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

[Minchan Kim <minchan@kernel.org>]

Hi Zlatko,

On Fri, Dec 28, 2012 at 02:29:11PM +0100, Zlatko Calusic wrote:
> 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@iskon.hr>
> >>
> >>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,

Argh, I didn't see 4ae0a48b in 3.8-rc1.

> 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.

Never mind. My confusing is caused my missing 4ae0a48b in lasest tree.
Thanks, Zlatko.

> 
> Regards,
> -- 
> Zlatko
> 
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-- 
Kind regards,
Minchan Kim

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