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



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