[PATCH] mm/selftests: Fix -Wtautological-compare warning in mremap_test.c

[PATCH] mm/selftests: Fix -Wtautological-compare warning in mremap_test.c

[Wake Liu]

The compiler warns about a tautological comparison in mremap_test.c:
"pointer comparison always evaluates to false [-Wtautological-compare]"

This occurs when checking for unsigned overflow:
  if (addr + c.dest_alignment < addr)

Cast 'addr' to 'unsigned long long' to ensure the comparison is performed
with a wider type, correctly detecting potential overflow and resolving
the warning.

Signed-off-by: Wake Liu <wakel@google.com>
---
 tools/testing/selftests/mm/mremap_test.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
index bf2863b102e3..c4933f4cbd48 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -1032,7 +1032,7 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 	/* Don't destroy existing mappings unless expected to overlap */
 	while (!is_remap_region_valid(addr, c.region_size) && !c.overlapping) {
 		/* Check for unsigned overflow */
-		if (addr + c.dest_alignment < addr) {
+		if ((unsigned long long)addr + c.dest_alignment < (unsigned long long)addr) {
 			ksft_print_msg("Couldn't find a valid region to remap to\n");
 			ret = -1;
 			goto clean_up_src;
-- 
2.51.2.1041.gc1ab5b90ca-goog

Re: [PATCH] mm/selftests: Fix -Wtautological-compare warning in mremap_test.c

[Andrew Morton]

On Tue, 11 Nov 2025 01:51:55 +0800 Wake Liu <wakel@google.com> wrote:

> The compiler warns about a tautological comparison in mremap_test.c:
> "pointer comparison always evaluates to false [-Wtautological-compare]"
> 
> This occurs when checking for unsigned overflow:
>   if (addr + c.dest_alignment < addr)
> 
> Cast 'addr' to 'unsigned long long' to ensure the comparison is performed
> with a wider type, correctly detecting potential overflow and resolving
> the warning.
> 

Thanks.  I recently merged

https://lore.kernel.org/all/20251108161829.25105-1-ankitkhushwaha.linux@gmail.com/T/#u

there has been some discussion...

> --- a/tools/testing/selftests/mm/mremap_test.c
> +++ b/tools/testing/selftests/mm/mremap_test.c
> @@ -1032,7 +1032,7 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
>  	/* Don't destroy existing mappings unless expected to overlap */
>  	while (!is_remap_region_valid(addr, c.region_size) && !c.overlapping) {
>  		/* Check for unsigned overflow */
> -		if (addr + c.dest_alignment < addr) {
> +		if ((unsigned long long)addr + c.dest_alignment < (unsigned long long)addr) {
>  			ksft_print_msg("Couldn't find a valid region to remap to\n");
>  			ret = -1;
>  			goto clean_up_src;

I wonder if we'd be better off borrowing ideas from
include/linux/overflow.h:check_add_overflow().  Did anyone try
__builtin_add_overflow() here?