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

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Message-ID: <3c15a093-7c19-4c2a-a571-56a5ed4b445f@redhat.com>
Date: Wed, 28 May 2025 12:48:59 +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 12:44, 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 think perhaps the only safe way to handle this is to parse /proc/iomem for a
>> region of "System RAM" that is at least 2 pages then use that for your read
>> tests. This would also solve the hypothetical issue of reading something that
>> has read size effects.
> 
> That sounds also plausible yes. I somehow remembered that mmap() would
> fail if "there is nothing".

Ah, my memory comes back, we perform checks only with CONFIG_STRICT_DEVMEM.

-- 
Cheers,

David / dhildenb