Re: Bug in 2.6.8-rc3 at mm/page_alloc.c:792 and mm/rmap.c:407

[Hugh Dickins]

On Sun, 8 Aug 2004, Hugh Dickins wrote:
> On Sun, 8 Aug 2004, Oskar Berggren wrote:
> > Two BUG's I've been seeing, one in page_alloc.c and one in rmap.c.
> 
> This is not the first report of an rmap.c:407,
> Denis reported one a week ago (on 2.6.7-bk20).
> 
> I don't know what's behind it, but I am wondering if PageReserved
> might be getting cleared while page is still mapped into userspace.

That was just a guess, I've no evidence, and now doubt that.

> You both have sound modules in, are you using audio?

You weren't actually using it (you mention in other mail),
even if Denis was, so audio no longer looks like a suspect.

> Could you mail me (privately) your /var/log/messages, Oskar, I don't
> have a clear picture of the relation between your page_alloc.c:792s,
> your rmap.c:407s and your other oopses.

Thanks a lot for the /var/log/messages.

The frustrating thing is that the most interesting lines are missing:
the "unqualified" printks, e.g. handle_BUG's "--- [ cut here ] ---" line
and stack traces do appear; but handle_BUG's KERN_ALERT "kernel BUG..."
and bad_page's very useful KERN_EMERG messages do not appear at all.

The "kernel BUG" messages you've already told us, but the missing
bad_page lines might, _might_ be really helpful.  Do you have some
klogd option set, not to print out the most important messages ;-?
I hope someone can tell us how to fix that.

I notice the page_remove_rmap BUG (which we know to be rmap.c:407
from your mail) was preceded 10 minutes earlier by a bad_page; and
one of the things bad_page will do is force page->mapcount to 0,
which would trigger the page_remove_rmap BUG, if that page being
freed was actually still in use in some process address space.

Most of the other BUGs (mostly page_alloc.c:792s, __free_pages called
from below shrink_cache or sock_release, finding page_count already 0)
were also preceded, less immediately, by bad_pages; though not all.

It's all consistent with pages being freed while still in use,
but I don't think I'm saying anything new there.  It doesn't look
to me like random corruption or bad memory, I don't think those would
show up so consistently as page freeing errors.  (Though if KERN_ERRs
aren't getting into /var/log/messages, there might be page table
corruption swap_free errors missing too.)

But I've no idea of where to look for the culprit: I'd better get
on with other things, and hope someone else can take this further.

Hugh

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