Page isolation currently sets MIGRATE_ISOLATE on a block, then drops
zone->lock and scans the block for straddling buddies to split up. 
Because this happens non-atomically wrt the page allocator, it's possible
for allocations to get a buddy whose first block is a regular pcp
migratetype but whose tail is isolated.  This means that in certain cases
memory can still be allocated after isolation.  It will also trigger the
freelist type hygiene warnings in subsequent patches.

start_isolate_page_range()
  isolate_single_pageblock()
    set_migratetype_isolate(tail)
      lock zone->lock
      move_freepages_block(tail) // nop
      set_pageblock_migratetype(tail)
      unlock zone->lock
                                                     __rmqueue_smallest()
                                                       del_page_from_freelist(head)
                                                       expand(head, head_mt)
                                                         WARN(head_mt != tail_mt)
    start_pfn = ALIGN_DOWN(MAX_ORDER_NR_PAGES)
    for (pfn = start_pfn, pfn < end_pfn)
      if (PageBuddy())
        split_free_page(head)

Introduce a variant of move_freepages_block() provided by the allocator
specifically for page isolation; it moves free pages, converts the block,
and handles the splitting of straddling buddies while holding zone->lock.

The allocator knows that pageblocks and buddies are always naturally
aligned, which means that buddies can only straddle blocks if they're
actually >pageblock_order.  This means the search-and-split part can be
simplified compared to what page isolation used to do.

Also tighten up the page isolation code around the expectations of which
pages can be large, and how they are freed.

Based on extensive discussions with and invaluable input from Zi Yan.

[hannes@cmpxchg.org: work around older gcc warning]
  Link: https://lkml.kernel.org/r/20240321142426.GB777580@cmpxchg.org
Link: https://lkml.kernel.org/r/20240320180429.678181-10-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>