[v3 11/24] mm: thp: handle split failure in userfaultfd move_pages()

Usama Arif
2026-03-27T02:15:24Z

The UFFDIO_MOVE ioctl's move_pages() loop splits a huge PMD when the
folio is pinned and can't be moved at PMD granularity.

If the split fails, the PMD stays huge and move_pages_pte() can't
process individual pages. Break out of the loop on split failure
and return -ENOMEM to the caller. This is similar to how other
allocation failures (__pte_alloc, mm_alloc_pmd) are handled in
move_pages().

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

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 481ec7eb44420..a04d62dd1e065 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1946,7 +1946,13 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
}

spin_unlock(ptl);
- split_huge_pmd(src_vma, src_pmd, src_addr);
+ /*
+ * If split fails, the PMD stays huge and
+ * move_pages_pte can't process it.
+ */
+ err = split_huge_pmd(src_vma, src_pmd, src_addr);
+ if (err)
+ break;
/* The folio will be split by move_pages_pte() */
continue;
}
--
2.52.0