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

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Date:	Tue, 2 Apr 2013 14:56:10 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
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 Mon, Apr 01, 2013 at 10:13:58PM -0700, Hugh Dickins wrote:
> 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.

Hey, Hugh, Please don't do apology.
It gives me a chance to look into that part in detail.
It never wasted my time.

And your deep insight and kind advise always makes everybody happier.

Looking forward to seeing you soon in LSF/MM.
Thanks!

-- 
Kind regards,
Minchan Kim
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