[PATCH] mm/uffd: Fix release hang over GUP

[Peter Xu]

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 fs/userfaultfd.c | 57 ++++++++++++++++++++++++++----------------------
 1 file changed, 31 insertions(+), 26 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index bbfaf5837a08..2358e6b00315 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -455,32 +455,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	if (!(vmf->flags & FAULT_FLAG_USER) && (ctx->flags & UFFD_USER_MODE_ONLY))
 		goto out;
 
-	/*
-	 * If it's already released don't get it. This avoids to loop
-	 * in __get_user_pages if userfaultfd_release waits on the
-	 * caller of handle_userfault to release the mmap_lock.
-	 */
-	if (unlikely(READ_ONCE(ctx->released))) {
-		/*
-		 * Don't return VM_FAULT_SIGBUS in this case, so a non
-		 * cooperative manager can close the uffd after the
-		 * last UFFDIO_COPY, without risking to trigger an
-		 * involuntary SIGBUS if the process was starting the
-		 * userfaultfd while the userfaultfd was still armed
-		 * (but after the last UFFDIO_COPY). If the uffd
-		 * wasn't already closed when the userfault reached
-		 * this point, that would normally be solved by
-		 * userfaultfd_must_wait returning 'false'.
-		 *
-		 * If we were to return VM_FAULT_SIGBUS here, the non
-		 * cooperative manager would be instead forced to
-		 * always call UFFDIO_UNREGISTER before it can safely
-		 * close the uffd.
-		 */
-		ret = VM_FAULT_NOPAGE;
-		goto out;
-	}
-
 	/*
 	 * Check that we can return VM_FAULT_RETRY.
 	 *
@@ -517,6 +491,37 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 		goto out;
 
+	/*
+	 * If it's already released don't get it. This avoids to loop
+	 * in __get_user_pages if userfaultfd_release waits on the
+	 * caller of handle_userfault to release the mmap_lock.
+	 */
+	if (unlikely(READ_ONCE(ctx->released))) {
+		/*
+		 * Don't return VM_FAULT_SIGBUS in this case, so a non
+		 * cooperative manager can close the uffd after the
+		 * last UFFDIO_COPY, without risking to trigger an
+		 * involuntary SIGBUS if the process was starting the
+		 * userfaultfd while the userfaultfd was still armed
+		 * (but after the last UFFDIO_COPY). If the uffd
+		 * wasn't already closed when the userfault reached
+		 * this point, that would normally be solved by
+		 * userfaultfd_must_wait returning 'false'.
+		 *
+		 * If we were to return VM_FAULT_SIGBUS here, the non
+		 * cooperative manager would be instead forced to
+		 * always call UFFDIO_UNREGISTER before it can safely
+		 * close the uffd.
+		 *
+		 * We release the mmap lock in this special case, just in
+		 * case we're in a gup to not dead loop, so the other uffd
+		 * handler thread/process can have a chance to take the
+		 * write lock and do the unregistration.
+		 */
+		release_fault_lock(vmf);
+		goto out;
+	}
+
 	/* take the reference before dropping the mmap_lock */
 	userfaultfd_ctx_get(ctx);
 
-- 
2.41.0
===8<===

-- 
Peter Xu