[PATCH v1 0/2] ring-buffer: Fix and unify size and page calculations

[PATCH v1 0/2] ring-buffer: Fix and unify size and page calculations

[Vincent Donnefort]

This short series unifies minimum page calculation for ring buffers and
fix a small issue with the number of pages calculation for trace
remotes.

Vincent Donnefort (2):
  tracing/remotes: Account for ring buffer page header in size
    calculation
  ring-buffer: Unify ring buffer minimum page calculations

 include/linux/ring_buffer.h       | 21 ++++++++++++++++++++-
 kernel/trace/ring_buffer.c        | 30 ++++++++++++------------------
 kernel/trace/simple_ring_buffer.c |  4 ++--
 kernel/trace/trace_remote.c       |  6 +++++-
 4 files changed, 39 insertions(+), 22 deletions(-)


base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.55.0.979.g7e5102b832-goog

[PATCH v1 1/2] tracing/remotes: Account for ring buffer page header in size calculation

[Vincent Donnefort]

trace_buffer_desc_size() and trace_remote_alloc_buffer undercount the
required pages because every ring buffer page contains a header
(BUF_PAGE_HDR_SIZE). Account for that header to ensure allocated remote
ring buffers aren't smaller than requested by the user.

While at it, ensure those functions catch nr_pages overflow.

Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index afc7daa6ee7d..7a1a92f87650 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -8,6 +8,8 @@
 
 #include <uapi/linux/trace_mmap.h>
 
+#include <linux/ring_buffer_types.h>
+
 struct trace_buffer;
 struct ring_buffer_iter;
 
@@ -281,9 +283,14 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
 
 static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
 {
-	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
+	unsigned long nr_pages =
+		max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
 	struct ring_buffer_desc *rbdesc;
 
+	/* Capped by ring_buffer_desc::nr_page_va */
+	if (nr_pages > UINT_MAX)
+		return SIZE_MAX;
+
 	return size_add(offsetof(struct trace_buffer_desc, __data),
 			size_mul(nr_cpus, struct_size(rbdesc, page_va, nr_pages)));
 }
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 75fa1ffc4c96..2e0fdbb730b7 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -980,9 +980,12 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
 			      const struct cpumask *cpumask)
 {
 	size_t min_desc_size = trace_buffer_desc_size(buffer_size, cpumask_weight(cpumask));
-	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
 	struct ring_buffer_desc *rb_desc;
 	int cpu, ret = -ENOMEM;
+	unsigned int nr_pages;
+
+	if (min_desc_size == SIZE_MAX)
+		return -E2BIG;
 
 	if (desc_size < min_desc_size)
 		return -EINVAL;
@@ -991,6 +994,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
 	desc->struct_len = min_desc_size;
 
 	rb_desc = __first_ring_buffer_desc(desc);
+	nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
 
 	for_each_cpu(cpu, cpumask) {
 		unsigned int id;
-- 
2.55.0.979.g7e5102b832-goog

[PATCH v1 2/2] ring-buffer: Unify ring buffer minimum page calculations

[Vincent Donnefort]

The codebase scatters hardcoded minimum page limits across multiple
places. Introduce RING_BUFFER_MIN_NR_PAGES and rb_size_to_nr_pages() to
formalize the requirement of two pages in a ring buffer.

Remote and persistent ring buffers must account for the reader page.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 7a1a92f87650..a9401b8ebc72 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -99,6 +99,13 @@ struct trace_buffer *__ring_buffer_alloc_range(unsigned long size, unsigned flag
 
 void *ring_buffer_meta_scratch(struct trace_buffer *buffer, unsigned int *size);
 
+/*
+ * A minimum of 2 pages ensures we can always swap a page out of the ring
+ * buffer. This does not account for the reader page or any other page
+ * containing meta data used by static ring buffers.
+ */
+#define RING_BUFFER_MIN_NR_PAGES 2UL
+
 /*
  * Because the ring buffer is generic, if other users of the ring buffer get
  * traced by ftrace, it can produce lockdep warnings. We need to keep each
@@ -281,10 +288,15 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
 	return (struct ring_buffer_desc *)(&desc->__data[0]);
 }
 
