[PATCH 1/2] pipe: change pipe_write() to never add a zero-sized buffer

[Oleg Nesterov <oleg@xxxxxxxxxx>]

a194dfe6e6f6 ("pipe: Rearrange sequence in pipe_write() to preallocate slot")
changed pipe_write() to increment pipe->head in advance.  IIUC to avoid the
race with the post_one_notification()-like code which can add another buffer
under pipe->rd_wait.lock without pipe->mutex.

This is no longer necessary after c73be61cede5 ("pipe: Add general notification
queue support"), pipe_write() checks pipe_has_watch_queue() and returns -EXDEV
at the start. And can't help in any case, pipe_write() no longer takes this
rd_wait.lock spinlock.

Change pipe_write() to call copy_page_from_iter() first and do nothing if it
fails. This way pipe_write() can't add a zero-sized buffer and we can simplify
pipe_read() which currently has to take care of this very unlikely case.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 fs/pipe.c                 | 47 +++++++++------------------------------
 include/linux/pipe_fs_i.h |  6 +++++
 2 files changed, 16 insertions(+), 37 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 2ae75adfba64..7f24d707f6a1 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -303,7 +303,7 @@ anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
 
 		if (!pipe_empty(head, tail)) {
 			struct pipe_buffer *buf = &pipe->bufs[tail & mask];
-			size_t chars = buf->len;
+			size_t chars = pipe_buf_assert_len(buf);
 			size_t written;
 			int error;
 
@@ -360,29 +360,9 @@ anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			break;
 		}
 		mutex_unlock(&pipe->mutex);
-
 		/*
 		 * We only get here if we didn't actually read anything.
 		 *
-		 * However, we could have seen (and removed) a zero-sized
-		 * pipe buffer, and might have made space in the buffers
-		 * that way.
-		 *
-		 * You can't make zero-sized pipe buffers by doing an empty
-		 * write (not even in packet mode), but they can happen if
-		 * the writer gets an EFAULT when trying to fill a buffer
-		 * that already got allocated and inserted in the buffer
-		 * array.
-		 *
-		 * So we still need to wake up any pending writers in the
-		 * _very_ unlikely case that the pipe was full, but we got
-		 * no data.
-		 */
-		if (unlikely(wake_writer))
-			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
-		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-
-		/*
 		 * But because we didn't read anything, at this point we can
 		 * just return directly with -ERESTARTSYS if we're interrupted,
 		 * since we've done any required wakeups and there's no need
@@ -391,7 +371,6 @@ anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
 			return -ERESTARTSYS;
 
-		wake_writer = false;
 		wake_next_reader = true;
 		mutex_lock(&pipe->mutex);
 	}
@@ -526,33 +505,27 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
 				pipe->tmp_page = page;
 			}
 
-			/* Allocate a slot in the ring in advance and attach an
-			 * empty buffer.  If we fault or otherwise fail to use
-			 * it, either the reader will consume it or it'll still
-			 * be there for the next write.
-			 */
-			pipe->head = head + 1;
+			copied = copy_page_from_iter(page, 0, PAGE_SIZE, from);
+			if (unlikely(copied < PAGE_SIZE && iov_iter_count(from))) {
+				if (!ret)
+					ret = -EFAULT;
+				break;
+			}
 
+			pipe->head = head + 1;
+			pipe->tmp_page = NULL;
 			/* Insert it into the buffer array */
 			buf = &pipe->bufs[head & mask];
 			buf->page = page;
 			buf->ops = &anon_pipe_buf_ops;
 			buf->offset = 0;
-			buf->len = 0;
 			if (is_packetized(filp))
 				buf->flags = PIPE_BUF_FLAG_PACKET;
 			else
 				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;
-			pipe->tmp_page = NULL;
 
-			copied = copy_page_from_iter(page, 0, PAGE_SIZE, from);
-			if (unlikely(copied < PAGE_SIZE && iov_iter_count(from))) {
-				if (!ret)
-					ret = -EFAULT;
-				break;
-			}
-			ret += copied;
 			buf->len = copied;
+			ret += copied;
 
 			if (!iov_iter_count(from))
 				break;
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 8ff23bf5a819..4174429a3e0e 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -31,6 +31,12 @@ struct pipe_buffer {
 	unsigned long private;
 };
 
+static inline unsigned int pipe_buf_assert_len(struct pipe_buffer *buf)
+{
+	WARN_ON_ONCE(!buf->len);
+	return buf->len;
+}
+
 /**
  *	struct pipe_inode_info - a linux kernel pipe
  *	@mutex: mutex protecting the whole thing
-- 
2.25.1.362.g51ebf55