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

[Minchan Kim <minchan@kernel.org>]

Hello,

On Tue, Aug 20, 2013 at 11:30:29PM +0800, Weijie Yang wrote:
> 2013/8/19 Minchan Kim <minchan@kernel.org>:
> > On Mon, Aug 19, 2013 at 10:17:38AM +0800, Bob Liu wrote:
> >> Hi Weijie,
> >>
> >> On 08/19/2013 12:14 AM, Weijie Yang wrote:
> >> > I found a few bugs in zswap when I review Linux-3.11-rc5, and I have
> >> > also some questions about it, described as following:
> >> >
> >> > BUG:
> >> > 1. A race condition when reclaim a page
> >> > when a handle alloced from zbud, zbud considers this handle is used
> >> > validly by upper(zswap) and can be a candidate for reclaim.
> >> > But zswap has to initialize it such as setting swapentry and addding
> >> > it to rbtree. so there is a race condition, such as:
> >> > thread 0: obtain handle x from zbud_alloc
> >> > thread 1: zbud_reclaim_page is called
> >> > thread 1: callback zswap_writeback_entry to reclaim handle x
> >> > thread 1: get swpentry from handle x (it is random value now)
> >> > thread 1: bad thing may happen
> >> > thread 0: initialize handle x with swapentry
> >
> > Nice catch!
> >
> >>
> >> Yes, this may happen potentially but in rare case.
> >> Because we have a LRU list for page frames, after Thread 0 called
> >> zbud_alloc the corresponding page will be add to the head of LRU
> >> list,While zbud_reclaim_page(Thread 1 called) is started from the tail
> >> of LRU list.
> >>
> >> > Of course, this situation almost never happen, it is a "theoretical
> >> > race condition" issue.
> >
> > But it's doable and we should prevent that although you feel it's rare
> > because system could go hang. When I look at the code, Why should zbud
> > have LRU logic instead of zswap? If I missed some history, sorry about that.
> > But at least to me, zbud is just allocator so it should have a role
> > to handle alloc/free object and how client of the allocator uses objects
> > depends on the upper layer so zbud should handle LRU. If so, we wouldn't
> > encounter this problem, either.
> >
> >> >
> >> > 2. Pollute swapcache data by add a pre-invalided swap page
> >> > when a swap_entry is invalidated, it will be reused by other anon
> >> > page. At the same time, zswap is reclaiming old page, pollute
> >> > swapcache of new page as a result, because old page and new page use
> >> > the same swap_entry, such as:
> >> > thread 1: zswap reclaim entry x
> >> > thread 0: zswap_frontswap_invalidate_page entry x
> >> > thread 0: entry x reused by other anon page
> >> > thread 1: add old data to swapcache of entry x
> >>
> >> I didn't get your idea here, why thread1 will add old data to entry x?
> >>
> >> > thread 0: swapcache of entry x is polluted
> >> > Of course, this situation almost never happen, it is another
> >> > "theoretical race condition" issue.
> >
> > Don't swapcache_prepare close the race?
> 
> Yes, I made a mistake, there is not a race here.
> However, I find another bug here after my more careful review. It is
> not only "theoretical", it will happen really. as:
> thread 1: zswap reclaim entry x (get the refcount, but not call
> zswap_get_swap_cache_page yet)
> thread 0: zswap_frontswap_invalidate_page entry x (finished, entry x
> and its zbud is not freed as its refcount != 0)
> now, the swap_map[x] = 0
> thread 1: zswap_get_swap_cache_page called, swapcache_prepare return
> -ENOENT because entry x is not used any more
> zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM
> zswap_writeback_entry do nothing except put refcount
> now, the memory of zswap_entry x leaks and its zpage become a zombie

It makes sense to me.
Maybe we should introduce ZSWAP_SWAPCACHE_NOENT and free the entry in
the case. Would you mind to send patches on the problems you found?

> 
> Best Regards,
> Weijie Yang
> 
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-- 
Kind regards,
Minchan Kim

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