Re: [PATCH v5 08/19] selftests/mm: add folio-order check for address ranges

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/8/26 8:50 PM, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> 
> An mTHP collapse test needs to know that a range is backed by folios of the
> target order, and that they sit where a collapse would put them.  Nothing
> answers that today: is_backed_by_folio() classifies the folio behind a
> single page, and check_huge_anon() reads smaps AnonHugePages, which only
> accounts PMD mappings.

Have you checked check_large_folios() in vm_util.c? It seems to meet 
your requirements, or am I missing something?

> Add is_range_backed_by_order().  It requires every folio-sized, folio-
> aligned part of the range to map one folio of that order, head to tail,
> with the head at the start of the part.
> 
> A part backed by two smaller folios fails, and so does a folio mapped off
> its natural alignment.  The mTHP cases need both to tell a collapsed range
> from the one beside it.
> 
> Assisted-by: LLM
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>   tools/testing/selftests/mm/vm_util.c | 47 ++++++++++++++++++++++++++++
>   tools/testing/selftests/mm/vm_util.h |  2 ++
>   2 files changed, 49 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 3ea42a7a2a3e..4947612e8b3d 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -552,6 +552,53 @@ bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
>   	return false;
>   }
>   
> +/**
> + * is_range_backed_by_order() - check that a range is backed by @order folios
> + * @start: start of the range, a multiple of the folio size
> + * @len: length of the range in bytes, a multiple of the folio size
> + * @order: the folio order to check for
> + * @pagemap_fd: open /proc/<pid>/pagemap of the range's owner
> + * @kpageflags_fd: open /proc/kpageflags
> + *
> + * Every folio-sized, folio-aligned part of the range must map one folio of
> + * @order, head to tail, with the head at the start of the part.  A part
> + * backed by several smaller folios fails, and so does a folio mapped off
> + * its natural alignment.
> + *
> + * Returns: true if the whole range is backed that way, false otherwise.
> + */
> +bool is_range_backed_by_order(char *start, size_t len, int order,
> +			      int pagemap_fd, int kpageflags_fd)
> +{
> +	const unsigned long nr_pages = 1UL << order;
> +	const size_t folio_size = nr_pages * psize();
> +	char *vaddr;
> +
> +	if ((uintptr_t)start % folio_size || len % folio_size)
> +		return false;
> +
> +	for (vaddr = start; vaddr < start + len; vaddr += folio_size) {
> +		const unsigned long pfn = pagemap_get_pfn(pagemap_fd, vaddr);
> +		unsigned long i;
> +
> +		/* Not present, or a tail page */
> +		if (pfn == -1UL || pfn % nr_pages)
> +			return false;
> +
> +		for (i = 1; i < nr_pages; i++) {
> +			char *page = vaddr + i * psize();
> +
> +			if (pagemap_get_pfn(pagemap_fd, page) != pfn + i)
> +				return false;
> +		}
> +
> +		if (!is_backed_by_folio(vaddr, order, pagemap_fd, kpageflags_fd))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>   /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
>   int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
>   			      bool miss, bool wp, bool minor, uint64_t *ioctls)
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index 56a28ce7d029..e509fc4012a5 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -99,6 +99,8 @@ int gather_folio_orders(char *vaddr_start, size_t len,
>   		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
>   bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
>   			int kpageflags_fd);
> +bool is_range_backed_by_order(char *start, size_t len, int order,
> +			      int pagemap_fd, int kpageflags_fd);
>   
>   int uffd_register(int uffd, void *addr, uint64_t len,
>   		  bool miss, bool wp, bool minor);