[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@xxxxxxxxxx>

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