ring-buffer: fix ring_buffer_read_page

The ring_buffer_read_page was broken if it were to only copy part
of the page. This patch fixes that up as well as adds a parameter
to allow a length field, in order to only copy part of the buffer page.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

2 files changed, 64 insertions, 35 deletions

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index f5e793d69bd..79fcbc4b09d 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -121,6 +121,9 @@ unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu);
 u64 ring_buffer_time_stamp(int cpu);
 void ring_buffer_normalize_time_stamp(int cpu, u64 *ts);
 
+size_t ring_buffer_page_len(void *page);
+
+
 /*
  * The below functions are fine to use outside the tracing facility.
  */
@@ -138,8 +141,8 @@ static inline int tracing_is_on(void) { return 0; }
 
 void *ring_buffer_alloc_read_page(struct ring_buffer *buffer);
 void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data);
-int ring_buffer_read_page(struct ring_buffer *buffer,
-                          void **data_page, int cpu, int full);
+int ring_buffer_read_page(struct ring_buffer *buffer, void **data_page,
+                          size_t len, int cpu, int full);
 
 enum ring_buffer_flags {
         RB_FL_OVERWRITE         = 1 << 0,
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9baad7ee4b3..2ad6bae95a3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -234,6 +234,11 @@ static void rb_init_page(struct buffer_data_page *bpage)
         local_set(&bpage->commit, 0);
 }
 
+size_t ring_buffer_page_len(void *page)
+{
+        return local_read(&((struct buffer_data_page *)page)->commit);
+}
+
 /*
  * Also stolen from mm/slob.c. Thanks to Mathieu Desnoyers for pointing
  * this issue out.
@@ -2378,8 +2383,8 @@ static void rb_remove_entries(struct ring_buffer_per_cpu *cpu_buffer,
  */
 void *ring_buffer_alloc_read_page(struct ring_buffer *buffer)
 {
-        unsigned long addr;
         struct buffer_data_page *bpage;
+        unsigned long addr;
 
         addr = __get_free_page(GFP_KERNEL);
         if (!addr)
@@ -2387,6 +2392,8 @@ void *ring_buffer_alloc_read_page(struct ring_buffer *buffer)
 
         bpage = (void *)addr;
 
+        rb_init_page(bpage);
+
         return bpage;
 }
 
@@ -2406,6 +2413,7 @@ void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
  * ring_buffer_read_page - extract a page from the ring buffer
  * @buffer: buffer to extract from
  * @data_page: the page to use allocated from ring_buffer_alloc_read_page
+ * @len: amount to extract
  * @cpu: the cpu of the buffer to extract
  * @full: should the extraction only happen when the page is full.
  *
@@ -2418,7 +2426,7 @@ void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
  *      rpage = ring_buffer_alloc_read_page(buffer);
  *      if (!rpage)
  *              return error;
- *      ret = ring_buffer_read_page(buffer, &rpage, cpu, 0);
+ *      ret = ring_buffer_read_page(buffer, &rpage, len, cpu, 0);
  *      if (ret >= 0)
  *              process_page(rpage, ret);
  *
@@ -2435,71 +2443,89 @@ void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
  *  <0 if no data has been transferred.
  */
 int ring_buffer_read_page(struct ring_buffer *buffer,
-                            void **data_page, int cpu, int full)
+                          void **data_page, size_t len, int cpu, int full)
 {
         struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
         struct ring_buffer_event *event;
         struct buffer_data_page *bpage;
+        struct buffer_page *reader;
         unsigned long flags;
+        unsigned int commit;
         unsigned int read;
         int ret = -1;
 
         if (!data_page)
-                return 0;
+                return -1;
 
         bpage = *data_page;
         if (!bpage)
-                return 0;
+                return -1;
 
         spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 
-        /*
-         * rb_buffer_peek will get the next ring buffer if
-         * the current reader page is empty.
-         */
-        event = rb_buffer_peek(buffer, cpu, NULL);
-        if (!event)
+        reader = rb_get_reader_page(cpu_buffer);
+        if (!reader)
                 goto out;
 
-        /* check for data */
-        if (!local_read(&cpu_buffer->reader_page->page->commit))
-                goto out;
+        event = rb_reader_event(cpu_buffer);
+
+        read = reader->read;
+        commit = rb_page_commit(reader);
 
-        read = cpu_buffer->reader_page->read;
         /*
-         * If the writer is already off of the read page, then simply
-         * switch the read page with the given page. Otherwise
-         * we need to copy the data from the reader to the writer.
+         * If len > what's left on the page, and the writer is also off of
+         * the read page, then simply switch the read page with the given
+         * page. Otherwise we need to copy the data from the reader to the
+         * writer.
          */
-        if (cpu_buffer->reader_page == cpu_buffer->commit_page) {
-                unsigned int commit = rb_page_commit(cpu_buffer->reader_page);
+        if ((len < (commit - read)) ||
+            cpu_buffer->reader_page == cpu_buffer->commit_page) {
                 struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
+                unsigned int pos = read;
+                unsigned int size;
 
                 if (full)
                         goto out;
-                /* The writer is still on the reader page, we must copy */
-                memcpy(bpage->data + read, rpage->data + read, commit - read);
 
-                /* consume what was read */
-                cpu_buffer->reader_page->read = commit;
+                if (len > (commit - read))
+                        len = (commit - read);
+
+                size = rb_event_length(event);
+
+                if (len < size)
+                        goto out;
+
+                /* Need to copy one event at a time */
+                do {
+                        memcpy(bpage->data + pos, rpage->data + pos, size);
+
+                        len -= size;
+
+                        rb_advance_reader(cpu_buffer);
+                        pos = reader->read;
+
+                        event = rb_reader_event(cpu_buffer);
+                        size = rb_event_length(event);
+                } while (len > size);
 
                 /* update bpage */
-                local_set(&bpage->commit, commit);
-                if (!read)
-                        bpage->time_stamp = rpage->time_stamp;
+                local_set(&bpage->commit, pos);
+                bpage->time_stamp = rpage->time_stamp;
+
         } else {
                 /* swap the pages */
                 rb_init_page(bpage);
-                bpage = cpu_buffer->reader_page->page;
-                cpu_buffer->reader_page->page = *data_page;
-                local_set(&cpu_buffer->reader_page->write, 0);
-                cpu_buffer->reader_page->read = 0;
+                bpage = reader->page;
+                reader->page = *data_page;
+                local_set(&reader->write, 0);
+                reader->read = 0;
                 *data_page = bpage;
+
+                /* update the entry counter */
+                rb_remove_entries(cpu_buffer, bpage, read);
         }
         ret = read;
 
-        /* update the entry counter */
-        rb_remove_entries(cpu_buffer, bpage, read);
  out:
         spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);