mm: page_isolation: avoid calling folio_hstate() without hugetlb_lock

I found a NULL pointer dereference as followed:

 BUG: kernel NULL pointer dereference, address: 0000000000000028
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0000 [linuxppc#1] SMP PTI
 CPU: 5 UID: 0 PID: 5964 Comm: sh Kdump: loaded Not tainted 6.13.0-dirty #20
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.
 RIP: 0010:has_unmovable_pages+0x184/0x360
 ...
 Call Trace:
  <TASK>
  set_migratetype_isolate+0xd1/0x180
  start_isolate_page_range+0xd2/0x170
  alloc_contig_range_noprof+0x101/0x660
  alloc_contig_pages_noprof+0x238/0x290
  alloc_gigantic_folio.isra.0+0xb6/0x1f0
  only_alloc_fresh_hugetlb_folio.isra.0+0xf/0x60
  alloc_pool_huge_folio+0x80/0xf0
  set_max_huge_pages+0x211/0x490
  __nr_hugepages_store_common+0x5f/0xe0
  nr_hugepages_store+0x77/0x80
  kernfs_fop_write_iter+0x118/0x200
  vfs_write+0x23c/0x3f0
  ksys_write+0x62/0xe0
  do_syscall_64+0x5b/0x170
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

As has_unmovable_pages() call folio_hstate() without hugetlb_lock, there
is a race to free the HugeTLB page between PageHuge() and folio_hstate(). 
There is no need to add hugetlb_lock here as the HugeTLB page can be freed
in lot of places.  So it's enough to unfold folio_hstate() and add a check
to avoid NULL pointer dereference for hugepage_migration_supported().

Link: https://lkml.kernel.org/r/20250122061151.578768-1-liushixin2@huawei.com
Fixes: 464c7ff ("mm/hugetlb: filter out hugetlb pages if HUGEPAGE migration is not supported.")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_isolation.c

@@ -83,7 +83,14 @@ static struct page *has_unmovable_pages(unsigned long start_pfn, unsigned long e
 			unsigned int skip_pages;
 
 			if (PageHuge(page)) {
-				if (!hugepage_migration_supported(folio_hstate(folio)))
+				struct hstate *h;
+
+				/*
+				 * The huge page may be freed so can not
+				 * use folio_hstate() directly.
+				 */
+				h = size_to_hstate(folio_size(folio));
+				if (h && !hugepage_migration_supported(h))
 					return page;
 			} else if (!folio_test_lru(folio) && !__folio_test_movable(folio)) {
 				return page;