Re: [PATCH] mm: fix null pointer dereference in wait_iff_congested()

[Zlatko Calusic]

On 28.12.2012 03:49, Minchan Kim wrote:
> Hello Zlatko,
>
> On Fri, Dec 28, 2012 at 03:16:38AM +0100, Zlatko Calusic wrote:
> > From: Zlatko Calusic <zlatko.calusic@xxxxxxxx>
> >
> > The unintended consequence of commit 4ae0a48b is that
> > wait_iff_congested() can now be called with NULL struct zone*
> > producing kernel oops like this:
>
> For good description, it would be better to write simple pseudo code
> flow to show how NULL-zone pass into wait_iff_congested because
> kswapd code flow is too complex.
>
> As I see the code, we have following line above wait_iff_congested.
>
> if (!unbalanced_zone || blah blah)
>          break;
>
> How can NULL unbalanced_zone reach wait_iff_congested?

Hello Minchan, and thanks for the comment.

That line was there before commit 4ae0a48b got in, and you're right, 
it's what was protecting wait_iff_congested() from being called with 
NULL zone*. But then all that logic got colapsed to a simple 
pgdat_balanced() call and that's when I introduced the bug, I lost the 
protection.

What I _think_ is happening (pseudo code following...) is that after 
scanning the zone in the dma->highmem direction, and concluding that all 
zones are balanced (unbalanced_zone remains NULL!), 
wake_up(&pgdat->pfmemalloc_wait) wakes up a lot of memory hungry 
processes (especially true in various aggressive test/benchmarks) that 
immediately drain and unbalance one or more zones. Then pgdat_balanced() 
call which immediately follows will be false, but we still have 
unbalanced_zone = NULL, rememeber? Oops...

But, all that is a speculation that I can't prove atm. Of course, if 
anybody thinks that's a credible explanation, I could add it as a commit 
comment, or even as a code comment, but I didn't want to be overly 
imaginative. The fix itself is simple and real.

Regards,
--
Zlatko
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