linux-kernel - Re: [PATCH v2 2/8] selftests/mm: protection

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Message-ID: <28d7d186-5068-4546-90af-f9a9a0147f21@arm.com>
Date: Mon, 18 Aug 2025 10:12:22 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
 Peter Xu <peterx@...hat.com>, Leon Romanovsky <leon@...nel.org>,
 Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, Shuah Khan <shuah@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>
Cc: kernel@...labora.com
Subject: Re: [PATCH v2 2/8] selftests/mm: protection_keys: Fix dead code

+Dave Hansen

On 31/07/2025 18:01, Muhammad Usama Anjum wrote:
> The while loop doesn't execute and following warning gets generated:
>
> protection_keys.c:561:15: warning: code will never be executed
> [-Wunreachable-code]
>                 int rpkey = alloc_random_pkey();
>
> Let's enable the while loop such that it gets executed nr_iterations
> times. Simplify the code a bit as well.
>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
>  tools/testing/selftests/mm/protection_keys.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/protection_keys.c b/tools/testing/selftests/mm/protection_keys.c
> index 23ebec367015f..6281d4c61b50e 100644
> --- a/tools/testing/selftests/mm/protection_keys.c
> +++ b/tools/testing/selftests/mm/protection_keys.c
> @@ -557,13 +557,11 @@ int mprotect_pkey(void *ptr, size_t size, unsigned long orig_prot,
>  	int nr_iterations = random() % 100;
>  	int ret;
>  
> -	while (0) {
> +	while (nr_iterations-- >= 0) {

Now that is a good catch! I had never realised this whole loop was dead
code...

The question is whether we really want it. This code looked exactly this
way when it was merged [1] so it has never been run. It looks
suspiciously like debug code.

Dave, should we just remove this?

- Kevin

[1]
https://lore.kernel.org/all/20160729163024.FC5A0C2D@viggo.jf.intel.com/T/#u

>  		int rpkey = alloc_random_pkey();
>  		ret = sys_mprotect_pkey(ptr, size, orig_prot, pkey);
>  		dprintf1("sys_mprotect_pkey(%p, %zx, prot=0x%lx, pkey=%ld) ret: %d\n",
>  				ptr, size, orig_prot, pkey, ret);
> -		if (nr_iterations-- < 0)
> -			break;
>  
>  		dprintf1("%s()::%d, ret: %d pkey_reg: 0x%016llx"
>  			" shadow: 0x%016llx\n",