Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

[Oleg Nesterov <oleg@redhat.com>]

On 02/03, K Prateek Nayak wrote:
>
> With the below patch on mainline, I see more improvements for a
> modified version of sched-messaging (sched-messaging is same as
> hackbench as you noted on the parallel thread) that uses
> pipe2(O_NOATIME)

Thanks,

> The original regression is still noticeable despite the improvements
> but if folks believe this is a corner case with the original changes
> exhibited by sched-messaging, I'll just continue further testing with
> the new baseline.

I still don't know if we should worry or not... But if we want to try
to improve the wake_writer logic, then I think it makes sense to cleanup
this code first.

IMO the (untested) patch below makes sense regardless, I am going to send
it after I grep fs/splice.c a bit more.

a194dfe6e6f6f ("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 c73be61cede ("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
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 bufer and we can simplify
pipe_read() which currently has to handle this very unlikely case.

Oleg.

diff --git a/fs/pipe.c b/fs/pipe.c
index baaa8c0817f1..0816070a5e7a 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -312,6 +312,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			size_t written;
 			int error;
 
+			WARN_ON_ONCE(chars == 0);
 			if (chars > total_len) {
 				if (buf->flags & PIPE_BUF_FLAG_WHOLE) {
 					if (ret == 0)
@@ -365,29 +366,9 @@ 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
@@ -396,7 +377,6 @@ 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);
 	}
@@ -524,31 +504,25 @@ 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;