linux-kernel - Re: [PATCH] ring-buffer: Add rb_check_bpage in __rb_allocate

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Date:   Wed, 14 Oct 2020 12:11:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Qiujun Huang <hqjagain@...il.com>
Cc:     mingo@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ring-buffer: Add rb_check_bpage in __rb_allocate_pages

On Wed, 14 Oct 2020 23:48:05 +0800
Qiujun Huang <hqjagain@...il.com> wrote:

> On Wed, Oct 14, 2020 at 11:38 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Wed, 14 Oct 2020 23:16:14 +0800
> > Qiujun Huang <hqjagain@...il.com> 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@...il.com>
> > > ---
> > >  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.

Also, did you see an issue? This check is more of me being paranoid to
make sure we don't crash later. I've honestly never seen it trigger.

-- Steve