Re: [BUG] kernel BUG at mm/vmacache.c:85!

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

On Mon, Apr 28, 2014 at 4:11 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> unuse_mm() leaves current->mm at NULL so we'd hear about it pretty
> quickly if a user task was running use_mm/unuse_mm.

Yes.

> I think so.  Maybe it's time to cook up a debug patch for Srivatsa to
> use?  Dump the vma cache when the bug hits, or wire up some trace
> points.  Or perhaps plain old printks - it seems to be happening pretty
> early in boot.

Well, I think Srivatsa has only seen it once, and wasn't able to
reproduce it, so we'd have to make it happen more first.

> Are there additional sanity checks we can perform at cache addition
> time?

I wouldn't really expect it to happen at cache addition time, since
that's really quite simple. There's only one caller of
vmacache_update(), namely find_vma(). And vmacache_update() does the
same sanity check that vmacache lookup does (ie check that the
passed-on mm is the current thread mm, and that we're not a kernel
thread).

I'd be more inclined to think it's a missing invalidate, but I can
only think of two reasons to invalidate:

 - the vma itself went away from the mm, got free'd/reused, and so
vm_mm changes..

   But then we'd have to remove it from the rb-tree, and both callers
of vma_rb_erase() do a vmacache_invalidate()

 - the mm of a thread changed

   This is exec, use_mm(), and fork() (and fork really only just
because we copy the vmacache).

   exec and fork do that "vmacache_flush(tsk)", which is why I was
looking at use_mm().

So it all looks sane. Which only means that I must obviously be
missing some case. Which case am I missing?

                  Linus

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