Re: [PATCH v5 2/3] kselftest: mm: replace usage of /proc/self/smaps for check_huge_xxx() helper

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

On 9/7/26 10:19, 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().
> 
> 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.
> 
> Instead of relying on /proc/self/smaps, use /proc/self/pagemap and
> /proc/kpageflags to detect huge-page mappings and large folios:
> 
>   1. If hpage_size == pmd_pagesize, check PAGE_IS_HUGE instead of
>      using check_large_folios(), since only the mapping type matters.
>      This identifies PMD-mapped huge pages.
>   2. Otherwise, use check_large_folios() to detect large folios. This
>      covers mTHP cases.
>   3. Check the folio flags according to the type of huge page.
> 
> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> Suggested-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  tools/testing/selftests/mm/vm_util.c | 145 ++++++++++++++++++++++-------------
>  tools/testing/selftests/mm/vm_util.h |   1 +
>  2 files changed, 92 insertions(+), 54 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 4821a3563036..dc62ce84e143 100644

[...]

>  
> -	if (hpage_size == pmd_pagesize)
> -		return __check_pmd_huge(addr, "AnonHugePages: ", nr_hpages, hpage_size);
> +static bool check_huge_type(uint64_t categories, uint64_t kpageflags,
> +			    enum check_huge_type type)
> +{
> +	const bool file = categories & PAGE_IS_FILE;
> +	const bool swapbacked = kpageflags & KPF_SWAPBACKED;
> +
> +	switch (type) {
> +	case CHECK_HUGE_ANON:
> +		return !file;
> +	case CHECK_HUGE_FILE:
> +		return file && !swapbacked;
> +	case CHECK_HUGE_SHMEM:
> +		return file & swapbacked;
> +	}

Why do we even need CHECK_HUGE_SHMEM?

That's really just a legacy thing for using smaps to identify huge pages.

It's sufficient to identify CHECK_HUGE_FILE if you know that you have shmem mapping.

You will not arbitrarily have non-shmem folios in a shmem mapping :)

>  
> -	return check_large_folios(addr, len, nr_hpages, hpage_size);
> +	return false;
>  }
>  
> -bool check_huge_file(void *addr, size_t len, int nr_hpages, uint64_t hpage_size)
> +static bool __check_huge(void *addr, size_t len, int nr_hpages,
> +			 uint64_t hpage_size, enum check_huge_type type)
>  {
> -	uint64_t pmd_pagesize = read_pmd_pagesize();
> +	bool ret = false;
> +	int pagemap_fd, kpageflags_fd;
> +	int nr_pmd_mappings = 0;
> +	uint64_t pmd_pagesize, scan_mapping_size;
> +	uint64_t categories, kpf;
> +	unsigned long pfn;
> +	bool check_pmd_mapping, allow_nonpresent;
> +	char *start = addr;
> +	char *end = start + len;
>  
> +	pmd_pagesize = read_pmd_pagesize();
>  	if (!pmd_pagesize)
>  		ksft_exit_fail_msg("reading PMD pagesize failed\n");
>  
> -	if (hpage_size == pmd_pagesize)
> -		return __check_pmd_huge(addr, "FilePmdMapped:", nr_hpages, hpage_size);
> +	check_pmd_mapping = (hpage_size == pmd_pagesize);
> +	scan_mapping_size = (nr_hpages > 0) ? hpage_size : psize();
> +	/* Some mTHP tests check a partially populated PMD-sized range. */
> +	allow_nonpresent = (uint64_t)nr_hpages * hpage_size < len;
>  
> -	return check_large_folios(addr, len, nr_hpages, hpage_size);
> +	pagemap_fd = open(PAGEMAP_PATH, O_RDONLY);
> +	if (pagemap_fd < 0)
> +		ksft_exit_fail_msg("open pagemap fail\n");
> +
> +	kpageflags_fd = open(KPAGEFLAGS_PATH, O_RDONLY);
> +	if (kpageflags_fd < 0) {
> +		close(pagemap_fd);

No need to cleanup when exiting.

> +		ksft_exit_fail_msg("open kpageflags fail\n");
> +	}
> +
> +	if (!check_pmd_mapping &&
> +	    !check_large_folios(pagemap_fd, kpageflags_fd,
> +				addr, len, nr_hpages, hpage_size))
> +		goto out;
> +
> +	for (; start < end; start += scan_mapping_size) {
> +		categories = pagemap_scan_get_categories(pagemap_fd, start);
> +		pfn = pagemap_get_pfn(pagemap_fd, start);
> +		if (pfn == -1UL) {
> +			if (!allow_nonpresent)
> +				goto out;
> +			else
> +				continue;
> +		}
> +		if (pageflags_get(pfn, kpageflags_fd, &kpf))
> +			ksft_exit_fail_msg("read kpageflags: %s\n", strerror(errno));
> +		if (check_pmd_mapping && (categories & PAGE_IS_HUGE))
> +			nr_pmd_mappings++;
> +		if (kpf & KPF_COMPOUND_TAIL)
> +			continue;
> +		if (!check_huge_type(categories, kpf, type))
> +			goto out;
> +	}
> +
> +	if (check_pmd_mapping && (nr_pmd_mappings != nr_hpages))
> +		goto out;
> +	ret = true;
> +
> +out:
> +	close(pagemap_fd);
> +	close(kpageflags_fd);
> +	return ret;
>  }



-- 
Cheers,

David