mm/compaction: fix low_pfn advance on isolating hugetlb

Commit 56ae0bb349 ("mm: compaction: convert to use a folio in
isolate_migratepages_block()") converts api from page to folio.  But the
low_pfn advance for hugetlb page seems wrong when low_pfn doesn't point to
head page.

Originally, if page is a hugetlb tail page, compound_nr() return 1, which
means low_pfn only advance one in next iteration.  After the change,
low_pfn would advance more than the hugetlb range, since folio_nr_pages()
always return total number of the large page.  This results in skipping
some range to isolate and then to migrate.

The worst case for alloc_contig is it does all the isolation and
migration, but finally find some range is still not isolated.  And then
undo all the work and try a new range.

Advance low_pfn to the end of hugetlb.

Link: https://lkml.kernel.org/r/20250910092240.3981-1-richard.weiyang@gmail.com
Fixes: 56ae0bb349 ("mm: compaction: convert to use a folio in isolate_migratepages_block()")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/compaction.c

@@ -989,7 +989,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				 * Hugepage was successfully isolated and placed
 				 * on the cc->migratepages list.
 				 */
-				low_pfn += folio_nr_pages(folio) - 1;
+				low_pfn += folio_nr_pages(folio) - folio_page_idx(folio, page) - 1;
 				goto isolate_success_no_list;
 			}