[PATCH v2 0/1] Put seq_buf on a diet

[PATCH v2 0/1] Put seq_buf on a diet

[Matthew Wilcox (Oracle)]

Prompted by the recent mails on ksummit, let's actually try to make this
work this time.  We need a container for manipulating strings easily,
and seq_buf is the closest thing we have to it.  The only problem I have
with it is the readpos that is only useful for the tracing code today.
So move it from the seq_buf to the tracing code.

We should go further with this patch series, including using seq_buf
within vsprintf, but if we can't get over this hurdle first, I'm not
going to waste my time on this again.

v2:
 - Add linux-trace-kernel@vger.kernel.org
 - Fix kernel-doc

Matthew Wilcox (Oracle) (1):
  trace: Move readpos from seq_buf to trace_seq

 include/linux/seq_buf.h   |  5 +----
 include/linux/trace_seq.h |  2 ++
 kernel/trace/trace.c      | 10 +++++-----
 kernel/trace/trace_seq.c  |  6 +++++-
 lib/seq_buf.c             | 22 ++++++++++------------
 5 files changed, 23 insertions(+), 22 deletions(-)

-- 
2.40.1

[PATCH v2 1/1] trace: Move readpos from seq_buf to trace_seq

[Matthew Wilcox (Oracle)]

To make seq_buf more lightweight as a string buf, move the readpos member
from seq_buf to its container, trace_seq.  That puts the responsibility
of maintaining the readpos entirely in the tracing code.  If some future
users want to package up the readpos with a seq_buf, we can define a
new struct then.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/seq_buf.h   |  5 +----
 include/linux/trace_seq.h |  2 ++
 kernel/trace/trace.c      | 10 +++++-----
 kernel/trace/trace_seq.c  |  6 +++++-
 lib/seq_buf.c             | 22 ++++++++++------------
 5 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 515d7fcb9634..a0fb013cebdf 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -14,19 +14,16 @@
  * @buffer:	pointer to the buffer
  * @size:	size of the buffer
  * @len:	the amount of data inside the buffer
- * @readpos:	The next position to read in the buffer.
  */
 struct seq_buf {
 	char			*buffer;
 	size_t			size;
 	size_t			len;
-	loff_t			readpos;
 };
 
 static inline void seq_buf_clear(struct seq_buf *s)
 {
 	s->len = 0;
-	s->readpos = 0;
 }
 
 static inline void
@@ -143,7 +140,7 @@ extern __printf(2, 0)
 int seq_buf_vprintf(struct seq_buf *s, const char *fmt, va_list args);
 extern int seq_buf_print_seq(struct seq_file *m, struct seq_buf *s);
 extern int seq_buf_to_user(struct seq_buf *s, char __user *ubuf,
-			   int cnt);
+			   size_t start, int cnt);
 extern int seq_buf_puts(struct seq_buf *s, const char *str);
 extern int seq_buf_putc(struct seq_buf *s, unsigned char c);
 extern int seq_buf_putmem(struct seq_buf *s, const void *mem, unsigned int len);
diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h
index 6be92bf559fe..3691e0e76a1a 100644
--- a/include/linux/trace_seq.h
+++ b/include/linux/trace_seq.h
@@ -14,6 +14,7 @@
 struct trace_seq {
 	char			buffer[PAGE_SIZE];
 	struct seq_buf		seq;
+	size_t			readpos;
 	int			full;
 };
 
@@ -22,6 +23,7 @@ trace_seq_init(struct trace_seq *s)
 {
 	seq_buf_init(&s->seq, s->buffer, PAGE_SIZE);
 	s->full = 0;
+	s->readpos = 0;
 }
 
 /**
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index abaaf516fcae..217cabd09c3e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1730,15 +1730,15 @@ static ssize_t trace_seq_to_buffer(struct trace_seq *s, void *buf, size_t cnt)
 {
 	int len;
 
