linux-kernel - Re: [PATCH 0/4] zswap bugfix: memory leaks and other problem

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Date:	Fri, 23 Aug 2013 10:22:29 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Weijie Yang <weijie.yang.kh@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, Bob Liu <bob.liu@...cle.com>,
	weijie.yang@...sung.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] zswap bugfix: memory leaks and other problem

On Fri, Aug 23, 2013 at 07:26:01PM +0800, Weijie Yang wrote:
> This patch series fix a few bugs in zswap based on Linux-3.11-rc6.
> 
> Corresponding mail thread see: lkml.org/lkml/2013/8/18/59 .
> 
> These issues fixed are:
>  1. memory leaks when re-swapon
>  2. potential problem which store and reclaim functions is called recursively
>  3. memory leaks when invalidate and reclaim occur simultaneously
>  4. unnecessary page scanning

Thanks for the patches!

Patches 2-4 have whitespace corruption (line wrapping) probably caused
by your mail agent.  You might check Documentation/email-clients.txt on
how to prevent this.

Seth

> 
> Issues discussed in that mail thread NOT fixed as it happens rarely or
> not a big problem:
>  1. a "theoretical race condition" when reclaim page
>  when a handle alloced from zbud, zbud considers this handle is used
> validly by upper(zswap) and can be a candidate for reclaim.
>  But zswap has to initialize it such as setting swapentry and adding
> it to rbtree. so there is a race condition, such as:
>  thread 0: obtain handle x from zbud_alloc
>  thread 1: zbud_reclaim_page is called
>  thread 1: callback zswap_writeback_entry to reclaim handle x
>  thread 1: get swpentry from handle x (it is random value now)
>  thread 1: bad thing may happen
>  thread 0: initialize handle x with swapentry
> 
> 2. frontswap_map bitmap not cleared after zswap reclaim
>  Frontswap uses frontswap_map bitmap to track page in "backend" implementation,
>  when zswap reclaim a page, the corresponding bitmap record is not cleared.
> 
> mm/zswap.c |   35 ++++++++++++++++++++++++-----------
>   1 files changed, 24 insertions(+), 11 deletions(-)
> 

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