linux-kernel - Re: [PATCH 16/16] selftests/mm: Create MAP_BELOW

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Message-ID: <71528354-4884-41e4-a4a9-318e26187f86@lucifer.local>
Date: Wed, 28 Aug 2024 18:48:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Arnd Bergmann <arnd@...db.de>, Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
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        Christophe Leroy <christophe.leroy@...roup.eu>,
        Naveen N Rao <naveen@...nel.org>, Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Huacai Chen <chenhuacai@...nel.org>,
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        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 16/16] selftests/mm: Create MAP_BELOW_HINT test

On Tue, Aug 27, 2024 at 10:49:22PM GMT, Charlie Jenkins wrote:
> Add a selftest for MAP_BELOW_HINT that maps until it runs out of space
> below the hint address.
>
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> ---
>  tools/testing/selftests/mm/Makefile         |  1 +
>  tools/testing/selftests/mm/map_below_hint.c | 29 +++++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index cfad627e8d94..4e2de85267b5 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -50,6 +50,7 @@ TEST_GEN_FILES += hugepage-shm
>  TEST_GEN_FILES += hugepage-vmemmap
>  TEST_GEN_FILES += khugepaged
>  TEST_GEN_FILES += madv_populate
> +TEST_GEN_FILES += map_below_hint
>  TEST_GEN_FILES += map_fixed_noreplace
>  TEST_GEN_FILES += map_hugetlb
>  TEST_GEN_FILES += map_populate
> diff --git a/tools/testing/selftests/mm/map_below_hint.c b/tools/testing/selftests/mm/map_below_hint.c
> new file mode 100644
> index 000000000000..305274c5af49
> --- /dev/null
> +++ b/tools/testing/selftests/mm/map_below_hint.c
> @@ -0,0 +1,29 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Test the MAP_BELOW_HINT mmap flag.
> + */
> +#include <sys/mman.h>
> +#include "../kselftest.h"
> +
> +#define ADDR 0x1000000UL
> +#define LENGTH (ADDR / 100)
> +
> +#define MAP_BELOW_HINT	  0x8000000	/* Not defined in all libc */
> +
> +/*
> + * Map memory with MAP_BELOW_HINT until no memory left. Ensure that all returned
> + * addresses are below the hint.
> + */
> +int main(int argc, char **argv)
> +{
> +	void *addr;
> +
> +	do {
> +		addr = mmap((void *)ADDR, LENGTH, MAP_ANONYMOUS, MAP_BELOW_HINT, -1, 0);

How can this be correct? mmap() has parameters:

       void *mmap(void addr[.length], size_t length, int prot, int flags,
                  int fd, off_t offset);

You'r setting prot = MAP_ANONYMOUS, flags = MAP_BELOW_HINT...

This surely should be:

mmap(..., PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE | MAP_BELOW_HINT, -1, 0);

> +	} while (addr == MAP_FAILED && (unsigned long)addr <= ADDR);

How can addr == MAP_FAILED (i.e. ~0) and addr <= ADDR? This will just loop
through once...

If you want to make sure you're getting mappings only below the hint until
you start getting MAP_FAILED's you'll need to handle this more robustly.

> +
> +	if (addr != MAP_FAILED && (unsigned long)addr > ADDR)
> +		ksft_exit_fail_msg("mmap returned address above hint with MAP_BELOW_HINT\n");

This is just going to fail because your flags are wrong then wrongly claim
to have passed...

> +
> +	ksft_test_result_pass("MAP_BELOW_HINT works\n");
> +}
>
> --
> 2.45.0
>