Commit a84edd52 authored by Vishal Moola (Oracle)'s avatar Vishal Moola (Oracle) Committed by Andrew Morton
Browse files

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 66fe1cf7 ("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: 369fa227 ("mm: make alloc_contig_range handle free hugetlb pages")
Signed-off-by: default avatarVishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: default avatarOscar Salvador <osalvador@suse.de>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 770c8d55

mm/compaction.c

+3 −3
Original line number Diff line number Diff line
@@ -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;

			}