RE: Questin about swap_slot free and invalidate page

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Ric Mason [mailto:ric.masonn@gmail.com]
> Sent: Tuesday, February 19, 2013 5:12 AM
> To: Dan Magenheimer
> Cc: Minchan Kim; Hugh Dickins; Nitin Gupta; Seth Jennings; Konrad Rzeszutek Wilk; linux-mm@kvack.org;
> linux-kernel@vger.kernel.org; Andrew Morton
> Subject: Re: Questin about swap_slot free and invalidate page
> 
> On 02/05/2013 05:28 AM, Dan Magenheimer wrote:
> >> From: Minchan Kim [mailto:minchan@kernel.org]
> >> Sent: Sunday, February 03, 2013 7:50 PM
> >> To: Hugh Dickins
> >> Cc: Nitin Gupta; Dan Magenheimer; Seth Jennings; Konrad Rzeszutek Wilk; linux-mm@kvack.org; linux-
> >> kernel@vger.kernel.org; Andrew Morton
> >> Subject: Re: Questin about swap_slot free and invalidate page
> >>
> >> Hi Hugh,
> >>
> >> On Sun, Feb 03, 2013 at 05:51:14PM -0800, Hugh Dickins wrote:
> >>> On Thu, 31 Jan 2013, Minchan Kim wrote:
> >>>
> >>>> When I reviewed zswap, I was curious about frontswap_store.
> >>>> It said following as.
> >>>>
> >>>>   * If frontswap already contains a page with matching swaptype and
> >>>>   * offset, the frontswap implementation may either overwrite the data and
> >>>>   * return success or invalidate the page from frontswap and return failure.
> >>>>
> >>>> It didn't say why it happens. we already have __frontswap_invalidate_page
> >>>> and call it whenever swap_slot frees. If we don't free swap slot,
> >>>> scan_swap_map can't find the slot for swap out so I thought overwriting of
> >>>> data shouldn't happen in frontswap.
> >>>>
> >> I am waiting Dan's reply(He will come in this week) and then, judge what's
> >> the best.
> > 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/ 
 
> > are never possible, I agree that the backend could avoid that
> > special case.  However, duplicates occur rarely enough and the
> > consequences (data loss) are bad enough that I think the case
> > should still be checked, at least with a BUG_ON.  I also wonder
> > if it is worth it to make changes to the core swap subsystem
> > to avoid code to implement a zswap corner case.
> >
> > Remember that zswap is an oversimplified special case of tmem
> > that handles only one frontend (Linux frontswap) and one backend
> > (zswap).  Tmem goes well beyond that and already supports other
> > more general backends including Xen and ramster, and could also
> > support other frontends such as a BSD or Solaris equivalent
> > of frontswap, for example with a Linux ramster/zcache backend.
> > I'm not sure how wise it is to tear out generic code and replace
> > it with simplistic code unless there is absolutely no chance that
> > the generic code will be necessary.

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