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

Weijie Yang
2013-09-26T07:26:35Z

On Thu, Sep 26, 2013 at 1:58 PM, Minchan Kim <minchan@kernel.org> wrote:
> Hello Weigie,
>
> On Wed, Sep 25, 2013 at 05:33:43PM +0800, Weijie Yang wrote:
>> 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.
>
> frontswap_tmem_exclusive_gets_enabled is just option to see tradeoff
> between CPU burining by frequent swapout and memory footprint by duplicate
> copy in swap cache and frontswap backend so it shouldn't affect the stability.

Thanks for explain this.
I don't mean to say this option affects the stability, but that zswap
only realize
one option. Maybe it's better to realize both options for different workloads.

>> this page is with PageSwapCache(page) and page_private(page) = entry.val
>> 3. change this page data, and it become dirty
>
> If non-shared swapin page become redirty, it should remove the page from
> swapcache. If shared swapin page become redirty, it should do CoW so it's a
> new page so that it doesn't live in swap cache. It means it should have new
> offset which is different with old's one for swap out.
>
> What's wrong with that?

It is really not a right scene for duplicate store. And I can not think out one.
If duplicate store is impossible, How about delete the handle code in zswap?
If it does exist, I think there is a potential issue as I described.

>> 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
>>
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>
> --
> Kind regards,
> Minchan Kim