Re: [PATCH] staging/zcache: Fix/improve zcache writeback code, tieto a config option

[Ric Mason]

On 02/26/2013 01:29 AM, Dan Magenheimer wrote:
> > From: Ric Mason [mailto:ric.masonn@xxxxxxxxx]
> > Subject: Re: [PATCH] staging/zcache: Fix/improve zcache writeback code, tie to a config option
> >
> > On 02/07/2013 02:27 AM, Dan Magenheimer wrote:
> > > It was observed by Andrea Arcangeli in 2011 that zcache can get "full"
> > > and there must be some way for compressed swap pages to be (uncompressed
> > > and then) sent through to the backing swap disk.  A prototype of this
> > > functionality, called "unuse", was added in 2012 as part of a major update
> > > to zcache (aka "zcache2"), but was left unfinished due to the unfortunate
> > > temporary fork of zcache.
> > >
> > > This earlier version of the code had an unresolved memory leak
> > > and was anyway dependent on not-yet-upstream frontswap and mm changes.
> > > The code was meanwhile adapted by Seth Jennings for similar
> > > functionality in zswap (which he calls "flush").  Seth also made some
> > > clever simplifications which are herein ported back to zcache.  As a
> > > result of those simplifications, the frontswap changes are no longer
> > > necessary, but a slightly different (and simpler) set of mm changes are
> > > still required [1].  The memory leak is also fixed.
> > >
> > > Due to feedback from akpm in a zswap thread, this functionality in zcache
> > > has now been renamed from "unuse" to "writeback".
> > >
> > > Although this zcache writeback code now works, there are open questions
> > > as how best to handle the policy that drives it.  As a result, this
> > > patch also ties writeback to a new config option.  And, since the
> > > code still depends on not-yet-upstreamed mm patches, to avoid build
> > > problems, the config option added by this patch temporarily depends
> > > on "BROKEN"; this config dependency can be removed in trees that
> > > contain the necessary mm patches.
> > >
> > > [1] https://lkml.org/lkml/2013/1/29/540/ https://lkml.org/lkml/2013/1/29/539/
> > This patch leads to backend interact with core mm directly,  is it core
> > mm should interact with frontend instead of backend? In addition,
> > frontswap has already have shrink funtion, should we can take advantage
> > of it?
> Good questions!
>
> If you have ideas (or patches) that handle the interaction with
> the frontend instead of backend, we can take a look at them.
> But for zcache (and zswap), the backend already interacts with
> the core mm, for example to allocate and free pageframes.
>
> The existing frontswap shrink function cause data pages to be sucked
> back from the backend.  The data pages are put back in the swapcache
> and they aren't marked in any way so it is possible the data page
> might soon (or immediately) be sent back to the backend.

Then can frontswap shrink work well?

> This code is used for backends that can't "callback" the frontend, such
> as the Xen tmem backend and ramster.  But I do agree that there
> might be a good use for the frontswap shrink function for zcache
> (and zswap).  Any ideas?
>
> Dan

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