Re: [PATCHv2] perf: Fix vmalloc ring buffer free function

[Jiri Olsa]

On Wed, Mar 06, 2013 at 04:20:15PM +0100, Frederic Weisbecker wrote:
> 2013/3/1 Jiri Olsa <jolsa@xxxxxxxxxx>:
> > If we allocate perf ring buffer with the size of single page,
> > we will get memory corruption when releasing it. It's caused
> > by rb_free_work function (CONFIG_PERF_USE_VMALLOC option).
> >
> > For single page sized ring buffer the page_order is -1 (because
> > nr_pages is 0). This needs to be recognized in the rb_free_work
> > function to release proper amount of pages.
> >
> > Introducing page_nr function (CONFIG_PERF_USE_VMALLOC only)
> > that returns number of allocated pages. Using it in rb_free_work
> > and perf_mmap_to_page functions.
> >
> > Also setting rb->nr_pages to 0 in case we have only user page
> > allocated, which will fail perf_output_begin function and
> > prevents sample storage.
> >
> > v2 changes:
> >  - fixed the perf_output_begin handling of single page buffer
> >
> > Reported-by: Jan Stancek <jstancek@xxxxxxxxxx>
> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx>
> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > ---
> >  kernel/events/ring_buffer.c | 40 +++++++++++++++++++++++++++++++++-------
> >  1 file changed, 33 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index 23cb34f..a802151 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -154,7 +154,8 @@ int perf_output_begin(struct perf_output_handle *handle,
> >         if (head - local_read(&rb->wakeup) > rb->watermark)
> >                 local_add(rb->watermark, &rb->wakeup);
> >
> > -       handle->page = offset >> (PAGE_SHIFT + page_order(rb));
> > +       /* page is allways 0 for CONFIG_PERF_USE_VMALLOC option */
> > +       handle->page = offset >> PAGE_SHIFT;
> >         handle->page &= rb->nr_pages - 1;
> >         handle->size = offset & ((PAGE_SIZE << page_order(rb)) - 1);
> >         handle->addr = rb->data_pages[handle->page];
> > @@ -312,11 +313,21 @@ void rb_free(struct ring_buffer *rb)
> >  }
> >
> >  #else
> > +/*
> > + * Returns the total number of pages allocated
> > + * by ring buffer including the user page.
> > + */
> > +static int page_nr(struct ring_buffer *rb)
> > +{
> > +       return page_order(rb) == -1 ?
> > +               1 :                        /* no data, just user page */
> > +               1 + (1 << page_order(rb)); /* user page + data pages */
> > +}
> 
> Could be simply rb->nr_page + 1 ?
> 
> Patch looks good in any case. Thanks.

nope, because CONFIG_PERF_USE_VMALLOC rb uses only 1st slot 
of rg->data_pages[], so rb->nr_page is either 0 or 1

the actuall number of pages is counted via rb->page_order
(which is -1 for case with no data pages)

thanks,
jirka
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