Re: BUG: Bad page map in process udevd (anon_vma: (null)) in 2.6.38-rc4

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Feb 17, 2011 at 1:09 AM, Michal Hocko <mhocko@suse.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 :(

                               Linus

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