linux-kernel - Re: [PATCH] tools/testing/selftests: fix guard region test tmpfs assumption

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Message-ID: <9f1cd31f-8e85-4ae6-9fa5-85db7a96d2fe@arm.com>
Date: Mon, 28 Apr 2025 11:23:35 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/testing/selftests: fix guard region test tmpfs
 assumption

On 25/04/2025 17:24, Lorenzo Stoakes wrote:
> The current implementation of the guard region tests assume that /tmp is
> mounted as tmpfs, that is shmem.
> 
> This isn't always the case, and at least one instance of a spurious test
> failure has been reported as a result.
> 
> This assumption is unsafe, rushed and silly - and easily remedied by simply
> using memfd, so do so.
> 
> We also have to fixup the readonly_file test to explicitly only be
> applicable to file-backed cases.
> 
> Reported-by: Ryan Roberts <ryan.roberts@....com>
> Closes: https://lore.kernel.org/linux-mm/a2d2766b-0ab4-437b-951a-8595a7506fe9@arm.com/
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  tools/testing/selftests/mm/guard-regions.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> index c39dd26c47a3..0cd9d236649d 100644
> --- a/tools/testing/selftests/mm/guard-regions.c
> +++ b/tools/testing/selftests/mm/guard-regions.c
> @@ -272,12 +272,16 @@ FIXTURE_SETUP(guard_regions)
>  	self->page_size = (unsigned long)sysconf(_SC_PAGESIZE);
>  	setup_sighandler();
>  
> -	if (variant->backing == ANON_BACKED)
> +	switch (variant->backing) {
> +	case ANON_BACKED:
>  		return;
> -
> -	self->fd = open_file(
> -		variant->backing == SHMEM_BACKED ? "/tmp/" : "",
> -		self->path);
> +	case LOCAL_FILE_BACKED:
> +		self->fd = open_file("", self->path);
> +		break;
> +	case SHMEM_BACKED:
> +		self->fd = memfd_create(self->path, 0);
> +		break;
> +	}
>  
>  	/* We truncate file to at least 100 pages, tests can modify as needed. */
>  	ASSERT_EQ(ftruncate(self->fd, 100 * self->page_size), 0);
> @@ -1697,7 +1701,7 @@ TEST_F(guard_regions, readonly_file)
>  	char *ptr;
>  	int i;
>  
> -	if (variant->backing == ANON_BACKED)
> +	if (variant->backing != LOCAL_FILE_BACKED)
>  		SKIP(return, "Read-only test specific to file-backed");
>  
>  	/* Map shared so we can populate with pattern, populate it, unmap. */