linux-kernel - Re: [PATCH v2 4/7] mm/selftests: Fix split_huge_page_test failure on systems with 64KB page size

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Message-ID: <0e099baa-0a36-4ffd-821f-a7a4856fd52f@linux.ibm.com>
Date: Thu, 3 Jul 2025 14:28:59 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>,
        Aboorva Devarajan <aboorvad@...ux.ibm.com>, akpm@...ux-foundation.org,
        Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, shuah@...nel.org,
        pfalcato@...e.de, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        baohua@...nel.org
Cc: linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, ritesh.list@...il.com
Subject: Re: [PATCH v2 4/7] mm/selftests: Fix split_huge_page_test failure on
 systems with 64KB page size


On 7/3/25 1:52 PM, David Hildenbrand wrote:
> On 03.07.25 08:06, Aboorva Devarajan wrote:
>> From: Donet Tom <donettom@...ux.ibm.com>
>>
>> The split_huge_page_test fails on systems with a 64KB base page size.
>> This is because the order of a 2MB huge page is different:
>>
>> On 64KB systems, the order is 5.
>>
>> On 4KB systems, it's 9.
>>
>> The test currently assumes a maximum huge page order of 9, which is only
>> valid for 4KB base page systems. On systems with 64KB pages, attempting
>> to split huge pages beyond their actual order (5) causes the test to 
>> fail.
>>
>> In this patch, we calculate the huge page order based on the system's 
>> base
>> page size. With this change, the tests now run successfully on both 64KB
>> and 4KB page size systems.
>>
>> Fixes: fa6c02315f745 ("mm: huge_memory: a new debugfs interface for 
>> splitting THP tests")
>> Signed-off-by: Donet Tom <donettom@...ux.ibm.com>
>> Signed-off-by: Aboorva Devarajan <aboorvad@...ux.ibm.com>
>> ---
>>   .../selftests/mm/split_huge_page_test.c       | 23 ++++++++++++++-----
>>   1 file changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c 
>> b/tools/testing/selftests/mm/split_huge_page_test.c
>> index aa7400ed0e99..38296a758330 100644
>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>> @@ -514,6 +514,15 @@ void split_thp_in_pagecache_to_order_at(size_t 
>> fd_size, const char *fs_loc,
>>       }
>>   }
>>   +static unsigned int get_order(unsigned int pages)
>> +{
>> +    unsigned int order = 0;
>> +
>> +    while ((1U << order) < pages)
>> +        order++;
>> +    return order;
>> +}
>
> I think this can simply be
>
> return 32 - __builtin_clz(pages - 1);
>
> That mimics what get_order() in the kernel does for BITS_PER_LONG == 32
>
> or simpler
>
> return 31 - __builtin_clz(pages);
>
> E.g., if pages=512, you get 31-22=9


Sure David, We will  change it.


>
>> +
>>   int main(int argc, char **argv)
>>   {
>>       int i;
>> @@ -523,6 +532,7 @@ int main(int argc, char **argv)
>>       const char *fs_loc;
>>       bool created_tmp;
>>       int offset;
>> +    unsigned int max_order;
>>         ksft_print_header();
>>   @@ -534,32 +544,33 @@ int main(int argc, char **argv)
>>       if (argc > 1)
>>           optional_xfs_path = argv[1];
>>   -    ksft_set_plan(1+8+1+9+9+8*4+2);
>> -
>>       pagesize = getpagesize();
>>       pageshift = ffs(pagesize) - 1;
>>       pmd_pagesize = read_pmd_pagesize();
>>       if (!pmd_pagesize)
>>           ksft_exit_fail_msg("Reading PMD pagesize failed\n");
>>   +    max_order = get_order(pmd_pagesize/pagesize);
>> + 
>> ksft_set_plan(1+(max_order-1)+1+max_order+max_order+(max_order-1)*4+2);
>
> Wow. Can we simplify that in any sane way?


It is counting test by test. Let me try to simplify it and send the next 
version.


>
>> +
>>       fd_size = 2 * pmd_pagesize;
>>         split_pmd_zero_pages();
>>   -    for (i = 0; i < 9; i++)
>> +    for (i = 0; i < max_order; i++)
>>           if (i != 1)
>>               split_pmd_thp_to_order(i);
>>         split_pte_mapped_thp();
>> -    for (i = 0; i < 9; i++)
>> +    for (i = 0; i < max_order; i++)
>>           split_file_backed_thp(i);
>>         created_tmp = prepare_thp_fs(optional_xfs_path, fs_loc_template,
>>               &fs_loc);
>> -    for (i = 8; i >= 0; i--)
>> +    for (i = (max_order-1); i >= 0; i--)
>
> "i = max_order - 1"


I will change it.


>
>> split_thp_in_pagecache_to_order_at(fd_size, fs_loc, i, -1);
>>   -    for (i = 0; i < 9; i++)
>> +    for (i = 0; i < max_order; i++)
>>           for (offset = 0;
>>                offset < pmd_pagesize / pagesize;
>>                offset += MAX(pmd_pagesize / pagesize / 4, 1 << i))
>
>