Re: Questin about swap_slot free and invalidate page

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

On 02/19/2013 11:27 PM, Dan Magenheimer wrote:
>> 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/

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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