Open HugeTLB allocation routine for more generic use

Hi,

The motivation for this patch series is guest_memfd, which would like
to use HugeTLB as a generic source of huge pages but not adopt
HugeTLB's reservation at mmap() time.

By refactoring alloc_hugetlb_folio() and some dependent functions,
there is now an option to allocate HugeTLB folios without providing a
VMA. Specifically, HugeTLB allocation used to be dependent on the VMA
to

1. Look up reservations in the resv_map
2. Get mpol, stored at vma->vm_policy

This refactoring provides hugetlb_alloc_folio(), which focuses on just
the allocation itself, and associated memory and HugeTLB charging
(cgroups). alloc_hugetlb_folio() still handles reservations in the
resv_map and subpools.

Regarding naming, I'm definitely open to alternative names :) I chose
hugetlb_alloc_folio() because I'm seeing this function as a general
allocation function that is provided by the HugeTLB subsystem (hence
the hugetlb_ prefix). I'm intending for alloc_hugetlb_folio() to be
later refactored as a static function for use just by HugeTLB, and
HugeTLBfs should probably use hugetlb_alloc_folio() directly.

To see how hugetlb_alloc_folio() is used by guest_memfd, the most
recent patch series that uses this more generic HugeTLB allocation
routine is at [1], and a newer revision of that patch series is at
[2].

Independently of guest_memfd, I believe this change is useful in
simplifying alloc_hugetlb_folio(). alloc_hugetlb_folio() was so
coupled to a VMA that even HugeTLBfs allocates HugeTLB folios using a
pseudo-VMA.

Testing:

+ libhugetlbfs tests pass
+ ./tools/testing/selftests/mm/ksft_hugetlb.sh passes

Andrew, thanks for nudging me to send a v4!

Oscar, you mentioned that you wanted to do a closer review on v3, hope you
find time to review v4 soon!

Changes in this revision:

+ Define gfp within hugetlb_alloc_folio() instead of having it as a
  parameter so users aren't able to override gfp arbitrarily as Oscar
  requested.
+ Addressed some of Sashiko's comments [4]. Some of the issues I didn't address
  were pre-existing issues. Since v3 of this series Sashiko was updated to
  actually send mails. I'd let Sashiko point them out again and then we should
  discuss those!

[1] https://lore.kernel.org/all/cover.1747264138.git.ackerley...
[2] https://github.com/googleprodkernel/linux-cc/tree/wip-gme...
[3] https://lore.kernel.org/all/agqaUcVp_hwH-VXr@localhost.lo...
[4] https://sashiko.dev/#/patchset/20260518-hugetlb-open-up-v...

RFC v1: https://lore.kernel.org/all/bb35a69a-5be9-45f5-a557-19024...
v2: https://lore.kernel.org/r/20260506-hugetlb-open-up-v2-0-8...
v3: https://lore.kernel.org/r/20260518-hugetlb-open-up-v3-0-e...

---
Ackerley Tng (6):
      mm: hugetlb: Consolidate interpretation of gbl_chg within alloc_hugetlb_folio()
      mm: hugetlb: Move mpol interpretation out of alloc_buddy_hugetlb_folio_with_mpol()
      mm: hugetlb: Move mpol interpretation out of dequeue_hugetlb_folio_vma()
      mm: hugetlb: Use error variable in alloc_hugetlb_folio
      mm: hugetlb: Move mem_cgroup_charge_hugetlb() earlier in allocation
      mm: hugetlb: Refactor out hugetlb_alloc_folio()

 include/linux/hugetlb.h        |  18 +++
 include/uapi/linux/mempolicy.h |   2 +-
 mm/hugetlb.c                   | 269 ++++++++++++++++++++++++-----------------
 3 files changed, 175 insertions(+), 114 deletions(-)
---
base-commit: 4a50a141f05a8d1737661b19ee22ff8455b94409
change-id: 20260504-hugetlb-open-up-eaba80571b09

Best regards,
--
Ackerley Tng <ackerleytng@google.com>