Re: [PATCH] libtracefs: Update the kbuf for previous read in trace_mmap_load_subbuf()

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Jan 24, 2024 at 11:56:51AM -0500, Steven Rostedt wrote:
> On Tue, 23 Jan 2024 17:41:59 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > On Tue, Jan 23, 2024 at 12:33:37PM -0500, Steven Rostedt wrote:
> > > From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> > > 
> > > The trace_mmap() checks if the mmapped data has any previously read data
> > > (the reader.read value of the meta page is non-zero), then it will pre-read
> > > the kbuf to move its internal reader pointer to the same value.
> > > 
> > > But when tracefs_cpu_read_buf() calls trace_mmap_load_subbuf(), its
> > > kbuf->subbuffer will not be the same as the mapped data and
> > > kbuffer_load_subbuffer() is called on it and it is returned. But that means
> > > its read pointer has not been updated, and the read data will restart again.
> > > 
> > > When the kbuf is updated in trace_mmap_load_subbuf() check the tmap->kbuf to
> > > see if it already read any of the data, and move the kbuffer forward just
> > > like the trace_mmap() does.
> > > 
> > > Link: https://lore.kernel.org/linux-trace-devel/Za-Md51snPcIoYFn@google.com/
> > > 
> > > Fixes: 2ed14b59 ("libtracefs: Add ring buffer memory mapping APIs")
> > > Reported-by: Vincent Donnefort <vdonnefort@google.com>
> > > Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>  
> > 
> > That fixes fast-forward, event already read are now skipped.
> 
> Can I also add your "Tested-by" tag then?
> 
> -- Steve

Of course, I wasn't sure you were using those tags for libtracefs. Will make it
more explicit next time!