Date:	Sun, 13 Feb 2011 18:45:13 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Alex Riesen <raa.lkml@...il.com>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Heads up Linux 2.6.38-rc4 compile problems.

On Sun, Feb 13, 2011 at 6:04 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> The build failures appear to have been due to a corrupted ccache. A
> coworker turned off using the ccache and the compiles started working
> again.  Unfortunately I can't qualify when my ccache got corrupted,
> or give a hint at which kernel bug caused the corrupted cache.  I
> expected it happened in whatever I tested just before -rc3.

Ok, that certainly explains how it was reproducible, and why it would
show up in rc4 despite there not being a lot of reasons for any of the
post-rc3 changes to introduce anything like that.

It does sound like memory corruption. I'm not at all sure that it's
the rcu lookup thing (although it's a possible case), and especially
if you've been playing around with some of the more experimental VM
features (memcg? transparent hugepage? migration/compaction?) it could
easily be something there. There's been several bug-fixes in those
areas.

Having SLUB debugging on would be a good start. Obviously,
CONFIG_DEBUG_PAGEALLOC would be wondeful, but it's expensive as heck,
so it can be a bit painful to use on a machine that is actually used
for real work. But it can really help pinpoint those kinds of
problems.

> There is something corrupting my page tables.
>
> messages:Feb 13 12:50:00 bs38 kernel: BUG: Bad page map in process [manager]  pte:ffff88028688b748 pmd:28688b067
> messages:Feb 13 12:50:00 bs38 kernel: BUG: Bad page map in process [manager]  pte:ffff88028688b748 pmd:28688b067
> messages:Feb 13 12:52:17 bs38 kernel: BUG: Bad page map in process [manager]  pte:ffff880011065748 pmd:11065067

Odd pattern. That is a totally invalid pte, and I do not see what the
pattern would come from. It's a kernel pointer, afaik, and obviously
shouldn't show up in the pte.

But it could be the result of a use-after-free. Or a double free.
Which I _think_ is that rcu lookup bug pattern, but I may be barking
up the wrong tree. Again, SLUB or PAGEALLOC debugging would probably
give more information.

I'm adding Andrew to the cc too, in case it's simply some of the VM patches.

> I have some unexpected kernel crashes as well.
> With 2.6.38-rc3 (something I think this was a git snapshot) I saw:
>
> <1>BUG: unable to handle kernel NULL pointer dereference at 0000000000000008

The instruction is the "lock xadd %ax,(%rdi)" that is the actual
locked spin-lock instruction. It's this:

   spin_lock(&root_anon_vma->lock);

in __page_lock_anon_vma(), and %rdi is 8. Which is consistent with
root_anon_vma being NULL.

> <0>Call Trace:
> <4> [<ffffffff813d0a0c>] _raw_spin_lock+0x9/0xb
> <4> [<ffffffff810d30cd>] __page_lock_anon_vma+0x3a/0x54
> <4> [<ffffffff810d3633>] page_referenced+0xaf/0x240
> <4> [<ffffffff810bcfda>] shrink_page_list+0x154/0x49e
> <4> [<ffffffff810bd762>] shrink_inactive_list+0x234/0x386
> <4> [<ffffffff810bdede>] shrink_zone+0x356/0x418
> <4> [<ffffffff810bed0e>] kswapd+0x4f6/0x84d
> <4> [<ffffffff81057de9>] kthread+0x7d/0x85
> <4> [<ffffffff810037a4>] kernel_thread_helper+0x4/0x10

It goes without saying that root_anon_vma shouldn't have been NULL
here. But maybe this triggers something for Andrew?

> With 2.6.38-rc4 I have seen:
> <0>general protection fault: 0000 [#1] SMP
> <4>RIP: 0010:[<ffffffff810326b0>]  [<ffffffff810326b0>] post_schedule+0x7/0x4e
> <4>RSP: 0000:ffff8802981c5bf8  EFLAGS: 00010287
> <4>RAX: 0000000000000006 RBX: ffff100367f45c28 RCX: ffff8801a6af0dc0
> <4>RDX: ffff8802981c5fd8 RSI: ffff8801a6af0dc0 RDI: ffff100367f45c28
> <0>Call Trace:
> <4> [<ffffffff813cf98c>] schedule+0x544/0x577
> <4> [<ffffffff813cfb4f>] schedule_timeout+0x22/0xbb
> <4> [<ffffffff813386e5>] __skb_recv_datagram+0x1ec/0x264
> <4> [<ffffffff8133877c>] skb_recv_datagram+0x1f/0x21
> <4> [<ffffffff813aefeb>] unix_accept+0x55/0x103
> <4> [<ffffffff8132efcb>] sys_accept4+0xf3/0x1c3
> <4> [<ffffffff81353b97>] compat_sys_socketcall+0x17d/0x186
> <4> [<ffffffff8102cd90>] sysenter_dispatch+0x7/0x2e
> <0>Code: 49 89 c4 8b 75 e8 48 89 df 31 c9 e8 a3 d4 ff ff 4c 89 e6 48 89 df e8 ae e3 39 00 48 83 c4 20 5b 41 5c c9 c3 55 48 89 e5 41 54 53 <83> bf 74 08 00 00 00 48 89 fb 74 36 e8 4d e3 39 00 49 89 c4 48
> <1>RIP  [<ffffffff810326b0>] post_schedule+0x7/0x4e

This is the very first memory access in post_schedule, the

   if (rq->post_schedule) {

load. (trapping instruction is "cmpl $0x0,0x874(%rdi)". With %rdi
being corrupt, and the resulting pointer being invalid, it looks like.

Odd, and looks pretty random. Maybe it really is just memory corruption.

> With 2.6.38-rc4 I have seen:
> <1>BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> <1>IP: [<ffffffff811016cb>] shrink_dcache_parent+0x104/0x23c
> <0>Call Trace:
> <4> [<ffffffff8113c8bc>] proc_flush_task+0xae/0x1d2
> <4> [<ffffffff8104061a>] release_task+0x35/0x3b9
> <4> [<ffffffff81040f53>] wait_consider_task+0x5b5/0x911
> <4> [<ffffffff810413a6>] do_wait+0xf7/0x222
> <4> [<ffffffff8104266f>] sys_wait4+0x99/0xbc
> <4> [<ffffffff81076155>] compat_sys_wait4+0x26/0xc3
> <4> [<ffffffff8102d9e0>] sys32_waitpid+0xb/0xd
> <4> [<ffffffff8102cd90>] sysenter_dispatch+0x7/0x2e
> <0>Code: 00 49 89 87 80 00 00 00 49 89 8f 88 00 00 00 48 89 11 49 8b 47 68 ff 05 28 04 72 00 ff 80 f0 00 00 00 eb 33 49 8b b7 88 00 00 00 <48> 89 72 08 48 89 16 48 8b 90 e8 00 00 00 48 89 88 e8 00 00 00
> <1>RIP  [<ffffffff811016cb>] shrink_dcache_parent+0x104/0x23c

I dunno. That instruction sequence looks like a list_del(), but I'm
not certain ("mov %rsi,0x8(%rdx) ; mov %rdx,(%rsi)"). With %rdx being
NULL. But shrink_dcache tends to be where a lot of random memory
corruption ends up then blowing up (because the dcache is very
pointer-intensive, and it can be a large cache), so again, I don't
think the oops really tells us anything. It looks more like the
symptom rather than a cause.

                                    Linus
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