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

[Minchan Kim]

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@xxxxxxxxx> wrote:
> > On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@xxxxxxxxx> 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.

>  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?


> 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
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