Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set without mapping

[Guenter Roeck]

On 6/11/24 12:28, Steven Rostedt wrote:

From: "Steven Rostedt (Google)" <rost...@goodmis.org>

In preparation for having the ring buffer mapped to a dedicated location,
which will have the same restrictions as user space memory mapped buffers,
allow it to use the "mapped" field of the ring_buffer_per_cpu structure
without having the user space meta page mapping.

When this starts using the mapped field, it will need to handle adding a
user space mapping (and removing it) from a ring buffer that is using a
dedicated memory range.

Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
---
  kernel/trace/ring_buffer.c | 11 ++++++++---
  1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 28853966aa9a..78beaccf9c8c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5224,6 +5224,9 @@ static void rb_update_meta_page(struct 
ring_buffer_per_cpu *cpu_buffer)
  {
        struct trace_buffer_meta *meta = cpu_buffer->meta_page;

+ if (!meta)

+               return;
+
        meta->reader.read = cpu_buffer->reader_page->read;
        meta->reader.id = cpu_buffer->reader_page->id;
        meta->reader.lost_events = cpu_buffer->lost_events;
@@ -6167,7 +6170,7 @@ rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)

mutex_lock(&cpu_buffer->mapping_lock); - if (!cpu_buffer->mapped) {

+       if (!cpu_buffer->mapped || !cpu_buffer->meta_page) {
                mutex_unlock(&cpu_buffer->mapping_lock);
                return ERR_PTR(-ENODEV);
        }
@@ -6359,12 +6362,13 @@ int ring_buffer_map(struct trace_buffer *buffer, int 
cpu,
         */
        raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
        rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
+

Picky again. Is that a leftover from something ? I don't see an immediate reason
for the added newline.

        raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

err = __rb_map_vma(cpu_buffer, vma);

        if (!err) {
                raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
-               cpu_buffer->mapped = 1;
+               cpu_buffer->mapped++;
                raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
        } else {
                kfree(cpu_buffer->subbuf_ids);
@@ -6403,7 +6407,8 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int 
cpu)
        mutex_lock(&buffer->mutex);
        raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);

- cpu_buffer->mapped = 0;

+       WARN_ON_ONCE(!cpu_buffer->mapped);
+       cpu_buffer->mapped--;

This will wrap to UINT_MAX if it was 0. Is that intentional ?

raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);