Re: [PATCH] selftests/cgroup: test_zswap: use the total RAM in bytes

[Tao Cui]

Hi Sasha,

在 2026/11/4 00:39, Sasha Levin 写道:
> test_no_kmem_bypass() uses the totalram field of struct sysinfo as a
> byte count, but it is in units of mem_unit, which can be larger than 1
> on 32-bit architectures. There totalram also can't exceed GB(4), so the
> size check never triggers:
> 
>   test_zswap.c:609:24: warning: result of comparison of constant 4294967296 with expression of type '__kernel_ulong_t' (aka 'unsigned long') is always false [-Wtautological-constant-out-of-range-compare]
> 
> Multiply totalram by mem_unit as an unsigned long long and use that for
> the check and the values derived from it. Nothing changes on 64-bit,
> where mem_unit is 1.
> 
Built test_zswap before and after on next-20261103 and ran both on a 4G
VM, identical results, no change on 64-bit as claimed.

One small thing: could you add a Fixes tag? The issue seems to date
back to a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem
bypass test"), which introduced the sysinfo block, and 6f615890b848
later reshaped it into the current GB(4) comparison with the warning.

Reviewed-by: Tao Cui <cuitao@xxxxxxxxxx>
Tested-by: Tao Cui <cuitao@xxxxxxxxxx>

Thanks,
Tao
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
> Found with 32-bit builds of the selftests (gcc 16 and clang 21 with
> -m32, with and without -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64);
> KernelCI's 32-bit arm builds show the same warnings. The changed files
> build without them now, and the x86_64 builds don't change.
> 
>  tools/testing/selftests/cgroup/test_zswap.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index d50acc1b83ac8..223e2eb1329ab 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -597,6 +597,7 @@ static int test_no_kmem_bypass(const char *root)
>  	size_t trigger_allocation_size;
>  	int wait_child_iteration = 0;
>  	long stored_pages_threshold;
> +	unsigned long long totalram;
>  	struct sysinfo sys_info;
>  	int ret = KSFT_FAIL;
>  	int child_status;
> @@ -606,10 +607,11 @@ static int test_no_kmem_bypass(const char *root)
>  	/* Read sys info and compute test values accordingly */
>  	if (sysinfo(&sys_info) != 0)
>  		return KSFT_FAIL;
> -	if (sys_info.totalram > GB(4)) {
> +	totalram = (unsigned long long)sys_info.totalram * sys_info.mem_unit;
> +	if (totalram > GB(4)) {
>  		ksft_print_msg(
> -			"requires less than 4GB total ram, sys_info.totalram: %.1fGB\n",
> -			(double)sys_info.totalram / GB(1));
> +			"requires less than 4GB total ram, total ram: %.1fGB\n",
> +			(double)totalram / GB(1));
>  		return KSFT_SKIP;
>  	}
>  	if (access(PATH_ZSWAP_STORED_PAGES, R_OK)) {
> @@ -622,12 +624,12 @@ static int test_no_kmem_bypass(const char *root)
>  		return KSFT_FAIL;
>  	if (read_min_free_kb(&min_free_kb_original))
>  		return KSFT_FAIL;
> -	min_free_kb_high = sys_info.totalram / 2000;
> -	min_free_kb_low = sys_info.totalram / 500000;
> -	values->target_alloc_bytes = (sys_info.totalram - min_free_kb_high * 1000) +
> -		sys_info.totalram * 5 / 100;
> -	stored_pages_threshold = sys_info.totalram / 5 / page_size;
> -	trigger_allocation_size = sys_info.totalram / 20;
> +	min_free_kb_high = totalram / 2000;
> +	min_free_kb_low = totalram / 500000;
> +	values->target_alloc_bytes = (totalram - min_free_kb_high * 1000) +
> +		totalram * 5 / 100;
> +	stored_pages_threshold = totalram / 5 / page_size;
> +	trigger_allocation_size = totalram / 20;
>  
>  	/* Set up test memcg */
>  	test_group = cg_name(root, "kmem_bypass_test");