Re: [BUG] seltests/iommu: runaway ./iommufd consuming 99% CPU after a failed assert()

[Shuah Khan]

On 3/27/24 14:04, Mirsad Todorovac wrote:
> On 3/27/24 11:41, Joao Martins wrote:
> > On 25/03/2024 13:52, Jason Gunthorpe wrote:
> > > On Mon, Mar 25, 2024 at 12:17:28PM +0000, Joao Martins wrote:
> > > > > However, I am not smart enough to figure out why ...
> > > > >
> > > > > Apparently, from the source, mmap() fails to allocate pages on the desired address:
> > > > >
> > > > >    1746         assert((uintptr_t)self->buffer % HUGEPAGE_SIZE == 0);
> > > > >    1747         vrc = mmap(self->buffer, variant->buffer_size, PROT_READ |
> > > > > PROT_WRITE,
> > > > >    1748                    mmap_flags, -1, 0);
> > > > > → 1749         assert(vrc == self->buffer);
> > > > >    1750
> > > > >
> > > > > But I am not that deep into the source to figure our what was intended and what
> > > > > went
> > > > > wrong :-/
> > > >
> > > > I can SKIP() the test rather assert() in here if it helps. Though there are
> > > > other tests that fail if no hugetlb pages are reserved.
> > > >
> > > > But I am not sure if this is problem here as the initial bug email had an
> > > > enterily different set of failures? Maybe all you need is an assert() and it
> > > > gets into this state?
> > >
> > > I feel like there is something wrong with the kselftest framework,
> > > there should be some way to fail the setup/teardown operations without
> > > triggering an infinite loop :(
> >
> > I am now wondering if the problem is the fact that we have an assert() in the
> > middle of FIXTURE_{TEST,SETUP} whereby we should be having ASSERT_TRUE() (or any
> > other kselftest macro that). The expect/assert macros from kselftest() don't do
> > asserts and it looks like we are failing mid tests in the assert().
> >
> > Maybe it is OK for setup_sizes(), but maybe not OK for the rest (i.e. during the
> > actual setup / tests). I can throw a patch there to see if this helps Mirsad.
>
> Well, we are in the job of making the kernel better and as bug free as we can.
>
> Maybe we should not delve too much into detail: is this a kernel bug, or the kselftest
> program bug?
>
> Some people already mentioned that I might have sysctl variable problems. I don't see
> what the mmap() HUGEPAGE allocation at fixed address was meant to prove?

I applied fix to this problem to linux-kselftest fixes branch for next rc.
Please give it a try.

thanks,
-- Shuah