On Mon, Apr 28, 2014 at 4:11 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> 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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