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

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

On 9/8/26 10:23, Yeoreum Yun wrote:
> On Mon, Sep 07, 2026 at 04:36:56PM +0200, David Hildenbrand (Arm) wrote:
>> 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
> 
> Acked. I thought it's enough to spell out in commit message but
> I'll spell out into comment too in this round not refactorying by me
> for ksm test.
> 
>>
>>>  # 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.
> 
> Okay.
> 
>> ... 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;
> 
> Hmm, most of tests care mainly two sizes and shiftes --
> psize()/pshift() and pmd_pagesize() and pmd_pageshift().
> 
> Would it be better to keep both kinda of helpers and
> add size_to_shift() helper for nother hpage size then?

Works for me as well. As long as it's consistent.

Maybe even pmd_psize() and pmd_pshift() to match our custom psize() and pshift()?

-- 
Cheers,

David