Re: [PATCH 1/2] memcg: move mem_cgroup_swappiness to memcontrol.h

[Ridong Chen <ridong.chen@linux.dev>]

On 7/14/2026 9:46 PM, Barry Song wrote:
> On Tue, Jul 14, 2026 at 7:31 PM Ridong Chen <ridong.chen@linux.dev> wrote:
> 
> [...]
>>>>>>
>>>>>> 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@gmail.com/
>>>>
>>>> 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?
>> 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?
> 
> We also have mm/swap.h, which is dedicated to exporting
> swap-related things. Is it a better place than mm/internal.h?
> 

Placing it in mm/swap.h is fine with me. Will update.

```
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -4,6 +4,7 @@

  #include <linux/atomic.h> /* for atomic_long_t */
  #include <linux/mm.h> /* for PAGE_SHIFT */
+#include <linux/memcontrol.h>
  struct mempolicy;
  struct swap_iocb;
  struct swap_memcg_table;
@@ -76,6 +77,25 @@ enum swap_cluster_flags {
         CLUSTER_FLAG_MAX,
  };

+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