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

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Wed, Apr 01, 2026 at 04:21:55PM +0200, David Hildenbrand (Arm) wrote:
> 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.

OK so digging in:

mremap -> ... -> vrm_set_new_addr() -> get_unmapped_area() -> ... (in ppc arch
code) -> slice_get_unmapped_area():

unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
				      unsigned long flags, unsigned int psize,
				      int topdown)
{
	...
	/* bunch of checks */

	/* If we have MAP_FIXED and failed the above steps, then error out */
	if (fixed)
		return -EBUSY;

	...
}

Is presumably where we hit the issue.

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

Yeah the weird bit I guess is that we _still_ invoke get_unmapped_area() but
with MAP_FIXED set to indicate that we want the specific address, so it's
subject to the above checks.

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

It seems the 'slices' check sees if the _target address_ has an equivalent page
size, presumably hugetlb-mandated, and fails if they're not equivalent, so this
change is just accounting for that.


>
> > 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.

Yeah I'm not sure about the MAP_POPULATE being all that important here.

>
> >
> > 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

Cheers, Lorenzo