[PATCH 10/11] mm/zsmalloc: Handle single object charge migration in migrate_zspage

Joshua Hahn
2026-03-11T19:52:20Z

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 single compressed object migration and
transfer lruvec and node statistics across the affected lruvecs / nodes.

Zsmalloc compressed objects, like slab objects, can span two pages.
When a spanning object is migrated, possibly to another zspage where
it spans two zpdescs, up to 4 nodes can be touched.

Instead of enumerating all possible combinations of node migrations,
simply uncharge entirely from the source (1 or 2 nodes) and charge
entirely to the destination (1 or 2 nodes).

s_off d_off
v v
----------+ +---- -----+ +---------
... ooo ooo xx| |x oo ... --> ... ooo x| |xx ooo oo ...
----------+ +---- -----+ +---------
pg1 pg2 pg3 pg4

s_zspage d_zspage

To do this, calculate how much of the compressed object lives on each
page and perform up to 4 uncharge-charges.

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 | 74 ++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 70 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index ab085961b0e2..f3508ff8b3ab 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1684,15 +1684,81 @@ static unsigned long find_alloced_obj(struct size_class *class,
return handle;
}

+#ifdef CONFIG_MEMCG
static void zs_migrate_objcg(struct zspage *s_zspage, struct zspage *d_zspage,
- unsigned long used_obj, unsigned long free_obj)
+ unsigned long used_obj, unsigned long free_obj,
+ struct zs_pool *pool, int size)
{
- unsigned int s_idx = used_obj & OBJ_INDEX_MASK;
- unsigned int d_idx = free_obj & OBJ_INDEX_MASK;
+ struct zpdesc *s_zpdesc, *d_zpdesc;
+ struct obj_cgroup *objcg;
+ struct mem_cgroup *memcg;
+ struct lruvec *l;
+ unsigned int s_idx, d_idx;
+ unsigned int s_off, d_off;
+ int charges[4], nids[4], partial;
+ int s_bytes_in_page, d_bytes_in_page;
+ int i;
+
+ if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ goto out;
+
+ obj_to_location(used_obj, &s_zpdesc, &s_idx);
+ obj_to_location(free_obj, &d_zpdesc, &d_idx);
+
+ objcg = s_zspage->objcgs[s_idx];
+ if (!objcg)
+ goto out;
+
+ /*
+ * The object migration here can touch up to 4 nodes.
+ * Instead of breaking down all possible combinations of node changes,
+ * just uncharge entirely from the source and charge entirely to the
+ * destination, even if there is are node overlaps between src and dst.
+ */
+ s_off = (s_idx * size) % PAGE_SIZE;
+ d_off = (d_idx * size) % PAGE_SIZE;
+ s_bytes_in_page = min_t(int, size, PAGE_SIZE - s_off);
+ d_bytes_in_page = min_t(int, size, PAGE_SIZE - d_off);
+
+ charges[0] = -s_bytes_in_page;
+ nids[0] = page_to_nid(zpdesc_page(s_zpdesc));
+ charges[1] = -(size - s_bytes_in_page); /* 0 if object doesn't span */
+ if (charges[1])
+ nids[1] = page_to_nid(zpdesc_page(get_next_zpdesc(s_zpdesc)));
+
+ charges[2] = d_bytes_in_page;
+ nids[2] = page_to_nid(zpdesc_page(d_zpdesc));
+ charges[3] = size - d_bytes_in_page; /* 0 if object doesn't span */
+ if (charges[3])
+ nids[3] = page_to_nid(zpdesc_page(get_next_zpdesc(d_zpdesc)));

+ rcu_read_lock();
+ memcg = obj_cgroup_memcg(objcg);
+ for (i = 0; i < 4; i++) {
+ if (!charges[i])
+ continue;
+
+ l = mem_cgroup_lruvec(memcg, NODE_DATA(nids[i]));
+ partial = (PAGE_SIZE * charges[i]) / size;
+ mod_memcg_lruvec_state(l, pool->compressed_stat, charges[i]);
+ mod_memcg_lruvec_state(l, pool->uncompressed_stat, partial);
+ }
+ rcu_read_unlock();
+
+ dec_node_page_state(zpdesc_page(s_zpdesc), pool->uncompressed_stat);
+ inc_node_page_state(zpdesc_page(d_zpdesc), pool->uncompressed_stat);
+
+out:
d_zspage->objcgs[d_idx] = s_zspage->objcgs[s_idx];
s_zspage->objcgs[s_idx] = NULL;
}
+#else
+static void zs_migrate_objcg(struct zspage *s_zspage, struct zspage *d_zspage,
+ unsigned long used_obj, unsigned long free_obj,
+ struct zs_pool *pool, int size)
+{
+}
+#endif

static void migrate_zspage(struct zs_pool *pool, struct zspage *src_zspage,
struct zspage *dst_zspage)
@@ -1719,7 +1785,7 @@ static void migrate_zspage(struct zs_pool *pool, struct zspage *src_zspage,

if (pool->memcg_aware)
zs_migrate_objcg(src_zspage, dst_zspage,
- used_obj, free_obj);
+ used_obj, free_obj, pool, class->size);

obj_idx++;
obj_free(class->size, used_obj);
--
2.52.0