Date:	Tue, 20 Aug 2013 23:30:29 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Bob Liu <bob.liu@...cle.com>, sjenning@...ux.vnet.ibm.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues

2013/8/19 Minchan Kim <minchan@...nel.org>:
> On Mon, Aug 19, 2013 at 10:17:38AM +0800, Bob Liu wrote:
>> Hi Weijie,
>>
>> On 08/19/2013 12:14 AM, Weijie Yang wrote:
>> > I found a few bugs in zswap when I review Linux-3.11-rc5, and I have
>> > also some questions about it, described as following:
>> >
>> > BUG:
>> > 1. A race condition when reclaim a 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 addding
>> > 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
>
> Nice catch!
>
>>
>> Yes, this may happen potentially but in rare case.
>> Because we have a LRU list for page frames, after Thread 0 called
>> zbud_alloc the corresponding page will be add to the head of LRU
>> list,While zbud_reclaim_page(Thread 1 called) is started from the tail
>> of LRU list.
>>
>> > Of course, this situation almost never happen, it is a "theoretical
>> > race condition" issue.
>
> But it's doable and we should prevent that although you feel it's rare
> because system could go hang. When I look at the code, Why should zbud
> have LRU logic instead of zswap? If I missed some history, sorry about that.
> But at least to me, zbud is just allocator so it should have a role
> to handle alloc/free object and how client of the allocator uses objects
> depends on the upper layer so zbud should handle LRU. If so, we wouldn't
> encounter this problem, either.
>
>> >
>> > 2. Pollute swapcache data by add a pre-invalided swap page
>> > when a swap_entry is invalidated, it will be reused by other anon
>> > page. At the same time, zswap is reclaiming old page, pollute
>> > swapcache of new page as a result, because old page and new page use
>> > the same swap_entry, such as:
>> > thread 1: zswap reclaim entry x
>> > thread 0: zswap_frontswap_invalidate_page entry x
>> > thread 0: entry x reused by other anon page
>> > thread 1: add old data to swapcache of entry x
>>
>> I didn't get your idea here, why thread1 will add old data to entry x?
>>
>> > thread 0: swapcache of entry x is polluted
>> > Of course, this situation almost never happen, it is another
>> > "theoretical race condition" issue.
>
> Don't swapcache_prepare close the race?

Yes, I made a mistake, there is not a race here.
However, I find another bug here after my more careful review. It is
not only "theoretical", it will happen really. as:
thread 1: zswap reclaim entry x (get the refcount, but not call
zswap_get_swap_cache_page yet)
thread 0: zswap_frontswap_invalidate_page entry x (finished, entry x
and its zbud is not freed as its refcount != 0)
now, the swap_map[x] = 0
thread 1: zswap_get_swap_cache_page called, swapcache_prepare return
-ENOENT because entry x is not used any more
zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM
zswap_writeback_entry do nothing except put refcount
now, the memory of zswap_entry x leaks and its zpage become a zombie

Best Regards,
Weijie Yang
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