Re: [PATCH 1/2] memcg: move mem_cgroup_swappiness to memcontrol.h
From: Ridong Chen
Date: Tue Jul 14 2026 - 07:31:43 EST
On 7/14/2026 6:21 PM, Barry Song wrote:
On Tue, Jul 14, 2026 at 3:43 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:I noticed that some swap-related macros are already defined in mm/internal.h, so I'm planning to place the code right after them for better grouping. Does that work for you?
On 7/14/2026 9:48 AM, Barry Song wrote:
On Tue, Jul 14, 2026 at 9:43 AM Barry Song <baohua@xxxxxxxxxx> wrote:
On Tue, Jul 14, 2026 at 9:20 AM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
On 7/13/2026 11:08 PM, Barry Song wrote:
On Sat, Jul 11, 2026 at 5:12 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:Thank you for your reply.
From: Ridong Chen <chenridong@xxxxxxxxxx>
The per-memcg swappiness knob is v1-only; v2 always uses global
vm_swappiness and ignores the per-cgroup field.
Guard memcg->swappiness with CONFIG_MEMCG_V1, and move the helper
to memcontrol.h where it belongs.
No functional change for v1; v2-only kernels drop the unused field.
Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
With some nits.
---[...]
struct mem_cgroup_per_node *nodeinfo[];
@@ -365,6 +366,9 @@ enum objext_flags {
#define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
+/* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
+extern int vm_swappiness;
This is a bit unusual. I'm not sure whether mm/swap.h would be
a more appropriate place for this.
The vm_swappiness variable is not utilized within mm/swap.c.
Furthermore, since memcontrol.h does not include swap.h, retaining the
extern int vm_swappiness declaration in mm/swap.h will result in a
compilation failure.
If this is the case, it still seems better to keep
extern int vm_swappiness in include/linux/swap.h.
Then we don't need the comment:
/* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
It also makes it clearer that vm_swappiness is an extern variable
belonging to the swap module, rather than the memcontrol module.
BTW, if mem_c_group_swappiness() and vm_swappiness are only used
within mm/, could all of these be moved to mm/swap.h and
mm/internal.h instead?
We are making a big effort to move many unrelated things out of
include/linux/swap.h recently. Could you check?
https://lore.kernel.org/linux-mm/20260708-ch-swap-series-plus-folio-lru-cleanup-v9-0-2bc72b4f8730@xxxxxxxxx/
Good suggestion. Moving them to mm/internal.h makes sense. Will update
in the next version.
Either mm/swap.h or mm/internal.h.
vm_swappiness probably belongs in mm/swap.h rather than
mm/internal.h, right?
BTW, I am not particularly eager about this cleanup;
it could be done later as a separate patch.
If you decide not to do the cleanup, I think it would be better to
leave "extern int vm_swappiness" in include/linux/swap.h rather than
declaring it in include/linux/memcontrol.h?
```
...
#define MEMCG_RECLAIM_MAY_SWAP (1 << 1)
#define MEMCG_RECLAIM_PROACTIVE (1 << 2)
#define MIN_SWAPPINESS 0
#define MAX_SWAPPINESS 200
/* Just reclaim from anon folios in proactive memory reclaim */
#define SWAPPINESS_ANON_ONLY (MAX_SWAPPINESS + 1)
extern int vm_swappiness;
static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
{
#ifdef CONFIG_MEMCG_V1
/* Cgroup2 doesn't have per-cgroup swappiness */
if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
return READ_ONCE(vm_swappiness);
/* root ? */
if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
return READ_ONCE(vm_swappiness);
return READ_ONCE(memcg->swappiness);
#else
return READ_ONCE(vm_swappiness);
#endif
}
...
```
--
Best regards
Ridong