mm/compaction: fix bug in hugetlb handling pathway

The compaction code doesn't take references on pages until we're certain
we should attempt to handle it.

In the hugetlb case, isolate_or_dissolve_huge_page() may return -EBUSY
without taking a reference to the folio associated with our pfn.  If our
folio's refcount drops to 0, compound_nr() becomes unpredictable, making
low_pfn and nr_scanned unreliable.  The user-visible effect is minimal -
this should rarely happen (if ever).

Fix this by storing the folio statistics earlier on the stack (just like
the THP and Buddy cases).

Also revert commit 66fe1cf ("mm: compaction: use helper compound_nr
in isolate_migratepages_block") to make backporting easier.

Link: https://lkml.kernel.org/r/20250401021025.637333-1-vishal.moola@gmail.com
Fixes: 369fa22 ("mm: make alloc_contig_range handle free hugetlb pages")
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/compaction.c

@@ -981,13 +981,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		}
 
 		if (PageHuge(page)) {
+			const unsigned int order = compound_order(page);
 			/*
 			 * skip hugetlbfs if we are not compacting for pages
 			 * bigger than its order. THPs and other compound pages
 			 * are handled below.
 			 */
 			if (!cc->alloc_contig) {
-				const unsigned int order = compound_order(page);
 
 				if (order <= MAX_PAGE_ORDER) {
 					low_pfn += (1UL << order) - 1;
@@ -1011,8 +1011,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				 /* Do not report -EBUSY down the chain */
 				if (ret == -EBUSY)
 					ret = 0;
-				low_pfn += compound_nr(page) - 1;
-				nr_scanned += compound_nr(page) - 1;
+				low_pfn += (1UL << order) - 1;
+				nr_scanned += (1UL << order) - 1;
 				goto isolate_fail;
 			}