[PATCH v2 1/1] pipe: change pipe_write() to never add a zero-sized buffer

Oleg Nesterov
2025-02-10T11:41:16Z

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.

Also, with this patch we can probably kill eat_empty_buffer() and more
"is this buffer empty" checks in fs/splice.c later.

Link: https://lore.kernel.org/all/20250209150718.GA17013@redhat.com/
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
---
fs/pipe.c | 45 +++++++++------------------------------------
1 file changed, 9 insertions(+), 36 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 2ae75adfba64..b0641f75b1ba 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -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;
--
2.25.1.362.g51ebf55