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

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Message-Id: <1253260528.4959.13.camel@penberg-laptop>
Date:	Fri, 18 Sep 2009 10:55:28 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Nitin Gupta <ngupta@...are.org>, Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ed Tomlinson <edt@....ca>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-mm-cc <linux-mm-cc@...top.org>,
	kamezawa.hiroyu@...fujitsu.com, nishimura@....nes.nec.co.jp
Subject: Re: [PATCH 2/4] send callback when swap slot is freed

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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