linux-kernel - Re: [PATCH v3] selftests/mm: use asm volatile to not optimize mmap read variable

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Date: Fri, 7 Jun 2024 21:39:36 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Shuah Khan <shuah@...nel.org>, linux-mm@...ck.org, gost.dev@...sung.com,
 David Hildenbrand <david@...hat.com>, willy@...radead.org,
 mcgrof@...nel.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, Zi Yan <zi.yan@...t.com>, Pankaj Raghav
 <p.raghav@...sung.com>
Subject: Re: [PATCH v3] selftests/mm: use asm volatile to not optimize mmap
 read variable

On Thu,  6 Jun 2024 20:36:19 +0000 "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com> wrote:

> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> create_pagecache_thp_and_fd() in split_huge_page_test.c used the
> variable dummy to perform mmap read.
> 
> However, this test was skipped even on XFS which has large folio
> support. The issue was compiler (gcc 13.2.0) was optimizing out the
> dummy variable, therefore, not creating huge page in the page cache.
> 
> Use asm volatile() trick to force the compiler not to optimize out
> the loop where we read from the mmaped addr. This is similar to what is
> being done in other tests (cow.c, etc)
> 
> As the variable is now used in the asm statement, remove the unused
> attribute.
> 

What are the runtime effects of this change?  An inappropriate test
failure?  If so, shouldn't we fix 6.9.x kernels also?  And is
fc4d182316bd ("mm: huge_memory: enable debugfs to split huge pages to
any order") an appropriate Fixes: target?