Re: [PATCH v2] selftests/mm/uffd: remove static address usage in shmem_allocate_area()

[Mike Rapoport]

On Thu, Nov 13, 2025 at 03:20:33PM +0100, Mehdi Ben Hadj Khelifa wrote:
> The current shmem_allocate_area() implementation uses a hardcoded virtual
> base address (BASE_PMD_ADDR) as a hint for mmap() when creating shmem-backed
> test areas. This approach is fragile and may fail on systems with ASLR or
> different virtual memory layouts, where the chosen address is unavailable.
> 
> Replace the static base address with a dynamically reserved address range
> obtained via mmap(NULL, ..., PROT_NONE). The memfd-backed areas and their
> alias are then mapped into that reserved region using MAP_FIXED, preserving
> the original layout and aliasing semantics while avoiding collisions with
> unrelated mappings.
> 
> This change improves robustness and portability of the test suite without
> altering its behavior or coverage.
> 
> Suggested-by: Mike Rapoport <rppt@xxxxxxxxxx>
> Signed-off-by: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@xxxxxxxxx>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
> Testing(Retested):
> A diff between running the mm selftests on 6.18-rc5 from before and after
> the change show no regression on x86_64 architecture with 32GB DDR5 RAM.
> 
> ChangeLog:
> 
> Changes from v1:
> 
> -Implemented Mike's suggestions to make cleanup code more clear.
> 
> Link:https://lore.kernel.org/all/20251111205739.420009-1-mehdi.benhadjkhelifa@xxxxxxxxx/
> 
>  tools/testing/selftests/mm/uffd-common.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
> index 994fe8c03923..edd02328f77b 100644
> --- a/tools/testing/selftests/mm/uffd-common.c
> +++ b/tools/testing/selftests/mm/uffd-common.c
> @@ -10,7 +10,6 @@
>  uffd_test_ops_t *uffd_test_ops;
>  uffd_test_case_ops_t *uffd_test_case_ops;
>  
> -#define BASE_PMD_ADDR ((void *)(1UL << 30))
>  
>  /* pthread_mutex_t starts at page offset 0 */
>  pthread_mutex_t *area_mutex(char *area, unsigned long nr, uffd_global_test_opts_t *gopts)
> @@ -142,30 +141,37 @@ static int shmem_allocate_area(uffd_global_test_opts_t *gopts, void **alloc_area
>  	unsigned long offset = is_src ? 0 : bytes;
>  	char *p = NULL, *p_alias = NULL;
>  	int mem_fd = uffd_mem_fd_create(bytes * 2, false);
> +	size_t region_size = bytes * 2 + hpage_size;
>  
> -	/* TODO: clean this up.  Use a static addr is ugly */
> -	p = BASE_PMD_ADDR;
> -	if (!is_src)
> -		/* src map + alias + interleaved hpages */
> -		p += 2 * (bytes + hpage_size);
> +	void *reserve = mmap(NULL, region_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS,
> +			-1, 0);
> +	if (reserve == MAP_FAILED) {
> +		close(mem_fd);
> +		return -errno;
> +	}
> +
> +	p = reserve;
>  	p_alias = p;
>  	p_alias += bytes;
>  	p_alias += hpage_size;  /* Prevent src/dst VMA merge */
>  
> -	*alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
> +	*alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED,
>  			   mem_fd, offset);
>  	if (*alloc_area == MAP_FAILED) {
>  		*alloc_area = NULL;
> +		munmap(reserve, region_size);
> +		close(mem_fd);
>  		return -errno;
>  	}
>  	if (*alloc_area != p)
>  		err("mmap of memfd failed at %p", p);
>  
> -	area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
> +	area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED,
>  			  mem_fd, offset);
>  	if (area_alias == MAP_FAILED) {
> -		munmap(*alloc_area, bytes);
>  		*alloc_area = NULL;
> +		munmap(reserve, region_size);
> +		close(mem_fd);
>  		return -errno;
>  	}
>  	if (area_alias != p_alias)
> -- 
> 2.51.2
> 

-- 
Sincerely yours,
Mike.