[LTP] [PATCH v2 1/1] overcommit_memory: Disable optimization for malloc to prevent false positives

[LTP] [PATCH v2 1/1] overcommit_memory: Disable optimization for malloc to prevent false positives

[Jan Polensky]

When compiling with GCC 15.1.1+, the default-enabled -fmalloc-dce=2 and
-fallocation-dce under -O2 can eliminate the entire malloc block because its
result is only compared to NULL and/or passed to free().

This leads to false positives in tests that expect malloc() to fail under memory
pressure, as the allocation is optimized away.

Disable this optimization for the affected function to preserve the intended
test behavior.

References: https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Optimize-Options.html
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
Changes since v1 (thanks to Cyril Hrubis):
- Use LTP_VAR_USED macro to prevent GCC from optimizing away malloc().

 testcases/kernel/mem/tunable/overcommit_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/testcases/kernel/mem/tunable/overcommit_memory.c b/testcases/kernel/mem/tunable/overcommit_memory.c
index b5beebbcd3c4..9b2cb479d5ee 100644
--- a/testcases/kernel/mem/tunable/overcommit_memory.c
+++ b/testcases/kernel/mem/tunable/overcommit_memory.c
@@ -62,6 +62,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <limits.h>
+#include "tst_common.h"
 #include "tst_test.h"

 #define DEFAULT_OVER_RATIO	50L
@@ -155,7 +156,7 @@ static int heavy_malloc(long size)
 {
 	char *p;

-	p = malloc(size * TST_KB);
+	LTP_VAR_USED(p) = malloc(size * TST_KB);
 	if (p != NULL) {
 		tst_res(TINFO, "malloc %ld kB successfully", size);
 		free(p);
--
2.49.0


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Re: [LTP] [PATCH v2 1/1] overcommit_memory: Disable optimization for malloc to prevent false positives

[Jan Stancek via ltp]

On Thu, May 22, 2025 at 5:21 PM Jan Polensky <japo@linux.ibm.com> wrote:
>
> When compiling with GCC 15.1.1+, the default-enabled -fmalloc-dce=2 and
> -fallocation-dce under -O2 can eliminate the entire malloc block because its
> result is only compared to NULL and/or passed to free().
>
> This leads to false positives in tests that expect malloc() to fail under memory
> pressure, as the allocation is optimized away.
>
> Disable this optimization for the affected function to preserve the intended
> test behavior.
>
> References: https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Optimize-Options.html
> Signed-off-by: Jan Polensky <japo@linux.ibm.com>

Pushed.

Thanks,
Jan

> ---
> Changes since v1 (thanks to Cyril Hrubis):
> - Use LTP_VAR_USED macro to prevent GCC from optimizing away malloc().
>
>  testcases/kernel/mem/tunable/overcommit_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/testcases/kernel/mem/tunable/overcommit_memory.c b/testcases/kernel/mem/tunable/overcommit_memory.c
> index b5beebbcd3c4..9b2cb479d5ee 100644
> --- a/testcases/kernel/mem/tunable/overcommit_memory.c
> +++ b/testcases/kernel/mem/tunable/overcommit_memory.c
> @@ -62,6 +62,7 @@
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <limits.h>
> +#include "tst_common.h"
>  #include "tst_test.h"
>
>  #define DEFAULT_OVER_RATIO     50L
> @@ -155,7 +156,7 @@ static int heavy_malloc(long size)
>  {
>         char *p;
>
> -       p = malloc(size * TST_KB);
> +       LTP_VAR_USED(p) = malloc(size * TST_KB);
>         if (p != NULL) {
>                 tst_res(TINFO, "malloc %ld kB successfully", size);
>                 free(p);
> --
> 2.49.0
>
>
> --
> Mailing list info: https://lists.linux.it/listinfo/ltp
>


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