linux-kernel - Re: [PATCH] zswap: update docs for runtime-changeable attributes

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Date:	Tue, 25 Aug 2015 08:22:03 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Dan Streetman <ddstreet@...e.org>
Cc:	Jonathan Corbet <corbet@....net>,
	Seth Jennings <sjennings@...iantweb.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] zswap: update docs for runtime-changeable attributes

On 25.8.2015 6:22, Sergey Senozhatsky wrote:
>>>> i'd argue that neither zbud nor zsmalloc are responsible for reacting
>>>> to memory pressure, they just store the pages.  It's zswap that has to
>>>> limit its size, which it does with max_percent_pool.
>>>
>>> Yeah but it's zbud that tracks the aging via LRU and reacts to reclaim requests
>>> from zswap when zswap hits the limit. Zswap could easily add a shrinker that
>>> would relay this requests in response to memory pressure as well. However,
>>> zsmalloc doesn't implement the reclaim, or LRU tracking.
>>
>> I wrote a patch for zsmalloc reclaim a while ago:
>>
>> https://lwn.net/Articles/611713/
>>
>> however it didn't make it in, due to the lack of zsmalloc LRU, or any
>> proven benefit to zsmalloc reclaim.
>>
>> It's not really possible to add LRU to zsmalloc, by the nature of its
>> design, using the struct page fields directly; there's no extra field
>> to use as a lru entry.
> 
> Just for information, zsmalloc now registers shrinker callbacks
> 
> https://lkml.org/lkml/2015/7/8/497

Yeah but that's just for compaction, not freeing. I think that ideally zswap
should track the LRU on the level of pages it receives as input, and then just
tell zswap/zbud to free them. Then zswap would use its compaction to make sure
that the reclaim results in actual freeing of page frames. Zbud could re-pair
the orphaned half-pages to the same effect.

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