Re: list_lru isolate callback question?

[Kairui Song <ryncsn@gmail.com>]

On Thu, Jun 5, 2025 at 10:17 AM Dave Airlie <airlied@gmail.com> wrote:
>
> I've hit a case where I think it might be valuable to have the nid +
> struct memcg for the item being iterated available in the isolate
> callback, I know in theory we should be able to retrieve it from the
> item, but I'm also not convinced we should need to since we have it
> already in the outer function?
>
> typedef enum lru_status (*list_lru_walk_cb)(struct list_head *item,
>                         struct list_lru_one *list,
>                         int nid,
>                         struct mem_cgroup *memcg,
>                         void *cb_arg);
>

Hi Dave,

> It's probably not essential (I think I can get the nid back easily,
> not sure about the memcg yet), but I thought I'd ask if there would be

If it's a slab object you should be able to get it easily with:
memcg = mem_cgroup_from_slab_obj(item));
nid = page_to_nid(virt_to_page(item));

> resistance against just adding them to the callback?

I'm not sure about the context here, I personally prefer to keep the
function minimized unless necessary, so things like !CONFIG_MEMCG or
single node builds won't have two dummy parameters here, and
most caller won't need them, the compiler can't optimize that
out IIUC.

>
> Dave.
>