linux-kernel - Re: [PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

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Date:	Wed, 25 Sep 2013 15:21:54 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Weijie Yang <weijie.yang@...sung.com>, akpm@...ux-foundation.org,
	sjenning@...ux.vnet.ibm.com, bob.liu@...cle.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, d.j.shin@...sung.com,
	heesub.shin@...sung.com, kyungmin.park@...sung.com,
	hau.chen@...sung.com, bifeng.tong@...sung.com, rui.xie@...sung.com
Subject: Re: [PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

On Tue, Sep 24, 2013 at 9:11 AM, Minchan Kim <minchan@...nel.org> wrote:
> On Mon, Sep 23, 2013 at 04:19:36PM +0800, Weijie Yang wrote:
>> This patch series fix a few bugs in mm/zswap based on Linux-3.11.
>>
>> v2 --> v3
>>       - keep GFP_KERNEL flag
>
> Why do you drop this?
>
> It's plain BUG. I read Bob's reply but it couldn't justify to let the pain
> remain. First of all, let's fix it and better idea could come later.

Hi, Minchan. Thanks for review

What I thought is that better idea could come sooner if I keep this
flag rather than fixing it

Yes, you are right. Fixing a bug has a higher priority than waiting
for better idea.

I will add it again in the next version patch set.

>>
>> v1 --> v2
>>       - free memory in zswap_frontswap_invalidate_area(in patch 1)
>>       - fix whitespace corruption (line wrapping)
>>
>> Corresponding mail thread: https://lkml.org/lkml/2013/8/18/59
>>
>> These issues fixed/optimized are:
>>
>>  1. memory leaks when re-swapon
>>
>>  2. memory leaks when invalidate and reclaim occur concurrently
>>
>>  3. avoid unnecessary page scanning
>>
>>
>> Issues discussed in that mail thread NOT fixed as it happens rarely or
>> not a big problem or controversial:
>>
>>  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.
>>
>> 3. the potential that zswap store and reclaim functions called recursively
>>
>>
>>  mm/zswap.c |   28 ++++++++++++++++++++--------
>>  1 file changed, 20 insertions(+), 8 deletions(-)
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@...ck.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
>
> --
> Kind regards,
> Minchan Kim
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