linux-kernel - Re: [PATCH v1] selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem

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Message-ID: <a0a54f02-bee3-41b8-8c4f-9cfd7ea524ed@redhat.com>
Date: Fri, 9 May 2025 12:43:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1] selftests/mm: add simple VM_PFNMAP tests based on
 mmap'ing /dev/mem

>> Is this not pretty much equivalent to a volatile read where you're forcing
>> the compiler to not optimise this unused thing away? In guard-regions I set:
>>
>> #define FORCE_READ(x) (*(volatile typeof(x) *)x)
>>
>> For this purpose, which would make this:
>>
>> FORCE_READ(addr);
>> FORCE_READ(&addr[pagesize]);
> 
> Hmmm, a compiler might be allowed to optimize out a volatile read.

Looking into this, the compiler should not be allowed to do that. So 
FORCE_READ() should work!

-- 
Cheers,

David / dhildenb