ring-buffer: Have dropped subbuffers be persistent across reboots

When the persistent ring buffer detects a corrupted subbuffer, it will
zero its size and report dropped pages in the dmesg, then it continues
normally.

But if a reboot happens without clearing or restarting tracing on the
persistent ring buffer, the next boot will show no pages are dropped.

If the persistent ring buffer is still the same, then it should still
report dropped pages so the user knows that the buffer has missing events.

Add the RB_MISSED_EVENTS flag to the commit value of the subbuffer so that
the next boot will still show that pages were dropped.

Link: https://patch.msgid.link/20260522171051.860780286@kernel.org
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@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 fd4a8dc5484ea3ca50c15905f4d3dcfd70f48fdd..db72969aefa9839661030734b015f7f340897c31 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1915,7 +1915,7 @@ static int __rb_validate_buffer(struct buffer_page *bpage, int cpu,
         * Even after clearing these bits, a commit value greater than the
         * subbuf_size is considered invalid.
         */
-       tail = rb_data_page_size(dpage);
+       tail = rb_data_page_commit(dpage);
        if (tail <= meta->subbuf_size - BUF_PAGE_HDR_SIZE)
                ret = rb_read_data_buffer(dpage, tail, cpu, &ts, &delta);
        else
@@ -1929,7 +1929,7 @@ static int __rb_validate_buffer(struct buffer_page *bpage, int cpu,
         */
        if (ret < 0 || (prev_ts && prev_ts > ts) || (next_ts && ts > next_ts)) {
                local_set(&bpage->entries, 0);
-               local_set(&dpage->commit, 0);
+               local_set(&dpage->commit, RB_MISSED_EVENTS);
                dpage->time_stamp = prev_ts ? prev_ts : next_ts;
                ret = -1;
        } else {
@@ -3451,7 +3451,7 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
         * is a mb(), which will synchronize with the rmb here.
         * (see rb_tail_page_update() and __rb_reserve_next())
         */
-       commit = rb_page_commit(iter_head_page);
+       commit = rb_page_size(iter_head_page);
        smp_rmb();
 
        /* An event needs to be at least 8 bytes in size */
@@ -3480,7 +3480,7 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
 
        /* Make sure the page didn't change since we read this */
        if (iter->page_stamp != iter_head_page->page->time_stamp ||
-           commit > rb_page_commit(iter_head_page))
+           commit > rb_page_size(iter_head_page))
                goto reset;
 
        iter->next_event = iter->head + length;
@@ -5651,7 +5651,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
         * (see rb_tail_page_update())
         */
        smp_rmb();
-       commit = rb_page_commit(commit_page);
+       commit = rb_page_size(commit_page);
        /* We want to make sure that the commit page doesn't change */
        smp_rmb();
 
@@ -5844,6 +5844,7 @@ __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
         */
        local_set(&cpu_buffer->reader_page->write, 0);
        local_set(&cpu_buffer->reader_page->entries, 0);
+       rb_init_data_page(cpu_buffer->reader_page->page);
        cpu_buffer->reader_page->real_end = 0;
 
  spin: