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

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

On Thu, Apr 02, 2026 at 09:33:29AM +0200, David Hildenbrand (Arm) wrote:
> On 4/1/26 22:39, Sayali Patil wrote:
> >
> >
> > On 01/04/26 20:10, Lorenzo Stoakes (Oracle) wrote:
> >> On Wed, Apr 01, 2026 at 04:21:55PM +0200, David Hildenbrand (Arm) wrote:
> >>
> >> 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.
> >>
> > Yes, this change accounts for that by ensuring the destination is
> > created with MAP_HUGETLB so it has the same page size as the source.
>
> Okay, weird, so it's the right thing to do to cover all odd arch behavior.
>
> >>
> >>>
> >>>
> >>> 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.
> >>
> > As far as I understand, without MAP_POPULATE, memory accesses would
> > trigger userfaults, and since the test is single-threaded and has no
> > background handler for the uffd, it would deadlock. MAP_POPULATE ensures
> > the test runs correctly by prefaulting all pages, but please let me know
> > if I’m mistaken.
>
> So you are saying the test would deadlock if you are not adding
> MAP_POPULATE? If so, please double check if that is actually the case.
>
> And if it's actually the case, please carefully document that in the
> patch description, and probably as a comment above the MAP_POPULATE usage.

Do keep in mind MAP_POPULATE is not _guaranteed_ to work :)

For guaranteed populate you need madvise(..., MADV_POPULATE_[READ/WRITE]) or to
directly fault in.

>
> --
> Cheers,
>
> David

Cheers, Lorenzo