linux-kernel - Re: [RFC] mm: remove swapcache page early

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Date:	Mon, 1 Apr 2013 22:13:58 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Minchan Kim <minchan@...nel.org>
cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Shaohua Li <shli@...nel.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Bob Liu <bob.liu@...cle.com>
Subject: Re: [RFC] mm: remove swapcache page early

On Tue, 2 Apr 2013, Minchan Kim wrote:
> On Fri, Mar 29, 2013 at 01:01:14PM -0700, Hugh Dickins wrote:
> > On Fri, 29 Mar 2013, Minchan Kim wrote:
> > > On Thu, Mar 28, 2013 at 11:19:12AM -0700, Dan Magenheimer wrote:
> > > > 
> > > > I wonder if something like this would have a similar result for zram?
> > > > (Completely untested... snippet stolen from swap_entry_free with
> > > > SetPageDirty added... doesn't compile yet, but should give you the idea.)
> > 
> > Be careful, although Dan is right that something like this can be
> > done for zram, I believe you will find that it needs a little more:
> > either a separate new entry point (not my preference) or a flags arg
> > (or boolean) added to swap_slot_free_notify.
> > 
> > Because this is a different operation: end_swap_bio_read() wants
> > to free up zram's compressed copy of the page, but the swp_entry_t
> > must remain valid until swap_entry_free() can clear up the rest.
> > Precisely how much of the work each should do, you will discover.
> 
> First of all, Thanks for noticing it for me!
> 
> If I parse your concern correctly, you are concerning about
> different semantic on two functions.
> (end_swap_bio_read's swap_slot_free_notify VS swap_entry_free's one).
> 
> But current implementatoin on zram_slot_free_notify could cover both cases
> properly with luck.
> 
> zram_free_page caused by end_swap_bio_read will free compressed copy
> of the page and zram_free_page caused by swap_entry_free later won't find
> right index from zram->table and just return.
> So I think there is no problem.
> 
> Remained problem is zram->stats.notify_free, which could be counted
> redundantly but not sure it's valuable to count exactly.
> 
> If I miss your point, please pinpoint your concern. :)

Looking at it again, I do believe you and Dan are perfectly correct,
and I was again the confused one.  Though I'd be happier if I could
see just how I was misreading it: makes me wonder if I had a great
insight that I can no longer grasp hold of!  I think I was paranoid
about a swp_entry_t getting recycled prematurely: but swap_entry_free
remains in control of that - freeing a swap entry is no part of what
notify_free gets up to.  Sorry for wasting your time.

Hugh
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