page_alloc: allow migration of smaller hugepages during contig_alloc

page_alloc: allow migration of smaller hugepages during contig_alloc

We presently skip regions with hugepages entirely when trying to do
contiguous page allocation.  This will cause otherwise-movable 2MB HugeTLB
pages to be considered unmovable, and makes 1GB gigantic page allocation
less reliable on systems utilizing both.

Commit 4d73ba5fa710 ("mm: page_alloc: skip regions with hugetlbfs pages
when allocating 1G pages") skipped all HugePage containing regions because
it can cause significant delays in 1G allocation (as HugeTLB migrations
may fail for a number of reasons).

Instead, if hugepage migration is enabled, consider regions with hugepages
smaller than the target contiguous allocation request as valid targets for
allocation.

We optimize for the existing behavior by searching for non-hugetlb regions
in a first pass, then retrying the search to include hugetlb only on
failure.  This allows the existing fast-path to remain the default case
with a slow-path fallback to increase reliability.

We only fallback to the slow path if a hugetlb region was detected, and we
do a full re-scan because the zones/blocks may have changed during the
first pass (and it's not worth further complexity).

isolate_migrate_pages_block() has similar hugetlb filter logic, and the
hugetlb code does a migratable check in folio_isolate_hugetlb() during
isolation.  The code servicing the allocation and migration already
supports this exact use case.

To test, allocate a bunch of 2MB HugeTLB pages (in this case 48GB) and
then attempt to allocate some 1G HugeTLB pages (in this case 4GB) (Scale
to your machine's memory capacity).

echo 24576 > .../hugepages-2048kB/nr_hugepages
echo 4 > .../hugepages-1048576kB/nr_hugepages

Prior to this patch, the 1GB page reservation can fail if no contiguous
1GB pages remain.  After this patch, the kernel will try to move 2MB pages
and successfully allocate the 1GB pages (assuming overall sufficient
memory is available).  Also tested this while a program had the 2MB
reservations mapped, and the 1GB reservation still succeeds.

folio_alloc_gigantic() is the primary user of alloc_contig_pages(),
other users are debug or init-time allocations and largely unaffected.
- ppc/memtrace is a debugfs interface
- x86/tdx memory allocation occurs once on module-init
- kfence/core happens once on module (late) init
- THP uses it in debug_vm_pgtable_alloc_huge_page at __init time

Link: https://lkml.kernel.org/r/20251221124656.2362540-1-gourry@gourry.net
Signed-off-by: Gregory Price <gourry@gourry.net>
Suggested-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/linux-mm/6fe3562d-49b2-4975-aa86-e139c535ad00@redhat.com/
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>