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

[Minchan Kim]

On Thu, Sep 26, 2013 at 03:26:33PM +0800, Weijie Yang wrote:
> On Thu, Sep 26, 2013 at 1:58 PM, Minchan Kim <minchan@xxxxxxxxxx> 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@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.
> 
> 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.

"zswap only relize one option"
What does it mena? Sorry. I couldn't parse your intention. :)
You mean zswap should do something special to support frontswap_tmem_exclusive_gets?

> 
> >>  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.

You mean "zswap_duplicate_entry"?
AFAIR, I already had a question to Seth when zswap was born but AFAIRC,
he said that he didn't know exact reason but he saw that case during
experiement so copy the code peice from zcache.

Do you see the case, too?

Anyway, we need to dive into that to know what happens and then open
our eyes for clear solution before dumping meaningless patch.

I hope Seth or Bob already know it.

> 
> >> 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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-- 
Kind regards,
Minchan Kim
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