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

[Yeoreum Yun <yeoreum.yun@arm.com>]

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@linux.alibaba.com/
> >>
> >
> > 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@linux.alibaba.com/
> 
> 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
and SWAPBACKED according to check_huge_xxx().

BTW, Should I do this after [1] is merged into mm-unstable?

> BTW, in your check_huge_shmem(), expect_huge seems to be inverted
> compared to other check_huge_xxx().

Yes. I've noticed. this was my typo while I change the variable name...

Thanks!

[1] https://lore.kernel.org/all/cover.1785985999.git.baolin.wang@linux.alibaba.com/


> 
> -- 
> Best Regards,
> Yan, Zi
> 

-- 
Sincerely,
Yeoreum Yun