Re: list_lru isolate callback question?

[Dave Airlie <airlied@gmail.com>]

On Thu, 5 Jun 2025 at 17:55, Kairui Song <ryncsn@gmail.com> wrote:
>
> 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));
>

It's in relation to some work trying to tie GPU system memory
allocations into memcg properly,

Not slab objects, but I do have pages so I'm using page_to_nid right now,
however these pages aren't currently setting p->memcg_data as I don't
need that for this, but maybe
this gives me a reason to go down that road.

> > 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.

I reconsidered and was wondering if

struct lru_walk_args {
    struct list_lru_one *list;
    int nid;
    struct mem_cgroup *memcg;
}
would also be an option instead of adding two unused args.

But I'll see if I can make it work once I get the memcg pieces of the
puzzle sorted out.

Dave.
>
> >
> > Dave.
> >