Message-ID: <1297960574.2769.20.camel@edumazet-laptop>
Date:	Thu, 17 Feb 2011 17:36:14 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Octavian Purdila <opurdila@...acom.com>,
	David Miller <davem@...emloft.net>
Subject: Re: BUG: Bad page map in process udevd (anon_vma: (null)) in
 2.6.38-rc4

Le jeudi 17 février 2011 à 08:13 -0800, Linus Torvalds a écrit :
> On Thu, Feb 17, 2011 at 1:09 AM, Michal Hocko <mhocko@...e.cz> wrote:
> >
> > I have seen that thread but I didn't think it is related. I thought
> > this is an another anon_vma issue. But you seem to be right that the
> > offset pattern can be related.
> 
> Hey, maybe it turns out to be about anon_vma's in the end, but I see
> no big reason to blame them per se. And we haven't had all that much
> churn wrt anon_vma's this release window, so I wouldn't expect
> anything exciting unless you're actively using transparent hugepages.
> And iirc, Eric was not using them (or memory compaction).
> 
> I'd be more likely to blame either the new path lookup (which uses
> totally new RCU freeing of inodes _and_
> INIT_LIST_HEAD(&inode->i_dentry)), but I'm not seeing how that could
> break either (I've gone through that patch many times).
> 
> And in addition, I don't see why others wouldn't see it (I've got
> DEBUG_PAGEALLOC and SLUB_DEBUG_ON turned on myself, and I know others
> do too).
> 
> So I'm wondering what triggers it. Must be something subtle.
> 
> > OK. I have just booted with the same kernel and the config turned on.
> > Let's see if I am able to reproduce.
> 
> Thanks. It might have been good to turn on SLUB_DEBUG_ON and
> DEBUG_LIST too, but PAGEALLOC is the big one.
> 
> > Btw.
> > $ objdump -d ./vmlinux-2.6.38-rc4-00001-g07409af-vmscan-test | grep 0x1e68
> >
> > didn't print out anything. Do you have any other way to find out the
> > structure?
> 
> Nope, that's roughly what I did to (in addition to doing all the .ko
> files and checking for 0xe68 too). Which made me worry that the 0x1e68
> offset is actually just the stack offset at some random code-path (it
> would stay constant for a particular kernel if there is only one way
> to reach that code, and it's always reached through some stable
> non-irq entrypoint).
> 
> People do use on-stack lists, and if you do it wrong I could imagine a
> stale list entry still pointing to the stack later. And while
> INIT_LIST_HEAD() is one pattern to get that "two consecutive words
> pointing to themselves", so is doing a "list_del()" on the last list
> entry that the head points to.
> 
> So _if_ somebody has a list_head on the stack, and leaves a stale list
> entry pointing to it, and then later on, when the stack has been
> released that stale list entry is deleted with "list_del()", you'd see
> the same memory corruption pattern. But I'm not aware of any new code
> that would do anything like that.
> 
> So I'm stumped, which is why I'm just hoping that extra debugging
> options would catch it closer to the place where it actually occurs.
> The "2kB allocation with a nice compile-time structure offset" sounded
> like _such_ a great way to catch it, but it clearly doesn't :(
> 
>  

Hmm, this rings a bell here.

Unfortunately I have to run so cannot check right now.

Please take a look at commit 443457242beb6716b43db4d (net: factorize
sync-rcu call in unregister_netdevice_many)

CC David and Octavian

dev_close_many() can apparently return with an non empty list






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