+static inline unsigned long __ring_buffer_size_to_desc_pages(size_t size)
+{
+	/* Takes into account the reader page */
+	return max(DIV_ROUND_UP(size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), RING_BUFFER_MIN_NR_PAGES) + 1;
+}
+
 static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
 {
-	unsigned long nr_pages =
-		max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
+	unsigned long nr_pages = __ring_buffer_size_to_desc_pages(buffer_size);
 	struct ring_buffer_desc *rbdesc;
 
 	/* Capped by ring_buffer_desc::nr_page_va */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c03a555a6ba..88e39d98332a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -642,6 +642,11 @@ static __always_inline unsigned int rb_subbuf_max_data_size(struct trace_buffer
 	return rb_subbuf_capacity(buffer) - RB_EVNT_HDR_SIZE - sizeof(event->array[0]);
 }
 
+static unsigned long rb_size_to_nr_pages(struct trace_buffer *buffer, unsigned long size)
+{
+	return max(DIV_ROUND_UP(size, rb_subbuf_capacity(buffer)), RING_BUFFER_MIN_NR_PAGES);
+}
+
 /**
  * rb_subbuf_start - Get the start address of a subbuffer
  * @buffer:	A trace buffer
@@ -2865,7 +2870,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 			goto fail_free_buffers;
 
 		/* Need at least two pages plus the reader page */
-		if (nr_pages < 3)
+		if (nr_pages < RING_BUFFER_MIN_NR_PAGES + 1)
 			goto fail_free_buffers;
 
  again:
@@ -2877,7 +2882,7 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 			ptr += subbuf_size * nr_pages;
 		}
 		if (ptr > end) {
-			if (nr_pages <= 3)
+			if (nr_pages <= RING_BUFFER_MIN_NR_PAGES + 1)
 				goto fail_free_buffers;
 			nr_pages--;
 			goto again;
@@ -2900,13 +2905,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		if (nr_pages > rb_static_max_pages())
 			goto fail_free_buffers;
 
-		if (nr_pages < 2)
+		if (nr_pages < RING_BUFFER_MIN_NR_PAGES)
 			goto fail_free_buffers;
 	} else {
-		/* need at least two pages */
-		nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
-		if (nr_pages < 2)
-			nr_pages = 2;
+		nr_pages = rb_size_to_nr_pages(buffer, size);
 	}
 
 	cpumask_set_cpu(cpu, buffer->cpumask);
@@ -3319,11 +3321,7 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 	mutex_lock(&buffer->mutex);
 	atomic_inc(&buffer->resizing);
 
-	nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
-
-	/* we need a minimum of two pages */
-	if (nr_pages < 2)
-		nr_pages = 2;
+	nr_pages = rb_size_to_nr_pages(buffer, size);
 
 	if (cpu_id == RING_BUFFER_ALL_CPUS) {
 		/*
@@ -7495,12 +7493,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 		}
 
 		/* Update the number of pages to match the new size */
-		nr_pages = old_capacity * buffer->buffers[cpu]->nr_pages;
-		nr_pages = DIV_ROUND_UP(nr_pages, rb_subbuf_capacity(buffer));
-
-		/* we need a minimum of two pages */
-		if (nr_pages < 2)
-			nr_pages = 2;
+		nr_pages = rb_size_to_nr_pages(buffer,
+					       old_capacity * buffer->buffers[cpu]->nr_pages);
 
 		cpu_buffer->nr_pages_to_update = nr_pages;
 
diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
index 49913bb0057a..abfaf9bee12e 100644
--- a/kernel/trace/simple_ring_buffer.c
+++ b/kernel/trace/simple_ring_buffer.c
@@ -383,8 +383,8 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
 	void *page;
 	int i;
 
