[PATCH 11/11] mm/zsmalloc: Handle charge migration in zpdesc substitution

Joshua Hahn
2026-03-11T19:52:23Z

In zsmalloc, there are two types of migrations: Migrations of single
compressed objects from one zspage to another, and substitutions of
zpdescs from zspages.

In both of these migrations, memcg association for the compressed
objects do not change. However, the physical location of the compressed
objects may change, which alters their lruvec association.

In this patch, handle the substitution of zpdescs from zspages, which
may change the node of all objects present (wholly or partially).

Take special care to address the partial compressed object at the
beginning of the swapped out zpdesc. "Ownership" of spanning objects
are associated to the zpdesc it begins on. Thus, when handling the
first compressed object, we must iterate through the (up to 4)
zpdescs present in the zspage to find the previous zpdesc, then
retrieve the object's zspage-wide index.

For the same reason, pool->uncompressed_stat, which can only be
accounted at PAGE_SIZE granularity for the node statistics, are
accounted for objects beginning in the zpdesc.

Likewise for the spanning object at the end of the replaced zpdesc,
account only the amount that lives on the zpdesc.

Note that these operations cannot call the existing
zs_{charge, uncharge}_objcg functions we introduced, since we are
holding the class spin lock and obj_cgroup_charge can sleep.

Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
mm/zsmalloc.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 92 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f3508ff8b3ab..a4c90447d28e 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1932,6 +1932,94 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
return page_zpdesc(page)->zspage;
}

+#ifdef CONFIG_MEMCG
+static void zs_migrate_lruvec(struct zs_pool *pool, struct obj_cgroup *objcg,
+ int old_nid, int new_nid, int charge,
+ int obj_size)
+{
+ struct mem_cgroup *memcg;
+ struct lruvec *old_lruvec, *new_lruvec;
+ int partial;
+
+ if (old_nid == new_nid || !objcg)
+ return;
+
+ /* Proportional (partial) uncompressed share for this portion */
+ partial = (PAGE_SIZE * charge) / obj_size;
+
+ rcu_read_lock();
+ memcg = obj_cgroup_memcg(objcg);
+ old_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(old_nid));
+ new_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(new_nid));
+
+ mod_memcg_lruvec_state(old_lruvec, pool->compressed_stat, -charge);
+ mod_memcg_lruvec_state(new_lruvec, pool->compressed_stat, charge);
+
+ mod_memcg_lruvec_state(old_lruvec, pool->uncompressed_stat, -partial);
+ mod_memcg_lruvec_state(new_lruvec, pool->uncompressed_stat, partial);
+ rcu_read_unlock();
+}
+
+/*
+ * Transfer per-lruvec and node-level stats when a zspage replaces a zpdesc
+ * with one from a different NUMA node. Must be called while old_zpdesc is
+ * still linked to the zspage. memcg-level charges are unchanged.
+ */
+static void zs_page_migrate_lruvec(struct zs_pool *pool, struct zspage *zspage,
+ struct zpdesc *old_zpdesc,
+ struct zpdesc *new_zpdesc,
+ struct size_class *class)
+{
+ int size = class->size;
+ int old_nid = page_to_nid(zpdesc_page(old_zpdesc));
+ int new_nid = page_to_nid(zpdesc_page(new_zpdesc));
+ unsigned int off, first_obj_offset, page_offset = 0;
+ unsigned int idx;
+ struct zpdesc *cursor = zspage->first_zpdesc;
+
+ if (old_nid == new_nid)
+ return;
+
+ while (cursor != old_zpdesc) {
+ cursor = get_next_zpdesc(cursor);
+ page_offset += PAGE_SIZE;
+ }
+
+ first_obj_offset = get_first_obj_offset(old_zpdesc);
+ idx = (page_offset + first_obj_offset) / size;
+
+ /* Boundary object spaning from the previous zpdesc*/
+ if (idx > 0 && zspage->objcgs[idx - 1])
+ zs_migrate_lruvec(pool, zspage->objcgs[idx - 1],
+ old_nid, new_nid, first_obj_offset, size);
+
+ for (off = first_obj_offset;
+ off < PAGE_SIZE && idx < class->objs_per_zspage;
+ idx++, off += size) {
+ struct obj_cgroup *objcg = zspage->objcgs[idx];
+ int bytes_on_page = min_t(int, size, PAGE_SIZE - off);
+
+ if (!objcg)
+ continue;
+
+ zs_migrate_lruvec(pool, objcg, old_nid, new_nid,
+ bytes_on_page, size);
+
+ dec_node_page_state(zpdesc_page(old_zpdesc),
+ pool->uncompressed_stat);
+ inc_node_page_state(zpdesc_page(new_zpdesc),
+ pool->uncompressed_stat);
+ }
+}
+#else
+static void zs_page_migrate_lruvec(struct zs_pool *pool, struct zspage *zspage,
+ struct zpdesc *old_zpdesc,
+ struct zpdesc *new_zpdesc,
+ struct size_class *class)
+{
+}
+#endif
+
static int zs_page_migrate(struct page *newpage, struct page *page,
enum migrate_mode mode)
{
@@ -2004,6 +2092,10 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
}
kunmap_local(s_addr);

+ /* Transfer lruvec/node stats while old zpdesc is still linked */
+ if (pool->memcg_aware)
+ zs_page_migrate_lruvec(pool, zspage, zpdesc, newzpdesc, class);
+
replace_sub_page(class, zspage, newzpdesc, zpdesc);
/*
* Since we complete the data copy and set up new zspage structure,
--
2.52.0