[PATCH 0/1] Put seq_buf on a diet

[PATCH 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.

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             | 13 +++++--------
 5 files changed, 18 insertions(+), 18 deletions(-)

-- 
2.40.1

[PATCH 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             | 13 +++++--------
 5 files changed, 18 insertions(+), 18 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..0d218f3835ae 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -340,7 +340,7 @@ int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
  *
  * 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 +350,17 @@ 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 0/1] Put seq_buf on a diet

[Steven Rostedt]

On Thu, 19 Oct 2023 20:45:13 +0100
"Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> 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.
> 
> 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             | 13 +++++--------
>  5 files changed, 18 insertions(+), 18 deletions(-)
> 

Thanks Matthew, I'll pull this in and add it to my for-next queue (after
testing)

-- Steve

Re: [PATCH 0/1] Put seq_buf on a diet

[Steven Rostedt]

On Thu, 19 Oct 2023 17:14:39 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 19 Oct 2023 20:45:13 +0100
> "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:
> 
> > 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.
> > 
> > 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             | 13 +++++--------
> >  5 files changed, 18 insertions(+), 18 deletions(-)
> >   
> 
> Thanks Matthew, I'll pull this in and add it to my for-next queue (after
> testing)

Actually,

Can you resend it, and Cc: linux-trace-kernel@vger.kernel.org so that it
goes into patchwork.

Thanks!

-- Steve

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

[kernel test robot]

Hi Matthew,

kernel test robot noticed the following build warnings:

[auto build test WARNING on kees/for-next/pstore]
[also build test WARNING on kees/for-next/kspp linus/master v6.6-rc6 next-20231019]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Matthew-Wilcox-Oracle/trace-Move-readpos-from-seq_buf-to-trace_seq/20231020-034718
base:   https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/pstore
patch link:    https://lore.kernel.org/r/20231019194514.2115506-2-willy%40infradead.org
patch subject: [PATCH 1/1] trace: Move readpos from seq_buf to trace_seq
config: m68k-allyesconfig (https://download.01.org/0day-ci/archive/20231020/202310200523.ggyHvT6w-lkp@intel.com/config)
compiler: m68k-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231020/202310200523.ggyHvT6w-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202310200523.ggyHvT6w-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> lib/seq_buf.c:344: warning: Function parameter or member 'start' not described in 'seq_buf_to_user'


vim +344 lib/seq_buf.c

3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  322) 
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  323) /**
9dbbc3b9d09d6d lib/seq_buf.c          Zhen Lei                 2021-07-07  324   * seq_buf_to_user - copy the sequence buffer to user space
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  325)  * @s: seq_buf descriptor
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  326)  * @ubuf: The userspace memory location to copy to
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  327)  * @cnt: The amount to copy
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  328)  *
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  329)  * Copies the sequence buffer into the userspace memory pointed to
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  330)  * by @ubuf. It starts from the last read position (@s->readpos)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  331)  * and writes up to @cnt characters or till it reaches the end of
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  332)  * the content in the buffer (@s->len), which ever comes first.
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  333)  *
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  334)  * On success, it returns a positive number of the number of bytes
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  335)  * it copied.
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  336)  *
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  337)  * On failure it returns -EBUSY if all of the content in the
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  338)  * sequence has been already read, which includes nothing in the
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  339)  * sequence (@s->len == @s->readpos).
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  340)  *
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  341)  * Returns -EFAULT if the copy to userspace fails.
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  342)  */
9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  343) int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25 @344) {
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  345) 	int len;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  346) 	int ret;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  347) 
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  348) 	if (!cnt)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  349) 		return 0;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  350) 
ff078d8fc64472 lib/seq_buf.c          Jerry Snitselaar         2015-11-16  351  	len = seq_buf_used(s);
ff078d8fc64472 lib/seq_buf.c          Jerry Snitselaar         2015-11-16  352  
9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  353) 	if (len <= start)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  354) 		return -EBUSY;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  355) 
9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  356) 	len -= start;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  357) 	if (cnt > len)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  358) 		cnt = len;
9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  359) 	ret = copy_to_user(ubuf, s->buffer + start, cnt);
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  360) 	if (ret == cnt)
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  361) 		return -EFAULT;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  362) 
9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  363) 	return cnt - ret;
3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  364) }
353cade3149c27 lib/seq_buf.c          Piotr Maziarz            2019-11-07  365  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

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

[Steven Rostedt]

On Fri, 20 Oct 2023 05:59:36 +0800
kernel test robot <lkp@intel.com> wrote:

> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  323) /**
> 9dbbc3b9d09d6d lib/seq_buf.c          Zhen Lei                 2021-07-07  324   * seq_buf_to_user - copy the sequence buffer to user space
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  325)  * @s: seq_buf descriptor
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  326)  * @ubuf: The userspace memory location to copy to
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  327)  * @cnt: The amount to copy

When you resend, I guess you should add the @start as well.

-- Steve

> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  328)  *
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  329)  * Copies the sequence buffer into the userspace memory pointed to
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  330)  * by @ubuf. It starts from the last read position (@s->readpos)
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  331)  * and writes up to @cnt characters or till it reaches the end of
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  332)  * the content in the buffer (@s->len), which ever comes first.
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  333)  *
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  334)  * On success, it returns a positive number of the number of bytes
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  335)  * it copied.
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  336)  *
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  337)  * On failure it returns -EBUSY if all of the content in the
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  338)  * sequence has been already read, which includes nothing in the
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  339)  * sequence (@s->len == @s->readpos).
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  340)  *
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  341)  * Returns -EFAULT if the copy to userspace fails.
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  342)  */
> 9377093a882be0 lib/seq_buf.c          Matthew Wilcox (Oracle   2023-10-19  343) int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25 @344) {
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  345) 	int len;
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  346) 	int ret;
> 3a161d99c43ce7 kernel/trace/seq_buf.c Steven Rostedt (Red Hat  2014-06-25  347) 

[PATCH 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             | 13 +++++--------
 5 files changed, 18 insertions(+), 18 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..0d218f3835ae 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -340,7 +340,7 @@ int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
  *
  * 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 +350,17 @@ 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