linux-kernel - Re: [BUG REPORT] ZSWAP: theoretical race condition issues

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Date:	Wed, 25 Sep 2013 18:02:15 +0800
From:	Bob Liu <lliubbo@...il.com>
To:	Weijie Yang <weijie.yang.kh@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, Bob Liu <bob.liu@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues

On Wed, Sep 25, 2013 at 5:33 PM, Weijie Yang <weijie.yang.kh@...il.com> wrote:
> On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu <lliubbo@...il.com> wrote:
>> On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@...il.com> wrote:
>>> I think I find a new issue, for integrity of this mail thread, I reply
>>> to this mail.
>>>
>>> It is a concurrence issue either, when duplicate store and reclaim
>>> concurrentlly.
>>>
>>> zswap entry x with offset A is already stored in zswap backend.
>>> Consider the following scenario:
>>>
>>> thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)
>>>
>>> thread 1: store new page with the same offset A, alloc a new zswap entry y.
>>>   store finished. shrink_page_list() call __remove_mapping(), and now
>>> it is not in swap_cache
>>>
>>
>> But I don't think swap layer will call zswap with the same offset A.
>
> 1. store page of offset A in zswap
> 2. some time later, pagefault occur, load page data from zswap.
>   But notice that zswap entry x is still in zswap because it is not

Sorry I didn't notice that zswap_frontswap_load() doesn't call rb_erase().

> frontswap_tmem_exclusive_gets_enabled.
>  this page is with PageSwapCache(page) and page_private(page) = entry.val
> 3. change this page data, and it become dirty
> 4. some time later again, swap this page on the same offset A.
>
> so, a duplicate store happens.
>

Then I think we should erase the entry from rbtree in zswap_frontswap_load().
After the page is decompressed and loaded from zswap, still storing
the compressed data in zswap is meanless.

-- 
Regards,
--Bob
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