linux-kernel - Re: [PATCH] selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"

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Message-ID: <d27e9451-8807-4c86-b2aa-063639f7801d@redhat.com>
Date: Thu, 17 Jul 2025 13:43:45 +0200
From: David Hildenbrand <david@...hat.com>
To: wang lian <lianux.mm@...il.com>, akpm@...ux-foundation.org
Cc: Liam.Howlett@...cle.com, brauner@...nel.org, broonie@...nel.org,
 gkwang@...x-info.com, jannh@...gle.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, lorenzo.stoakes@...cle.com, ludovico.zy.wu@...il.com,
 p1ucky0923@...il.com, ryncsn@...il.com, shuah@...nel.org, sj@...nel.org,
 vbabka@...e.cz, zijing.zhang@...ton.me, ziy@...dia.com
Subject: Re: [PATCH] selftests/mm: reuse FORCE_READ to replace "asm
 volatile("" : "+r" (XXX));"

On 17.07.25 12:48, wang lian wrote:
>> On Wed, 16 Jul 2025 20:31:26 +0800 wang lian <lianux.mm@...il.com> wrote:
> 
>>> Several mm selftests use the `asm volatile("" : "+r" (variable));`
>>> construct to force a read of a variable, preventing the compiler from
>>> optimizing away the memory access. This idiom is cryptic and duplicated
>>> across multiple test files.
>>>
>>> Following a suggestion from David[1], this patch refactors this
>>> common pattern into a FORCE_READ() macro
>>>
>>>   tools/testing/selftests/mm/cow.c              | 30 +++++++++----------
>>>   tools/testing/selftests/mm/hugetlb-madvise.c  |  5 +---
>>>   tools/testing/selftests/mm/migration.c        | 13 ++++----
>>>   tools/testing/selftests/mm/pagemap_ioctl.c    |  4 +--
>>>   .../selftests/mm/split_huge_page_test.c       |  4 +--
>>>   5 files changed, 24 insertions(+), 32 deletions(-)
> 
>> The patch forgot to move the FORCE_READ definition into a header?
> 
> Hi Andrew,
> You are absolutely right. My apologies for the inconvenience.
> This patch was sent standalone based on a suggestion from David during
> the discussion of a previous, larger patch series. In that original series,
> I had already moved the FORCE_READ() macro definition into vm_util.h.
> 
> You can find the original patch series and discussion at this link:
> https://lore.kernel.org/lkml/20250714130009.14581-1-lianux.mm@gmail.com/
> It should also be in your mailing list archive.
> 
> To make this easier to review and apply, I can send a new, small patch series
> that first introduces the FORCE_READ() macro in vm_util.h and then applies this refactoring.

Please simply perform the move of FORCE_READ() in this very patch where 
you also use it elswehere.

I missed that when skimming over this patch.

-- 
Cheers,

David / dhildenb