Re: [PATCH v4] selftests/mm: hugetlb_madv_vs_map: add underflow test

[Mike Rapoport <rppt@kernel.org>]

On Fri, Sep 04, 2026 at 09:34:19PM +0200, Guillaume Morin wrote:
> Add a test that checks for underflows when a parent unmaps the page
> first. Also check that when the child exits the reserve count is
> correct.
> 
> Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
> Signed-off-by: Guillaume Morin <guillaume@morinfr.org
> Cc: Breno Leitao <leitao@debian.org>
> Cc: Mike Rapoport <rppt@kernel.org>

With some small nits below fixed

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> v4: address Mike's comments
> v3: address Mike's comments
> v2: refactor tests into 2 functions
> 
> @@ -100,7 +98,7 @@ int main(void)
>  				-1, 0);
>  
>  		if ((unsigned long)huge_ptr == -1)
> -			ksft_exit_fail_msg("Failed to allocate huge page\n");
> +			ksft_exit_fail_perror("Failed to allocate huge page\n");

Nit: _perror() does not need "\n".

>  		pthread_create(&thread1, NULL, madv, NULL);
>  		pthread_create(&thread2, NULL, touch, NULL);
> @@ -120,5 +118,110 @@ int main(void)
>  	}
>  
>  	ksft_test_result_pass("No unexpected huge page allocations\n");
> +}
> +
> +/*  We create a child process, then unmap the page in the parent while the child
> + *  waits and verify that there is no underflow of the reserved count. We also
> + *  verify that after the child exits, the reserved count is properly restored.
> + */
> +void test_underflow(void)
> +{
> +	pid_t pid;
> +	int pipe_fds[2];
> +	unsigned long nr_reserved = 0;
> +
> +	huge_ptr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
> +			MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0);
> +
> +	if ((unsigned long)huge_ptr == -1)
> +		ksft_exit_fail_perror("Failed to allocate huge page\n");

Ditto.

> +
> +	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
> +	if (nr_reserved != 1)
> +		ksft_exit_fail_msg("Unexpected number of reserved pages: %lu, expected 1\n",
> +				   nr_reserved);
> +

-- 
Sincerely yours,
Mike.