RE: [PATCH] staging/zcache: Fix/improve zcache writeback code, tie toa config option

From: Dan Magenheimer
Date: Tue Feb 26 2013 - 15:18:14 EST

> From: Ric Mason [mailto:ric.masonn@xxxxxxxxx]
> Subject: Re: [PATCH] staging/zcache: Fix/improve zcache writeback code, tie to a config option
> 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]
> >> 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?

Sorry, I'm not sure what you mean. The frontswap shrink code
and the zcache writeback code do different things and both work
well for what they are intended to do.

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