Re: [Qemu-devel] post-copy is broken?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Andrea Arcangeli (aarcange@redhat.com) wrote:

> The next suspect is the massive THP refcounting change that went
> upstream recently:

> As further debug hint, can you try to disable THP and see if that
> makes the problem go away?

Yep, this seems to be the problem (cc'ing in Kirill).

122afea9626ab3f717b250a8dd3d5ebf57cdb56c - works (just before Kirill disables THP)
61f5d698cc97600e813ca5cf8e449b1ea1c11492 - breaks (when THP is reenabled)

It's pretty reliable; as you say disabling THP makes it work again
and putting it back to THP/madvise mode makes it break.  And you need
to test on a machine with some free ram to make sure THP has a chance
to have happened.

I'm not sure of all of the rework that happened in that series,
but my reading of it is that splitting of THP pages gets deferred;
so I wonder if when I do the madvise to turn THP off, if it's actually
still got THP pages and thus we end up with a whole THP mapped
when I'm expecting to be userfaulting those pages.

Dave

> 
> Thanks,
> Andrea
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK