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

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Date: Thu, 4 Apr 2024 10:54:12 -0600
From: Shuah Khan <skhan@...uxfoundation.org>
To: Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
 Joao Martins <joao.m.martins@...cle.com>, Jason Gunthorpe <jgg@...dia.com>
Cc: iommu@...ts.linux.dev, Kevin Tian <kevin.tian@...el.com>,
 Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [BUG] seltests/iommu: runaway ./iommufd consuming 99% CPU after a
 failed assert()

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