[PATCH v2] selftests/mm: use global variable to not optimize mmap read variable

[PATCH v2] selftests/mm: use global variable to not optimize mmap read variable

[Pankaj Raghav (Samsung)]

From: Pankaj Raghav <p.raghav@samsung.com>

create_pagecache_thp_and_fd() in split_huge_page_test.c used the
variable dummy to perform mmap read.

However, this test was skipped even on XFS which has large folio
support. The issue was compiler (gcc 13.2.0) was optimizing out the
dummy variable, therefore, not creating huge page in the page cache.

Make it as a global variable to force the compiler not to optimize out
the loop where we read from the mmaped addr.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
Changes since v1:
- Make the dummy variable as a global variable(willy).

 tools/testing/selftests/mm/split_huge_page_test.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index d3c7f5fb3e7b..c4857de2c042 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -23,6 +23,11 @@
 uint64_t pagesize;
 unsigned int pageshift;
 uint64_t pmd_pagesize;
+/*
+ * Used by create_pagecache_thp_and_fd() to do mmap read.
+ * Made it as global to avoid compiler optimizing out the variable.
+ */
+int dummy;
 
 #define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages"
 #define SMAP_PATH "/proc/self/smaps"
@@ -300,7 +305,6 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 		char **addr)
 {
 	size_t i;
-	int __attribute__((unused)) dummy = 0;
 
 	srand(time(NULL));
 

base-commit: d97496ca23a2d4ee80b7302849404859d9058bcd
-- 
2.44.1

Re: [PATCH v2] selftests/mm: use global variable to not optimize mmap read variable

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 868 bytes --]

On 6 Jun 2024, at 8:44, Pankaj Raghav (Samsung) wrote:

> From: Pankaj Raghav <p.raghav@samsung.com>
>
> create_pagecache_thp_and_fd() in split_huge_page_test.c used the
> variable dummy to perform mmap read.
>
> However, this test was skipped even on XFS which has large folio
> support. The issue was compiler (gcc 13.2.0) was optimizing out the
> dummy variable, therefore, not creating huge page in the page cache.
>
> Make it as a global variable to force the compiler not to optimize out
> the loop where we read from the mmaped addr.
>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
> Changes since v1:
> - Make the dummy variable as a global variable(willy).
>
>  tools/testing/selftests/mm/split_huge_page_test.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>

Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks.

Best Regards,
Yan, Zi

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]