[PATCH 0/2] mm/hugetlb Try to respect mempolicy when calculating surplus huge pages

[Charles Haithcock <chaithco@redhat.com>]

Presently, when calculating how many huge pages are needed when
reserving surplus huge pages, the global count of free huge pages
is used. When reserving with a mempolicy, the global count of free huge
pages is used even if some/all of those free huge pages are on numa
nodes outside of the mempolicy.

Reserving surplus huge pages is ultimately best effort even without a
mempolicy. Restrictions from cpusets and mempolicies further complicate
calculating correct numbers of surplus huge pages to reserve and 
maintaining which nodes those reservations belong to (see the comment in
`hugetlb_acct_memory`).

However, we can do a little better when reserving surplus huge pages
with a mempolicy. This patch changes how to calculate the necessary
amount of surplus huge pages to reserve by considering the max of either
the amount of free huge pages on nodes in the mempolicy or the global
amount of free huge pages. We may still attempt to reserve huge pages
outside the mempolicy, however, we end up being more likely to reserve
from nodes in the mempolicy.

- v1: Modified `needed` calculation to use `allowed_mems_nr(h)` in order
  to consider free hugetlb pages in our mempolicy.
- v2: Folded in Joshua Hahn's recommendation [1] to further modify
  `needed` calculation to take the max of either the available hugetlb
  pages in the mempolicy or the globally available hugetlb pages. Allows
  allocations to prioritize nodes in the mempolicy but can still fall
  back to offnode allocations. Also added selftests to check only for 
  the edgecase which caused this to initially be reported and sanity
  checks.
- v3: Split the change and tests into separate patches and clean up code
  to better align with kernel style guidelines. Likewise, fold in Usama
  Arif's correction [2].

[1] https://lore.kernel.org/all/20260602152022.2673803-1-joshua.hahnjy@gmail.com/
[2] https://lore.kernel.org/linux-mm/20260624144600.8159-1-usama.arif@linux.dev/

Charles Haithcock (2):
  mm/hugetlb: Prefer mempolicy nodes for calcuating surplus hugepage
    reservation
  selftests/mm: add surplus hugetlb mempolicy reservation tests

 mm/hugetlb.c                                  |  42 +-
 tools/testing/selftests/mm/Makefile           |   3 +
 .../selftests/mm/hugetlb_surplus_mempolicy.c  | 491 ++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh     |   1 +
 4 files changed, 517 insertions(+), 20 deletions(-)
 create mode 100644 tools/testing/selftests/mm/hugetlb_surplus_mempolicy.c

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2.55.0