Re: [PATCH] ring-buffer: Add rb_check_bpage in __rb_allocate_pages

[Qiujun Huang]

On Thu, Oct 15, 2020 at 12:11 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 14 Oct 2020 23:48:05 +0800
> Qiujun Huang <hqjagain@xxxxxxxxx> wrote:
>
> > On Wed, Oct 14, 2020 at 11:38 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >
> > > On Wed, 14 Oct 2020 23:16:14 +0800
> > > Qiujun Huang <hqjagain@xxxxxxxxx> wrote:
> > >
> > > > It may be better to check each page is aligned by 4 bytes. The 2
> > > > least significant bits of the address will be used as flags.
> > > >
> > > > Signed-off-by: Qiujun Huang <hqjagain@xxxxxxxxx>
> > > > ---
> > > >  kernel/trace/ring_buffer.c | 11 +++++++----
> > > >  1 file changed, 7 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > > > index 93ef0ab6ea20..9dec7d58b177 100644
> > > > --- a/kernel/trace/ring_buffer.c
> > > > +++ b/kernel/trace/ring_buffer.c
> > > > @@ -1420,7 +1420,8 @@ static int rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
> > > >       return 0;
> > > >  }
> > > >
> > > > -static int __rb_allocate_pages(long nr_pages, struct list_head *pages, int cpu)
> > > > +static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
> > > > +             long nr_pages, struct list_head *pages, int cpu)
> > > >  {
> > > >       struct buffer_page *bpage, *tmp;
> > > >       bool user_thread = current->mm != NULL;
> > > > @@ -1464,6 +1465,8 @@ static int __rb_allocate_pages(long nr_pages, struct list_head *pages, int cpu)
> > > >               if (!bpage)
> > > >                       goto free_pages;
> > > >
> > > > +             rb_check_bpage(cpu_buffer, bpage);
> > > > +
> > > >
> > >
> > > Why add it here, and not just add this check to the scan in
> > > rb_check_pages()?
> >
> > rb_head_page_deactivate() in rb_check_pages() will clear the 2 LSB first.
> >
>
> Well, you could just add another scan there, but if you want to do it this
> way, then remove passing the int cpu to these functions, and use the
> cpu_buffer->cpu, as keeping the cpu is just redundant.
Get it.
>
> Also, did you see an issue? This check is more of me being paranoid to
No, I'm a little paranoid too following the code :-)
> make sure we don't crash later. I've honestly never seen it trigger.
>
> -- Steve