Re: [PATCH] perf/ring_buffer: Fix invalid page order

[Takao Indoh]

On 2016/11/15 17:48, Ingo Molnar wrote:
> * Takao Indoh <indou.takao@xxxxxxxxxxxxxx> wrote:
>
> > In rb_alloc_aux_page(), a page order is set to MAX_ORDER when order is
> > greater than MAX_ORDER, but page order should be less than MAX_ORDER,
> > therefore alloc_pages_node fails at least once. This patch fixes page
> > order so that it can be always less than MAX_ORDER.
> >
> > Signed-off-by: Takao Indoh <indou.takao@xxxxxxxxxxxxxx>
> > ---
> >  kernel/events/ring_buffer.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index 257fa46..3f76fdd 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -502,8 +502,8 @@ static struct page *rb_alloc_aux_page(int node, int order)
> >  {
> >  	struct page *page;
> >
> > -	if (order > MAX_ORDER)
> > -		order = MAX_ORDER;
> > +	if (order >= MAX_ORDER)
> > +		order = MAX_ORDER - 1;
> >
> >  	do {
> >  		page = alloc_pages_node(node, PERF_AUX_GFP, order);
>
> I'm wondering under what circumstances this allocation failure was seen in
> practice - why did others not hit this?

I found this when I ran perf with -m,2048. I think in the most cases
users use default buffer size hence they does not notice.

Thanks,
Takao Indoh