-	if (trace_seq_used(s) <= s->seq.readpos)
+	if (trace_seq_used(s) <= s->readpos)
 		return -EBUSY;
 
-	len = trace_seq_used(s) - s->seq.readpos;
+	len = trace_seq_used(s) - s->readpos;
 	if (cnt > len)
 		cnt = len;
-	memcpy(buf, s->buffer + s->seq.readpos, cnt);
+	memcpy(buf, s->buffer + s->readpos, cnt);
 
-	s->seq.readpos += cnt;
+	s->readpos += cnt;
 	return cnt;
 }
 
@@ -7006,7 +7006,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 
 	/* Now copy what we have to the user */
 	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
-	if (iter->seq.seq.readpos >= trace_seq_used(&iter->seq))
+	if (iter->seq.readpos >= trace_seq_used(&iter->seq))
 		trace_seq_init(&iter->seq);
 
 	/*
diff --git a/kernel/trace/trace_seq.c b/kernel/trace/trace_seq.c
index bac06ee3b98b..7be97229ddf8 100644
--- a/kernel/trace/trace_seq.c
+++ b/kernel/trace/trace_seq.c
@@ -370,8 +370,12 @@ EXPORT_SYMBOL_GPL(trace_seq_path);
  */
 int trace_seq_to_user(struct trace_seq *s, char __user *ubuf, int cnt)
 {
+	int ret;
 	__trace_seq_init(s);
-	return seq_buf_to_user(&s->seq, ubuf, cnt);
+	ret = seq_buf_to_user(&s->seq, ubuf, s->readpos, cnt);
+	if (ret > 0)
+		s->readpos += ret;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(trace_seq_to_user);
 
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 45c450f423fa..b7477aefff53 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -324,23 +324,24 @@ int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
  * seq_buf_to_user - copy the sequence buffer to user space
  * @s: seq_buf descriptor
  * @ubuf: The userspace memory location to copy to
+ * @start: The first byte in the buffer to copy
  * @cnt: The amount to copy
  *
  * Copies the sequence buffer into the userspace memory pointed to
- * by @ubuf. It starts from the last read position (@s->readpos)
- * and writes up to @cnt characters or till it reaches the end of
- * the content in the buffer (@s->len), which ever comes first.
+ * by @ubuf. It starts from @start and writes up to @cnt characters
+ * or until it reaches the end of the content in the buffer (@s->len),
+ * whichever comes first.
  *
  * On success, it returns a positive number of the number of bytes
  * it copied.
  *
  * On failure it returns -EBUSY if all of the content in the
  * sequence has been already read, which includes nothing in the
- * sequence (@s->len == @s->readpos).
+ * sequence (@s->len == @start).
  *
  * Returns -EFAULT if the copy to userspace fails.
  */
-int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
+int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
 {
 	int len;
 	int ret;
@@ -350,20 +351,17 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
 
 	len = seq_buf_used(s);
 
-	if (len <= s->readpos)
+	if (len <= start)
 		return -EBUSY;
 
-	len -= s->readpos;
+	len -= start;
 	if (cnt > len)
 		cnt = len;
-	ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);
+	ret = copy_to_user(ubuf, s->buffer + start, cnt);
 	if (ret == cnt)
 		return -EFAULT;
 
-	cnt -= ret;
-
-	s->readpos += cnt;
-	return cnt;
+	return cnt - ret;
 }
 
 /**
-- 
2.40.1

Re: [PATCH v2 1/1] trace: Move readpos from seq_buf to trace_seq

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2 1/1] trace: Move readpos from seq_buf to trace_seq

[Greg KH]

On Fri, Oct 20, 2023 at 04:35:45AM +0100, Matthew Wilcox (Oracle) wrote:
> To make seq_buf more lightweight as a string buf, move the readpos member
> from seq_buf to its container, trace_seq.  That puts the responsibility
> of maintaining the readpos entirely in the tracing code.  If some future
> users want to package up the readpos with a seq_buf, we can define a
> new struct then.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>