linux-kernel - RE: Questin about swap_slot free and invalidate page

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Message-ID: <622b6670-f311-40d5-9c8d-f5dd3e03633c@default>
Date:	Mon, 25 Feb 2013 09:20:55 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Ric Mason <ric.masonn@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	Nitin Gupta <ngupta@...are.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: Questin about swap_slot free and invalidate page

> From: Ric Mason [mailto:ric.masonn@...il.com]
> Subject: Re: Questin about swap_slot free and invalidate page
> 
> On 02/22/2013 05:42 AM, Dan Magenheimer wrote:
> >> From: Ric Mason [mailto:ric.masonn@...il.com]
> >> Subject: Re: Questin about swap_slot free and invalidate page
> >>
> >> On 02/19/2013 11:27 PM, Dan Magenheimer wrote:
> >>>> From: Ric Mason [mailto:ric.masonn@...il.com]
> >>>>> Hugh is right that handling the possibility of duplicates is
> >>>>> part of the tmem ABI.  If there is any possibility of duplicates,
> >>>>> the ABI defines how a backend must handle them to avoid data
> >>>>> coherency issues.
> >>>>>
> >>>>> The kernel implements an in-kernel API which implements the tmem
> >>>>> ABI.  If the frontend and backend can always agree that duplicate
> >>>> Which ABI in zcache implement that?
> >>> https://oss.oracle.com/projects/tmem/dist/documentation/api/tmemspec-v001.pdf
> >>>
> >>> The in-kernel APIs are frontswap and cleancache.  For more information about
> >>> tmem, see http://lwn.net/Articles/454795/
> >> But you mentioned that you have in-kernel API which can handle
> >> duplicate.  Do you mean zcache_cleancache/frontswap_put_page? I think
> >> they just overwrite instead of optional flush the page on the
> >> second(duplicate) put as mentioned in your tmemspec.
> > Maybe I am misunderstanding your question...  The spec allows
> > overwrite (and return success) OR flush the page (and return
> > failure).  Zcache does the latter (flush).  The code that implements
> > it is in tmem_put.
> 
> Thanks for your point out.  Pers pages can have duplicate put since swap
> cache page can be reused. Can eph pages also have duplicate put? If yes,
> when can happen?

Yes, I have seen it.  I am not sure of the exact circumstances
when it happens as I am not an expert in the VFS subsystem.
(Chris Mason wrote the VFS cleancache hooks in 2009.)

Dan
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