Re: [PATCH 2/4] send callback when swap slot is freed

Pekka Enberg
2009-09-18T07:55:28Z

Hi Hugh,

On Fri, 2009-09-18 at 08:17 +0100, Hugh Dickins wrote:
> On Fri, 18 Sep 2009, Pekka Enberg wrote:
> > On Fri, 2009-09-18 at 04:13 +0530, Nitin Gupta wrote:
> > > +EXPORT_SYMBOL_GPL(set_swap_free_notify);
> > > +
> > > static int swap_entry_free(struct swap_info_struct *p,
> > > swp_entry_t ent, int cache)
> > > {
> > > @@ -585,6 +617,8 @@ static int swap_entry_free(struct swap_info_struct *p,
> > > swap_list.next = p - swap_info;
> > > nr_swap_pages++;
> > > p->inuse_pages--;
> > > + if (p->swap_free_notify_fn)
> > > + p->swap_free_notify_fn(p->bdev, offset);
> > > }
> > > if (!swap_count(count))
> > > mem_cgroup_uncharge_swap(ent);
> >
> > OK, this hits core kernel code so we need to CC some more mm/swapfile.c
> > people. The set_swap_free_notify() API looks strange to me. Hugh, I
> > think you mentioned that you're okay with an explicit hook. Any
> > suggestions how to do this cleanly?
>
> No, no better suggestion. I quite see Nitin's point that ramzswap
> would benefit significantly from a callback here, though it's not a
> place (holding swap_lock) where we'd like to offer a callback at all.
>
> I think I would prefer the naming to make it absolutely clear that
> it's a special for ramzswap or compcache, rather than dressing it
> up in the grand generality of a swap_free_notify_fn: giving our
> hacks fancy names doesn't really make them better.
>
> (Does the bdev matching work out if there are any regular swapfiles
> around? I've not checked, might or might not need refinement there.)

The *hook* looks OK to me but set_swap_free_notify() looks like an ugly
hack. I don't understand why we're setting up the hook lazily in
ramzswap_read() nor do I understand why we need to look up struct
swap_info_struct with a bdev. Surely there's a cleaner way to do all
this? Probably somewhere in sys_swapon()?

Pekka