[RFC v2 04/21] mm: thp: handle split failure in do_huge_pmd_wp_page()

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

do_huge_pmd_wp_page() splits the PMD when a COW of the entire huge page
fails (e.g., can't allocate a new folio or the folio is pinned).  It then
returns VM_FAULT_FALLBACK so the fault can be retried at PTE granularity.

If the split fails, the PMD is still huge.  Returning VM_FAULT_FALLBACK
would re-enter the PTE fault path, which expects a PTE page table at the
PMD entry — not a huge PMD.

Return VM_FAULT_OOM on split failure, which signals the fault handler to
invoke the OOM killer or return -ENOMEM to userspace.

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 d9fb5875fa59e..e82b8435a0b7f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2153,7 +2153,13 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 	folio_unlock(folio);
 	spin_unlock(vmf->ptl);
 fallback:
-	__split_huge_pmd(vma, vmf->pmd, vmf->address, false);
+	/*
+	 * Split failure means the PMD is still huge; returning
+	 * VM_FAULT_FALLBACK would re-enter the PTE path with a
+	 * huge PMD, causing incorrect behavior.
+	 */
+	if (__split_huge_pmd(vma, vmf->pmd, vmf->address, false))
+		return VM_FAULT_OOM;
 	return VM_FAULT_FALLBACK;
 }
 
-- 
2.47.3