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Date: Thu, 6 Jun 2024 15:28:58 +0000
From: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	gost.dev@samsung.com, mcgrof@kernel.org,
	linux-kselftest@vger.kernel.org, Zi Yan <zi.yan@sent.com>,
	Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH] selftests/mm: use volatile keyword to not optimize mmap
 read variable
Message-ID: <20240606152858.hmtnmieyesv7o5i3@quentin>
References: <20240606135835.600022-1-kernel@pankajraghav.com>
 <ZmHJMyo37QdtTWee@casper.infradead.org>
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In-Reply-To: <ZmHJMyo37QdtTWee@casper.infradead.org>

On Thu, Jun 06, 2024 at 03:35:31PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 06, 2024 at 01:58:35PM +0000, Pankaj Raghav (Samsung) wrote:
> > +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> > @@ -300,7 +300,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
> >  		char **addr)
> >  {
> >  	size_t i;
> > -	int __attribute__((unused)) dummy = 0;
> > +	volatile int __attribute__((unused)) dummy = 0;
> 
> The mistake made by whoever wrote this test was making 'dummy' a stack
> variable.  That lets the compiler figure out that it's unused.  If you
> make it a top-level variable (not static) so the compiler can't tell
> whether it's referenced by a different compilation unit, it can't make
> that deduction.  And you don't need the stupid attibute or volatile on it.

I did cringe a little before adding the volatile keyword. While not a 
fan of global variables, that might be better than all these keywords.

I will send a v2 right away! Thanks.

--
Pankaj