Re: [PATCH v3 08/13] selftests/mm: ensure destination is hugetlb-backed in hugepage-mremap

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

On 3/27/26 08:16, Sayali Patil wrote:
> The hugepage-mremap selftest reserves the destination address using a
> anonymous base-page mapping before calling mremap() with MREMAP_FIXED,
> while the source region is hugetlb-backed.
> 
> When remapping a hugetlb mapping into a base-page VMA may fail with:
> 
>     mremap: Device or resource busy
> 
> This is observed on powerpc hash MMU systems where slice constraints
> and page size incompatibilities prevent the remap.
> 

That is weird. An mremap(MREMAP_FIXED) is really just an munmap() + move.

Are we sure this is not some actual problem in the hugetlb implementation?

> Ensure the destination region is created using MAP_HUGETLB so that both
> source and destination VMAs are hugetlb-backed and compatible. Also add
> MAP_POPULATE to the destination mapping to prefault hugepages,
> matching the behaviour used for other hugetlb mapping in the test and
> ensuring deterministic behaviour.

But then the test suddenly requires more hugetlb pages, no? I don't see
a good reason for the MAP_POPULATE, really. It will be discarded either way.

> 
> Update the FLAGS macro to include MAP_HUGETLB | MAP_SHARED |
> MAP_POPULATE so that both mappings are hugetlb-backed and compatible.
> Also use the macro for the mmap() calls to avoid repeating
> the flag combination.
> 
> This ensures the test reliably exercises hugetlb mremap instead of
> failing due to VMA type mismatch.
> 
> Fixes: 12b613206474 ("mm, hugepages: add hugetlb vma mremap() test")
> Acked-by: Zi Yan <ziy@nvidia.com>
> Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
> ---
>  tools/testing/selftests/mm/hugepage-mremap.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/hugepage-mremap.c b/tools/testing/selftests/mm/hugepage-mremap.c
> index e611249080d6..48c24a4ba9a7 100644
> --- a/tools/testing/selftests/mm/hugepage-mremap.c
> +++ b/tools/testing/selftests/mm/hugepage-mremap.c
> @@ -31,7 +31,7 @@
>  #define MB_TO_BYTES(x) (x * 1024 * 1024)
>  
>  #define PROTECTION (PROT_READ | PROT_WRITE | PROT_EXEC)
> -#define FLAGS (MAP_SHARED | MAP_ANONYMOUS)
> +#define FLAGS (MAP_HUGETLB | MAP_SHARED | MAP_POPULATE)
>  
>  static void check_bytes(char *addr)
>  {
> @@ -121,23 +121,20 @@ int main(int argc, char *argv[])
>  
>  	/* mmap to a PUD aligned address to hopefully trigger pmd sharing. */
>  	unsigned long suggested_addr = 0x7eaa40000000;
> -	void *haddr = mmap((void *)suggested_addr, length, PROTECTION,
> -			   MAP_HUGETLB | MAP_SHARED | MAP_POPULATE, fd, 0);
> +	void *haddr = mmap((void *)suggested_addr, length, PROTECTION, FLAGS, fd, 0);
>  	ksft_print_msg("Map haddr: Returned address is %p\n", haddr);
>  	if (haddr == MAP_FAILED)
>  		ksft_exit_fail_msg("mmap1: %s\n", strerror(errno));
>  
>  	/* mmap again to a dummy address to hopefully trigger pmd sharing. */
>  	suggested_addr = 0x7daa40000000;
> -	void *daddr = mmap((void *)suggested_addr, length, PROTECTION,
> -			   MAP_HUGETLB | MAP_SHARED | MAP_POPULATE, fd, 0);
> +	void *daddr = mmap((void *)suggested_addr, length, PROTECTION, FLAGS, fd, 0);
>  	ksft_print_msg("Map daddr: Returned address is %p\n", daddr);
>  	if (daddr == MAP_FAILED)
>  		ksft_exit_fail_msg("mmap3: %s\n", strerror(errno));
>  
>  	suggested_addr = 0x7faa40000000;
> -	void *vaddr =
> -		mmap((void *)suggested_addr, length, PROTECTION, FLAGS, -1, 0);
> +	void *vaddr = mmap((void *)suggested_addr, length, PROTECTION, FLAGS, fd, 0);
>  	ksft_print_msg("Map vaddr: Returned address is %p\n", vaddr);
>  	if (vaddr == MAP_FAILED)
>  		ksft_exit_fail_msg("mmap2: %s\n", strerror(errno));


-- 
Cheers,

David