The patch titled Subject: mm/compaction: fix bug in hugetlb handling pathway has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-compaction-fix-bug-in-hugetlb-handling-pathway.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: "Vishal Moola (Oracle)" vishal.moola@gmail.com Subject: mm/compaction: fix bug in hugetlb handling pathway Date: Mon, 31 Mar 2025 19:10:24 -0700

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.

Link: https://lkml.kernel.org/r/20250401021025.637333-1-vishal.moola@gmail.com Fixes: 369fa227c219 ("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 | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/compaction.c~mm-compaction-fix-bug-in-hugetlb-handling-pathway +++ a/mm/compaction.c @@ -981,13 +981,13 @@ isolate_migratepages_block(struct compac }

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 compac /* 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; }

_

Patches currently in -mm which might be from vishal.moola@gmail.com are

mm-compaction-fix-bug-in-hugetlb-handling-pathway.patch