[RFC v2 06/21] mm: thp: handle split failure in wp_huge_pmd()

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

wp_huge_pmd() splits the PMD when COW or write-notify must be handled at
PTE level (e.g., shared/file VMAs, userfaultfd).  It then returns
VM_FAULT_FALLBACK so the fault handler retries at PTE granularity.
If the split fails, the PMD is still huge.  The PTE fault path cannot
handle a huge PMD entry.
Return VM_FAULT_OOM on split failure, which signals the fault handler to
invoke the OOM killer or return -ENOMEM to userspace. This is similar to
what __handle_mm_fault would do if p4d_alloc or pud_alloc fails.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/memory.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7ba1221c63792..51d2717e3f1b4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6161,8 +6161,13 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
 	}
 
 split:
-	/* COW or write-notify handled on pte level: split pmd. */
-	__split_huge_pmd(vma, vmf->pmd, vmf->address, false);
+	/*
+	 * COW or write-notify handled on pte level: split pmd.
+	 * If split fails, the PMD is still huge so falling back
+	 * to PTE handling would be incorrect.
+	 */
+	if (__split_huge_pmd(vma, vmf->pmd, vmf->address, false))
+		return VM_FAULT_OOM;
 
 	return VM_FAULT_FALLBACK;
 }
-- 
2.47.3