[PATCH RFC v2 05/12] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare

[Peter Xu <peterx@redhat.com>]

We can take the hugetlb walker lock, here taking vma lock directly.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 fs/userfaultfd.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 07c81ab3fd4d..a602f008dde5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -376,7 +376,8 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
  */
 vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 {
-	struct mm_struct *mm = vmf->vma->vm_mm;
+	struct vm_area_struct *vma = vmf->vma;
+	struct mm_struct *mm = vma->vm_mm;
 	struct userfaultfd_ctx *ctx;
 	struct userfaultfd_wait_queue uwq;
 	vm_fault_t ret = VM_FAULT_SIGBUS;
@@ -403,7 +404,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	 */
 	mmap_assert_locked(mm);
 
-	ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
+	ctx = vma->vm_userfaultfd_ctx.ctx;
 	if (!ctx)
 		goto out;
 
@@ -493,6 +494,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	blocking_state = userfaultfd_get_blocking_state(vmf->flags);
 
+	/*
+	 * This stablizes pgtable for hugetlb on e.g. pmd unsharing.  Need
+	 * to be before setting current state.
+	 */
+	if (is_vm_hugetlb_page(vma))
+		hugetlb_vma_lock_read(vma);
+
 	spin_lock_irq(&ctx->fault_pending_wqh.lock);
 	/*
 	 * After the __add_wait_queue the uwq is visible to userland
@@ -507,13 +515,15 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	set_current_state(blocking_state);
 	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
 
-	if (!is_vm_hugetlb_page(vmf->vma))
+	if (!is_vm_hugetlb_page(vma))
 		must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
 						  reason);
 	else
-		must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
+		must_wait = userfaultfd_huge_must_wait(ctx, vma,
 						       vmf->address,
 						       vmf->flags, reason);
+	if (is_vm_hugetlb_page(vma))
+		hugetlb_vma_unlock_read(vma);
 	mmap_read_unlock(mm);
 
 	if (likely(must_wait && !READ_ONCE(ctx->released))) {
-- 
2.37.3