linux-kernel - Re: [PATCH v2] selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem

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Message-ID: <1916ff1c-93e7-442a-b1be-e0e35190a6ba@redhat.com>
Date: Wed, 28 May 2025 13:03:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Ingo Molnar
 <mingo@...hat.com>, Peter Xu <peterx@...hat.com>, Dev Jain
 <dev.jain@....com>, Aishwarya TCV <Aishwarya.TCV@....com>
Subject: Re: [PATCH v2] selftests/mm: add simple VM_PFNMAP tests based on
 mmap'ing /dev/mem

On 28.05.25 12:53, Ryan Roberts wrote:
> On 28/05/2025 11:48, David Hildenbrand wrote:
>> On 28.05.25 12:44, David Hildenbrand wrote:
>>> On 28.05.25 12:34, Ryan Roberts wrote:
>>>> Hi David,
>>>>
>>>>
>>>> On 09/05/2025 16:30, David Hildenbrand wrote:
>>>>> Let's test some basic functionality using /dev/mem. These tests will
>>>>> implicitly cover some PAT (Page Attribute Handling) handling on x86.
>>>>>
>>>>> These tests will only run when /dev/mem access to the first two pages
>>>>> in physical address space is possible and allowed; otherwise, the tests
>>>>> are skipped.
>>>>
>>>> We are seeing really horrible RAS errors with this test when run on arm64 tx2
>>>> machine. Based solely on reviewing the code, I think the problem is that tx2
>>>> doesn't have anything at phys address 0, so test_read_access() is trying to put
>>>> trasactions out to a bad address on the bus.
>>>>
>>>> tx2 /proc/iomem:
>>>>
>>>> $ sudo cat /proc/iomem
>>>> 30000000-37ffffff : PCI ECAM
>>>> 38000000-3fffffff : PCI ECAM
>>>> 40000000-5fffffff : PCI Bus 0000:00
>>>> ...
>>>>
>>>> Whereas my x86 box has some reserved memory:
>>>>
>>>> $ sudo cat /proc/iomem
>>>> 00000000-00000fff : Reserved
>>>> 00001000-0003dfff : System RAM
>>>> ...
>>>>
>>>
>>> A quick fix would be to make this test specific to x86 (the only one I
>>> tested on). We should always have the lower two pages IIRC (BIOS stuff etc).
> 
> I'm not sure how far along this patch is? I'm guessing mm-stable? Perhaps you
> can do the quick fix, then I'd be happy to make this more robust for arm64 later?

Already hacking on the parsing :)

-- 
Cheers,

David / dhildenb