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

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Message-ID: <20250228163347.GB17761@redhat.com>
Date: Fri, 28 Feb 2025 17:33:48 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Sapkal, Swapnil" <swapnil.sapkal@....com>
Cc: Mateusz Guzik <mjguzik@...il.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	David Howells <dhowells@...hat.com>,
	WangYuli <wangyuli@...ontech.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Shenoy, Gautham Ranjal" <gautham.shenoy@....com>,
	Neeraj.Upadhyay@....com
Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still
 full

And... I know, I know you already hate me ;)

but if you have time, could you check if this patch (with or without the
previous debugging patch) makes any difference? Just to be sure.

Oleg.
---

diff --git a/fs/pipe.c b/fs/pipe.c
index 4336b8cccf84..524b8845523e 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -445,7 +445,7 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		return 0;
 
 	mutex_lock(&pipe->mutex);
-
+again:
 	if (!pipe->readers) {
 		send_sig(SIGPIPE, current, 0);
 		ret = -EPIPE;
@@ -467,20 +467,24 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		unsigned int mask = pipe->ring_size - 1;
 		struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
 		int offset = buf->offset + buf->len;
+		int xxx;
 
 		if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
 		    offset + chars <= PAGE_SIZE) {
-			ret = pipe_buf_confirm(pipe, buf);
-			if (ret)
+			xxx = pipe_buf_confirm(pipe, buf);
+			if (xxx) {
+				if (!ret) ret = xxx;
 				goto out;
+			}
 
-			ret = copy_page_from_iter(buf->page, offset, chars, from);
-			if (unlikely(ret < chars)) {
-				ret = -EFAULT;
+			xxx = copy_page_from_iter(buf->page, offset, chars, from);
+			if (unlikely(xxx < chars)) {
+				if (!ret) ret = -EFAULT;
 				goto out;
 			}
 
-			buf->len += ret;
+			ret += xxx;
+			buf->len += xxx;
 			if (!iov_iter_count(from))
 				goto out;
 		}
@@ -567,6 +571,7 @@ atomic_inc(&WR_SLEEP);
 		mutex_lock(&pipe->mutex);
 		was_empty = pipe_empty(pipe->head, pipe->tail);
 		wake_next_writer = true;
+		goto again;
 	}
 out:
 	if (pipe_full(pipe->head, pipe->tail, pipe->max_usage))