ring-buffer: Show persistent buffer dropped events in trace file

When the persistent ring buffer is validated on boot up, if a subbuffer is
deemed invalid, it resets the buffer and continues. Currently, these lost
events are not shown in the trace file output.

Have the trace iterator look for subbuffers that have the RB_MISSED_EVENTS
set and set the iter->missed_events flag when it is detected. This will
then have the trace file shows "LOST EVENTS" when it reads across a
subbuffer that was corrupted and invalidated.

For example:

 <...>-1016    [005] ...1.  6230.660403: preempt_disable: caller=__mod_memcg_state+0x1c8/0x200 parent=__mod_memcg_state+0x1c8/0x200
CPU:5 [LOST EVENTS]
 <...>-1016    [005] .....  6230.660673: kmem_cache_alloc: call_site=__anon_vma_prepare+0x1ad/0x1e0 ptr=000000006e40294c name=anon_vma bytes_req=200 bytes_alloc=208 gfp_flags=GFP_KERNEL node=-1 accounted=true

Link: https://patch.msgid.link/20260522171052.006276604@kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index db72969aefa9839661030734b015f7f340897c31..988915f035c792f9768f439c543e81712d1e539f 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3525,6 +3525,9 @@ static void rb_inc_iter(struct ring_buffer_iter *iter)
        else
                rb_inc_page(&iter->head_page);
 
+       if (rb_page_commit(iter->head_page) & RB_MISSED_EVENTS)
+               iter->missed_events = -1;
+
        iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
        iter->head = 0;
        iter->next_event = 0;
@@ -7061,7 +7064,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
        struct ring_buffer_event *event;
        struct buffer_data_page *dpage;
        struct buffer_page *reader;
-       unsigned long missed_events;
+       long missed_events;
        unsigned int commit;
        unsigned int read;
        u64 save_timestamp;
@@ -7187,6 +7190,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
                local_set(&reader->entries, 0);
                reader->read = 0;
                data_page->data = dpage;
+               if (!missed_events && rb_data_page_commit(dpage) & RB_MISSED_EVENTS)
+                       missed_events = -1;
 
                /*
                 * Use the real_end for the data size,
@@ -7204,10 +7209,12 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
         * Set a flag in the commit field if we lost events
         */
        if (missed_events) {
-               /* If there is room at the end of the page to save the
+               /*
+                * If there is room at the end of the page to save the
                 * missed events, then record it there.
                 */
-               if (buffer->subbuf_size - commit >= sizeof(missed_events)) {
+               if (missed_events > 0 &&
+                   buffer->subbuf_size - commit >= sizeof(missed_events)) {
                        memcpy(&dpage->data[commit], &missed_events,
                               sizeof(missed_events));
                        local_add(RB_MISSED_STORED, &dpage->commit);