[PATCH] mm/compaction: Fix bug in hugetlb handling pathway

Vishal Moola (Oracle)
2025-04-01T02:11:10Z

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 66fe1cf7f581 ("mm: compaction: use helper compound_nr
in isolate_migratepages_block")
to make backporting easier.

Fixes: 369fa227c219 ("mm: make alloc_contig_range handle free hugetlb pages")
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
---
mm/compaction.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 139f00c0308a..ca71fd3c3181 100644
--- a/mm/compaction.c
+++ b/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;
}

--
2.48.1