Re: [BUG REPORT] ZSWAP: theoretical race condition issues

Weijie Yang
2013-09-25T09:33:46Z

On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu <lliubbo@gmail.com> wrote:
> On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@gmail.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
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.

what I can think is that use flags and CAS to protect store and reclaim on
the same offset happens concurrentlly.

>> thread 0: zswap_get_swap_cache_page called. old page data is added to swap_cache
>>
>> Now, swap cache has old data rather than new data for offset A.
>> error will happen If do_swap_page() get page from swap_cache.
>>
>
> --
> Regards,
> --Bob