From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx105.postini.com [74.125.245.105]) by kanga.kvack.org (Postfix) with SMTP id D5A096B005A for ; Fri, 28 Dec 2012 08:29:18 -0500 (EST) Date: Fri, 28 Dec 2012 14:29:11 +0100 From: Zlatko Calusic MIME-Version: 1.0 References: <50D24AF3.1050809@iskon.hr> <20121220111208.GD10819@suse.de> <20121220125802.23e9b22d.akpm@linux-foundation.org> <50D601C9.9060803@iskon.hr> <50D71166.6030608@iskon.hr> <50DB129E.7010000@iskon.hr> <50DD0106.7040001@iskon.hr> <20121228024928.GA19720@blaptop> In-Reply-To: <20121228024928.GA19720@blaptop> Message-ID: <50DD9EA7.6050309@iskon.hr> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [PATCH] mm: fix null pointer dereference in wait_iff_congested() Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Andrew Morton , Mel Gorman , Linus Torvalds , Hugh Dickins , linux-mm , Linux Kernel Mailing List , Zhouping Liu , Sedat Dilek 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 >> >> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org