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

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

On Mon, Apr 28, 2014 at 3:39 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
>
> Is this perhaps a KVM guest? fwiw I see CONFIG_KVM_ASYNC_PF=y which is a
> user of use_mm().

So I tried to look through these guys, and that was one of the ones I looked at.

It's using use_mm(), but it's only called through schedule_work().
Which *should* mean that it's in a kernel thread and
vmacache_valid_mm() will not be true.

HOWEVER.

The whole "we don't use the vma cache on kernel threads" does seem to
be a pretty fragile approach to the whole workqueue etc issue. I think
we always use a kernel thread for workqueue entries, but at the same
time I'm not 100% convinced that we should *rely* on that kind of
behavior. I don't think that it's necessarily fundamentally guaranteed
conceptually - I could see, for example, some user of "flush_work()"
deciding to run the work *synchronously* within the context of the
process that does the flushing.

Now, I don't think we actually do that, but my point is that I think
it's a bit dangerous to just say "only kernel threads do use_mm(), and
work entries are always done by kernel threads, so let's disable vma
caching for kernel threads". It may be *true*, but it's a very
indirect kind of true.

That's why I think we might be better off saying "let's just
invalidate the vmacache in use_mm(), and not care about who does it".
No subtle indirect logic about why the caching is safe in one context
but not another.

But quite frankly, I grepped for things that set "tsk->mm", and apart
from clearing it on exit, the only uses I found was copy_mm() (which
does that vmacache_flush()) and use_mm(). And all the use_mm() cases
_seem_ to be in kernel threads, and that first BUG_ON() didn't have a
very complex call chain at all, just a regular page fault from udevd.

So it might just be some really nasty corruption totally unrelated to
the vmacache, and those preceding odd udevd-work and kdump faults
could be related.

             Linus

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