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

[Ric Mason <ric.masonn@gmail.com>]

On 02/26/2013 01:29 AM, Dan Magenheimer wrote:
>> From: Ric Mason [mailto:ric.masonn@gmail.com]
>> 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