_min_seq(struct lruvec *lruvec, int type, int
swappiness, bool force_anon_aging)
{
int zone;
int remaining = MAX_LRU_BATCH;
@@ -4063,7 +4063,7 @@ static bool inc_min_seq(struct lruvec *lruvec, int
type, int swappiness)
goto done;
/* For anon type, skip the check if swappiness is zero (file
only) */
- if (!type && !swappiness)
+ if (!type && !swappiness && !force_anon_aging)
goto done;
VM_WARN_ON_ONCE(get_nr_gens(lruvec, type) != MAX_NR_GENS);
@@ -4183,7 +4183,20 @@ static void try_to_inc_min_seq(struct lruvec
*lruvec, int swappiness)
}
}
-static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int
swappiness)
+static bool is_force_anon_aging(struct scan_control *sc, struct lruvec
*lruvec,
+ int swappiness)
+{
+ if (swappiness || (sc->gfp_mask & __GFP_IO))
+ return false;
As above, GFP_NOIO doesn't imply swappiness=0 if there are many
swapcache folios to reclaim. So we should check
reclaimable_anon_is_low() here.
+
+ /*
+ * Explain why we neeed to force increase anon min seq.
+ * xxxxxx
+ */
+ return swappiness != sc_swappiness(sc, lruvec_memcg(lruvec));
+}
+
+static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int
swappiness, bool force_anon_aging)
{
bool success;
int prev, next;
@@ -4205,7 +4218,7 @@ static bool inc_max_seq(struct lruvec *lruvec,
unsigned long seq, int swappiness
if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
continue;
- if (inc_min_seq(lruvec, type, swappiness))
+ if (inc_min_seq(lruvec, type, swappiness, force_anon_aging))
continue;
lruvec_unlock_irq(lruvec);
@@ -4249,7 +4262,7 @@ static bool inc_max_seq(struct lruvec *lruvec,
unsigned long seq, int swappiness
}
static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long seq,
- int swappiness, bool force_scan)
+ int swappiness, bool force_scan, bool
force_anon_aging)
{
bool success;
struct lru_gen_mm_walk *walk;
@@ -4260,7 +4273,7 @@ static bool try_to_inc_max_seq(struct lruvec
*lruvec, unsigned long seq,
VM_WARN_ON_ONCE(seq > READ_ONCE(lrugen->max_seq));
if (!mm_state)
- return inc_max_seq(lruvec, seq, swappiness);
+ return inc_max_seq(lruvec, seq, swappiness,
force_anon_aging);
/* see the comment in iterate_mm_list() */
if (seq <= READ_ONCE(mm_state->seq))
@@ -4295,7 +4308,7 @@ static bool try_to_inc_max_seq(struct lruvec
*lruvec, unsigned long seq,
} while (mm);
done:
if (success) {
- success = inc_max_seq(lruvec, seq, swappiness);
+ success = inc_max_seq(lruvec, seq, swappiness,
force_anon_aging);
WARN_ON_ONCE(!success);
}
@@ -5262,7 +5275,9 @@ static bool try_to_shrink_lruvec(struct lruvec
*lruvec, struct scan_control *sc)
}
if (should_run_aging(lruvec, max_seq, sc, swappiness)) {
- if (try_to_inc_max_seq(lruvec, max_seq,
swappiness, false))
This is quite tricky. If we pass swappiness=0, anon aging will be
skipped. Do we want that for GFP_NOIO cases? I mean, these are not
real cases where anon aging should be disabled.
No, we have real use cases where 'vm_swappiness' is set to 0, which means the workload never reclaims anon folios. So in this case we don't need to age anon folios.
static int should_skip_vma(unsigned long start, unsigned long end, struct mm_walk *args)
{
...
if (vma == get_gate_vma(vma->vm_mm))
return true;
...
}
+ bool force_anon_aging = is_force_anon_aging(sc,
lruvec, swappiness);
Adding a force_anon_aging argument to inc_max_seq() seems like
quite a burden. Maybe a cleaner approach is to pass sc all the
way to inc_max_seq() and let it detect GFP_NOIO cases itself?
OK, still look good to me.
BTW, I guess we don't need to worry about swappiness at all. We can
simply bypass anon reclaim?
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fc823fe3bd1f..fa6d9860156d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4903,6 +4903,8 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
int skipped = 0;
unsigned long remaining = nr_to_scan;
struct lru_gen_folio *lrugen = &lruvec->lrugen;
+ struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
VM_WARN_ON_ONCE(!list_empty(list));
@@ -4910,6 +4912,9 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
return 0;
+ if (!type && reclaimable_anon_is_low(memcg, pgdat->node_id, sc))
+ return 0;
+
gen = lru_gen_from_seq(lrugen->min_seq[type]);
for (i = MAX_NR_ZONES; i > 0; i--) {
@@ -5164,12 +5169,19 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
struct scan_control *sc, int swappiness)
{
+ struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
DEFINE_MIN_SEQ(lruvec);
/* have to run aging, since eviction is not possible anymore */
if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
return true;
+ /* run aging when GFP_NOIO limits anon reclaim with few swapcache folios */
+ if (swappiness && min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq &&
+ reclaimable_anon_is_low(memcg, pgdat->node_id, sc))
+ return true;
I've considered this, but we'd still need to add some code in get_nr_to_scan()/lruvec_is_sizable() to filter out the anon folios for GFP_NOIO case. Rather than spreading the logic across multiple places, I'd prefer adding a helper with proper comments to handle the aging issue, like the sample code I showed.