[PATCH mm-unstable fix] mm: userfaultfd: add new UFFDIO_POISON ioctl: fix

[Hugh Dickins <hughd@google.com>]

Smatch has observed that pte_offset_map_lock() is now allowed to fail,
and then ptl should not be unlocked.  Use -EAGAIN here like elsewhere.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
Axel, Peter: this seems right as a fix to the patch in mm-unstable;
but in preparing this, I noticed mfill_atomic()'s code before calling
mfill_atomic_pte(), and think that my original choice of -EFAULT was
therefore better than -EAGAIN for all of these; and that mfill_atomic()'s
BUG_ONs there would be better deleted (and is its BUG_ON(folio) safe??).
Something one of us should address, after this fixup is in akpm's tree.

 mm/userfaultfd.c | 4 ++++
 1 file changed, 4 insertions(+)

--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -300,7 +300,10 @@ static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
 	spinlock_t *ptl;
 
 	_dst_pte = make_pte_marker(PTE_MARKER_POISONED);
+	ret = -EAGAIN;
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
 
 	if (mfill_file_over_size(dst_vma, dst_addr)) {
 		ret = -EFAULT;
@@ -319,6 +322,7 @@ static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
 	ret = 0;
 out_unlock:
 	pte_unmap_unlock(dst_pte, ptl);
+out:
 	return ret;
 }