-	/* At least 1 reader page and two pages in the ring-buffer */
-	if (desc->nr_page_va < 3)
+	/* At least RING_BUFFER_MIN_NR_PAGES and 1 reader page */
+	if (desc->nr_page_va < RING_BUFFER_MIN_NR_PAGES + 1)
 		return -EINVAL;
 
 	memset(cpu_buffer, 0, sizeof(*cpu_buffer));
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 2e0fdbb730b7..0d90e02b1f4b 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -994,7 +994,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
 	desc->struct_len = min_desc_size;
 
 	rb_desc = __first_ring_buffer_desc(desc);
-	nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
+	nr_pages = __ring_buffer_size_to_desc_pages(buffer_size);
 
 	for_each_cpu(cpu, cpumask) {
 		unsigned int id;
-- 
2.55.0.979.g7e5102b832-goog

Re: [PATCH v1 1/2] tracing/remotes: Account for ring buffer page header in size calculation

[Steven Rostedt]

On Mon,  7 Sep 2026 20:26:42 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> trace_buffer_desc_size() and trace_remote_alloc_buffer undercount the
> required pages because every ring buffer page contains a header
> (BUF_PAGE_HDR_SIZE). Account for that header to ensure allocated remote
> ring buffers aren't smaller than requested by the user.
> 
> While at it, ensure those functions catch nr_pages overflow.

Fixes patches should never have "while at it". That means it belongs as a
separate patch. The only "while at it" that is acceptable is white space
fixes or other formatting changes along with new code that is not a fix.

> 
> Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index afc7daa6ee7d..7a1a92f87650 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -8,6 +8,8 @@
>  
>  #include <uapi/linux/trace_mmap.h>
>  
> +#include <linux/ring_buffer_types.h>

Hmm, why is this after the uapi header? It should be part of the linux/
headers.

> +
>  struct trace_buffer;
>  struct ring_buffer_iter;
>  
> @@ -281,9 +283,14 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
>  
>  static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
>  {
> -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
> +	unsigned long nr_pages =
> +		max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;

This is getting a little unwieldy. Let's break it up:

	unsigned long pages_req = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
	unsigned long nr_pages = max(pages_req, 2UL) + 1;

Or better yet, let's add a new helper function:

static inline unsigned long calculate_nr_pages(size_t buffer_size)
{
	unsigned long req_pages = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
	return max(pages_req, 2UL) + 1;
}

Then this could be simply:

	unsigned long nr_pages = calculate_nr_pages(buffer_size);

>  	struct ring_buffer_desc *rbdesc;
>  
> +	/* Capped by ring_buffer_desc::nr_page_va */
> +	if (nr_pages > UINT_MAX)
> +		return SIZE_MAX;

Separate patch.

> +
>  	return size_add(offsetof(struct trace_buffer_desc, __data),
>  			size_mul(nr_cpus, struct_size(rbdesc, page_va, nr_pages)));
>  }
> diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
> index 75fa1ffc4c96..2e0fdbb730b7 100644
> --- a/kernel/trace/trace_remote.c
> +++ b/kernel/trace/trace_remote.c
> @@ -980,9 +980,12 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
>  			      const struct cpumask *cpumask)
>  {
>  	size_t min_desc_size = trace_buffer_desc_size(buffer_size, cpumask_weight(cpumask));
> -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
>  	struct ring_buffer_desc *rb_desc;
>  	int cpu, ret = -ENOMEM;
> +	unsigned int nr_pages;
> +


> +	if (min_desc_size == SIZE_MAX)
> +		return -E2BIG;

Looks like this should be a separate patch too.

>  
>  	if (desc_size < min_desc_size)
>  		return -EINVAL;
> @@ -991,6 +994,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
>  	desc->struct_len = min_desc_size;
>  
>  	rb_desc = __first_ring_buffer_desc(desc);
> +	nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;

And here we can have;

	nr_pages = calculate_nr_pages(buffer_size);

-- Steve

>  
>  	for_each_cpu(cpu, cpumask) {
>  		unsigned int id;

Re: [PATCH v1 2/2] ring-buffer: Unify ring buffer minimum page calculations

[Steven Rostedt]

On Mon,  7 Sep 2026 20:26:43 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> The codebase scatters hardcoded minimum page limits across multiple
> places. Introduce RING_BUFFER_MIN_NR_PAGES and rb_size_to_nr_pages() to
> formalize the requirement of two pages in a ring buffer.

