[RFC v2 05/21] mm: thp: handle split failure in zap_pmd_range()

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

zap_pmd_range() splits a huge PMD when the zap range doesn't cover the
full PMD (partial unmap).  If the split fails, the PMD stays huge.
Falling through to zap_pte_range() would dereference the huge PMD entry
as a PTE page table pointer.

Skip the range covered by the PMD on split failure instead.

The skip is safe across all call paths into zap_pmd_range():

- exit_mmap() and OOM reaper: the zap range covers entire VMAs, so
  every PMD is fully covered (next - addr == HPAGE_PMD_SIZE).  The
  zap_huge_pmd() branch handles these without splitting.  The split
  failure path is unreachable.

- munmap / mmap overlay: vma_adjust_trans_huge() (called from
  __split_vma) splits any PMD straddling the VMA boundary before the
  VMA is split.  If that PMD split fails, __split_vma() returns
  -ENOMEM and the munmap is aborted before reaching zap_pmd_range().
  The split failure path is unreachable.

- MADV_DONTNEED: advisory hint, the kernel is allowed to ignore it.
  The pages remain valid and accessible.  A subsequent access returns
  existing data without faulting.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/memory.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9385842c35034..7ba1221c63792 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1983,9 +1983,18 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
 	do {
 		next = pmd_addr_end(addr, end);
 		if (pmd_is_huge(*pmd)) {
-			if (next - addr != HPAGE_PMD_SIZE)
-				__split_huge_pmd(vma, pmd, addr, false);
-			else if (zap_huge_pmd(tlb, vma, pmd, addr)) {
+			if (next - addr != HPAGE_PMD_SIZE) {
+				/*
+				 * If split fails, the PMD stays huge.
+				 * Skip the range to avoid falling through
+				 * to zap_pte_range, which would treat the
+				 * huge PMD entry as a page table pointer.
+				 */
+				if (__split_huge_pmd(vma, pmd, addr, false)) {
+					addr = next;
+					continue;
+				}
+			} else if (zap_huge_pmd(tlb, vma, pmd, addr)) {
 				addr = next;
 				continue;
 			}
-- 
2.47.3