Re: Questin about swap_slot free and invalidate page

[Ric Mason]

On 02/19/2013 11:27 PM, Dan Magenheimer wrote:
> > From: Ric Mason [mailto:ric.masonn@xxxxxxxxx]
> > 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@xxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; 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@xxxxxxxxxx]
> > > > Sent: Sunday, February 03, 2013 7:50 PM
> > > > To: Hugh Dickins
> > > > Cc: Nitin Gupta; Dan Magenheimer; Seth Jennings; Konrad Rzeszutek Wilk; linux-mm@xxxxxxxxx; linux-
> > > > kernel@xxxxxxxxxxxxxxx; 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/

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.

>   
> > > 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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