Re: [PATCH v2] kselftest: mm: remove HPAGE_SIZE and HPAGE_SHIFT macro

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/7/26 10:15, Yeoreum Yun wrote:
> HPAGE_SIZE and HPAGE_SHIFT macro is written based on the 4KB PAGE_SIZE.
> When this macro is used in some test, test result would be strange in
> the system where PAGE_SIZE is more then 4KB.
> 
> Here is the example with transhuge-stress test with 16KB PAGE_SIZE:
> 
>   transhuge-stress: allocate 61073 transhuge pages, using 122146 MiB virtual memory and 1908 MiB of ram
>     3.292 s/loop, 0.054 ms/page,  37106.002 MiB/s	2566 succeed, 58507 failed, 2566 different pages
>     0.591 s/loop, 0.010 ms/page, 206850.792 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     0.527 s/loop, 0.009 ms/page, 231895.107 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     0.527 s/loop, 0.009 ms/page, 231839.704 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     0.528 s/loop, 0.009 ms/page, 231544.782 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     0.528 s/loop, 0.009 ms/page, 231462.074 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     0.527 s/loop, 0.009 ms/page, 231770.300 MiB/s	   0 succeed, 61073 failed,    0 different pages
>     ...
>   ok 1 Completed
> 
> Remove the HPAGE_SIZE and HPAGE_SHIFT macros and introduce pmd_page_shift()
> helper to get the HPAGE_SHIFT properly. For HPAGE_SIZE, use pre-existing
> helper, read_pmd_pagesize().
> 
> Also, run the KSM_MERGE_TIME_HUGE_PAGES test with a size of 512 MiB,
> which is the least common multiple of the PMD sizes for 4 KiB, 16 KiB,
> and 64 KiB base pages. Since allocate_transhuge() allocates mappings in
> PMD-sized units, the test may fail with the previous size of 100 MiB,
> which is not a multiple of the PMD size when the base page size is
> 16 KiB or 64 KiB.
> 
> After this patch, output of transhuge-stress:
> 
>   transhuge-stress: allocate 3817 transhuge pages, using 122146 MiB virtual memory and 119 MiB of ram
>     2.558 s/loop, 0.670 ms/page,  47755.759 MiB/s	2585 succeed, 1232 failed, 2585 different pages
>     2.640 s/loop, 0.692 ms/page,  46268.432 MiB/s	2585 succeed, 1232 failed, 2585 different pages
>     2.635 s/loop, 0.690 ms/page,  46360.298 MiB/s	2585 succeed, 1232 failed, 2585 different pages
>     2.782 s/loop, 0.729 ms/page,  43899.795 MiB/s	2616 succeed, 1201 failed, 2616 different pages
>     2.692 s/loop, 0.705 ms/page,  45380.876 MiB/s	2627 succeed, 1190 failed, 2627 different pages
>     2.612 s/loop, 0.684 ms/page,  46765.812 MiB/s	2628 succeed, 1189 failed, 2628 different pages
>     2.683 s/loop, 0.703 ms/page,  45520.990 MiB/s	2630 succeed, 1187 failed, 2630 different pages
>     2.727 s/loop, 0.714 ms/page,  44789.321 MiB/s	2631 succeed, 1186 failed, 2631 different pages
>     ...
>   ok 1 Completed
> 
> Suggested-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---


[...]

> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
> index d09f9f6a384e..24aa1de27c25 100755
> --- a/tools/testing/selftests/mm/run_vmtests.sh
> +++ b/tools/testing/selftests/mm/run_vmtests.sh
> @@ -360,8 +360,8 @@ fi
>  CATEGORY="memfd_secret" run_test ./memfd_secret
>  fi
>  
> -# KSM KSM_MERGE_TIME_HUGE_PAGES test with size of 100
> -CATEGORY="ksm" run_test ./ksm_tests -H -s 100
> +# KSM KSM_MERGE_TIME_HUGE_PAGES test with size of 512
> +CATEGORY="ksm" run_test ./ksm_tests -H -s 512


Why the magic value 512? (desrves a comment)

A clear sign that the ksm tests must be rewritten to be standalone and just
handle this internally. @Sarthak

>  # KSM KSM_MERGE_TIME test with size of 100
>  CATEGORY="ksm" run_test ./ksm_tests -P -s 100
>  # KSM MADV_MERGEABLE test with 10 identical pages

[...]

> index 4821a3563036..f75bf0bacd18 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -21,6 +21,8 @@
>  
>  unsigned int __page_size;
>  unsigned int __page_shift;
> +uint64_t __pmd_page_size;
> +uint64_t __pmd_page_shift;
>  
>  uint64_t pagemap_get_entry(int fd, char *start)
>  {
> @@ -161,6 +163,9 @@ uint64_t read_pmd_pagesize(void)
>  	char buf[20];
>  	ssize_t num_read;
>  
> +	if (__pmd_page_size)
> +		return __pmd_page_size;
> +
>  	fd = open(PMD_SIZE_FILE_PATH, O_RDONLY);
>  	if (fd == -1)
>  		return 0;
> @@ -173,7 +178,25 @@ uint64_t read_pmd_pagesize(void)
>  	buf[num_read] = '\0';
>  	close(fd);
>  
> -	return strtoul(buf, NULL, 10);
> +	__pmd_page_size = strtoul(buf, NULL, 10);
> +
> +	return __pmd_page_size;
> +}
> +
> +uint64_t pmd_page_shift(void)
> +{
> +	if (__pmd_page_shift)
> +		return __pmd_page_shift;
> +
> +	if (!__pmd_page_size)
> +		__pmd_page_size = read_pmd_pagesize();
> +
> +	if (!__pmd_page_size)
> +		return 0;
> +
> +	__pmd_page_shift = (ffsl(__pmd_page_size) - 1);
> +
> +	return __pmd_page_shift;
>  }
The interface is a bit inconsistent.

read_pmd_pagesize() vs. pmd_page_shift().

Consider renaming read_pmd_pagesize() to pmd_pagesize() in a cleanup patch.

... or (better?) adding a size_to_shift() helper instead?

I think most places that want the shift also want the size.

So this can just be

const uint64_t hpage_size = read_pmd_pagesize();
const uint64_t hpage_shift = size_to_shift(hpage_size);


In size_to_shift(), you can just handle

if (size == 0)
	return 0;

etc.

-- 
Cheers,

David