Now I read this after I replied to patch 1 :-)

Anyway, I like my calculate_nr_pages() better than rb_size_to_nr_pages()
but I'm not really picky ;-)

I would make the helper function part of the first patch as you introduce
the duplication of the code there. Whenever I introduce a duplicate code, I
will usually do it by creating a helper function and using that function in
the places of the duplication instead of making it into two steps.

The use of the MIN_NR_PAGES macro can stay as a separate patch.

-- Steve

Re: [PATCH v1 1/2] tracing/remotes: Account for ring buffer page header in size calculation

[Vincent Donnefort]

On Wed, Sep 09, 2026 at 06:13:16PM -0400, Steven Rostedt wrote:
> On Mon,  7 Sep 2026 20:26:42 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > trace_buffer_desc_size() and trace_remote_alloc_buffer undercount the
> > required pages because every ring buffer page contains a header
> > (BUF_PAGE_HDR_SIZE). Account for that header to ensure allocated remote
> > ring buffers aren't smaller than requested by the user.
> > 
> > While at it, ensure those functions catch nr_pages overflow.
> 
> Fixes patches should never have "while at it". That means it belongs as a
> separate patch. The only "while at it" that is acceptable is white space
> fixes or other formatting changes along with new code that is not a fix.

ack.

> 
> > 
> > Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > 
> > diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> > index afc7daa6ee7d..7a1a92f87650 100644
> > --- a/include/linux/ring_buffer.h
> > +++ b/include/linux/ring_buffer.h
> > @@ -8,6 +8,8 @@
> >  
> >  #include <uapi/linux/trace_mmap.h>
> >  
> > +#include <linux/ring_buffer_types.h>
> 
> Hmm, why is this after the uapi header? It should be part of the linux/
> headers.
> 
> > +
> >  struct trace_buffer;
> >  struct ring_buffer_iter;
> >  
> > @@ -281,9 +283,14 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
> >  
> >  static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
> >  {
> > -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
> > +	unsigned long nr_pages =
> > +		max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
> 
> This is getting a little unwieldy. Let's break it up:
> 
> 	unsigned long pages_req = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
> 	unsigned long nr_pages = max(pages_req, 2UL) + 1;
> 
> Or better yet, let's add a new helper function:
> 
> static inline unsigned long calculate_nr_pages(size_t buffer_size)
> {
> 	unsigned long req_pages = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
> 	return max(pages_req, 2UL) + 1;
> }

I'll introduce the helper from this patch as suggested.

> 
> Then this could be simply:
> 
> 	unsigned long nr_pages = calculate_nr_pages(buffer_size);
> 
> >  	struct ring_buffer_desc *rbdesc;
> >  
> > +	/* Capped by ring_buffer_desc::nr_page_va */
> > +	if (nr_pages > UINT_MAX)
> > +		return SIZE_MAX;
> 
> Separate patch.
> 
> > +
> >  	return size_add(offsetof(struct trace_buffer_desc, __data),
> >  			size_mul(nr_cpus, struct_size(rbdesc, page_va, nr_pages)));
> >  }
> > diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
> > index 75fa1ffc4c96..2e0fdbb730b7 100644
> > --- a/kernel/trace/trace_remote.c
> > +++ b/kernel/trace/trace_remote.c
> > @@ -980,9 +980,12 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
> >  			      const struct cpumask *cpumask)
> >  {
> >  	size_t min_desc_size = trace_buffer_desc_size(buffer_size, cpumask_weight(cpumask));
> > -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
> >  	struct ring_buffer_desc *rb_desc;
> >  	int cpu, ret = -ENOMEM;
> > +	unsigned int nr_pages;
> > +
> 
> 
> > +	if (min_desc_size == SIZE_MAX)
> > +		return -E2BIG;
> 
> Looks like this should be a separate patch too.
> 
> >  
> >  	if (desc_size < min_desc_size)
> >  		return -EINVAL;
> > @@ -991,6 +994,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
> >  	desc->struct_len = min_desc_size;
> >  
> >  	rb_desc = __first_ring_buffer_desc(desc);
> > +	nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
> 
> And here we can have;
> 
> 	nr_pages = calculate_nr_pages(buffer_size);
> 
> -- Steve
> 
> >  
> >  	for_each_cpu(cpu, cpumask) {
> >  		unsigned int id;
> 

-- 
Vincent