[RFC v2 03/21] mm: thp: handle split failure in copy_huge_pmd()

[Usama Arif <usama.arif@linux.dev>]

copy_huge_pmd() splits the source PMD when a folio is pinned and can't
be COW-shared at PMD granularity.  It then returns -EAGAIN so
copy_pmd_range() falls through to copy_pte_range().

If the split fails, the PMD is still huge.  Returning -EAGAIN would cause
copy_pmd_range() to call copy_pte_range(), which would dereference the
huge PMD entry as if it were a pointer to a PTE page table.
Return -ENOMEM on split failure instead (which is already done in
copy_huge_pmd() if pte_alloc_one() fails), which causes copy_page_range()
to abort the fork with -ENOMEM, similar to how copy_pmd_range() would
be aborted if pmd_alloc() and copy_pte_range() fail.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/huge_memory.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a979aa5bd2995..d9fb5875fa59e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1929,7 +1929,13 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		pte_free(dst_mm, pgtable);
 		spin_unlock(src_ptl);
 		spin_unlock(dst_ptl);
-		__split_huge_pmd(src_vma, src_pmd, addr, false);
+		/*
+		 * If split fails, the PMD is still huge so copy_pte_range
+		 * (via -EAGAIN) would misinterpret it as a page table
+		 * pointer.  Return -ENOMEM directly to copy_pmd_range.
+		 */
+		if (__split_huge_pmd(src_vma, src_pmd, addr, false))
+			return -ENOMEM;
 		return -EAGAIN;
 	}
 	add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
-- 
2.47.3