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Date: Mon, 19 Jan 2026 14:30:45 +0100
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Subject: Re: [PATCH v2 6/8] selftests/mm: fix faulting-in code in
 pagemap_ioctl test
To: "David Hildenbrand (Red Hat)" <david@kernel.org>, linux-mm@kvack.org,
 linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@oracle.com>, Mark Brown
 <broonie@kernel.org>, Ryan Roberts <ryan.roberts@arm.com>,
 Shuah Khan <shuah@kernel.org>, Usama Anjum <Usama.Anjum@arm.com>
References: <20260107164842.3289559-1-kevin.brodsky@arm.com>
 <20260107164842.3289559-7-kevin.brodsky@arm.com>
 <cd9e1fca-81ef-4eaa-aea1-aac6dd00623d@kernel.org>
From: Kevin Brodsky <kevin.brodsky@arm.com>
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On 19/01/2026 12:09, David Hildenbrand (Red Hat) wrote:
> On 1/7/26 17:48, Kevin Brodsky wrote:
>> One of the pagemap_ioctl tests attempts to fault in pages by
>> memcpy()'ing them to an unused buffer. This probably worked
>> originally, but since commit 46036188ea1f ("selftests/mm: build with
>> -O2") the compiler is free to optimise away that unused buffer and
>> the memcpy() with it. As a result there might not be any resident
>> page in the mapping and the test may fail.
>
> Yes, I assume so. Using FORCE_READ() etc is the way to go.
>
>
> Should we add
>
> Fixes: 46036188ea1f ("selftests/mm: build with -O2") 

Yes that seems reasonable, the test was arguably broken to start with
but it is this commit that revealed it ;)

- Kevin