Re: [PATCH 2/2] kselftest: mm: replace usage of /proc/self/smaps for check_huge_xxx() helper

[Zi Yan]

On Thu Aug 20, 2026 at 8:26 PM EDT, Yeoreum Yun wrote:
> On Thu, Aug 20, 2026 at 08:09:12PM -0400, Zi Yan wrote:
>> On Thu Aug 20, 2026 at 5:26 PM EDT, Yeoreum Yun wrote:
>> >> On Thu Aug 20, 2026 at 3:40 PM EDT, Yeoreum Yun wrote:
>> >> > Since glibc commit 321e1fc73f (“malloc: Enable 2MB THP by default on AArch64”),
>> >> > glibc may call madvise(MADV_HUGEPAGE) for sufficiently large allocations
>> >> > made by memalign().
>> 
>> I just googled the commit and find that Dev did that. ;)
>> 
>> >> >
>> >> > The underlying VMA may start at a different address from the aligned
>> >> > address returned by memalign(). Furthermore, a subsequent
>> >> > madvise(MADV_HUGEPAGE) call does not split the VMA because the flag is
>> >> > already set.
>> >> >
>> >> > This causes split_huge_page_test to fail because the check_huge_xxx()
>> >> > helpers incorrectly require the address returned by memalign() to
>> >> > match the VMA start address reported in /proc/self/smaps.
>> >> >
>> >> > Fix this by using /proc/self/pagemap and /proc/kpageflags instead of
>> >> > /proc/self/smaps to detect huge pages and change the meaning of
>> >> > check_huge_xxx()'s nr_hpages argument:
>> >> 
>> >> Have you checked Baolin's patches[1] in mm-new? They resue
>> >> gather_after_split_folio_orders() to reimplement check_huge_xxx(), also
>> >> based on pagemap and kpageflags. Does it fix the issue?
>> >> 
>> >> [1] https://lore.kernel.org/all/cover.1785985999.git.baolin.wang@xxxxxxxxxxxxxxxxx/
>> >>
>> >
>> > Unfortunately, No. since __check_pmd_huge() in check_huge_xxx() still use
>> > /proc/self/smaps [1] for pmd THP, it still has problem though ths patch
>> > series applied.
>> >
>> > [1] https://lore.kernel.org/all/56b16691f605426b33b5cf47319233de6127a6b3.1785985999.git.baolin.wang@xxxxxxxxxxxxxxxxx/
>> 
>> In that case, is it possible to use and extend check_large_folios() for
>> all check_huge_xxx()? You still need pagemap_scan_get_categories() to
>> check PAGE_IS_HUGE to identify huge mappings. Or at least
>> check_huge_xxx() in your patch can share most of the code.
>
> Agree. but TBH, I think check_large_folios() can replace checking
> PAGE_IS_HUGE and keep the later part to check wehther PAGE_IS_FILE

IIUC, PAGE_IS_HUGE comes from checking page table entry (see
pagemap_thp_category()), whereas check_large_folios() checks folios,
namely physical memory. A PTE-mapped order-9 (4KB base page) folio is
!PAGE_IS_HUGE but check_large_folios() returns true for PMD_SIZE.
check_huge_xxx() are checking PMD-mapped folios, so PAGE_IS_HUGE is
functionally equivalent.

> and SWAPBACKED according to check_huge_xxx().
>


-- 
Best Regards,
Yan, Zi