Re: [PATCH v2] perf/ring_buffer: Prefer struct_size over open coded arithmetic

[Erick Archer]

On Sun, May 05, 2024 at 05:24:55PM +0200, Christophe JAILLET wrote:
> Le 05/05/2024 à 16:15, Erick Archer a écrit :
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index 4013408ce012..080537eff69f 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -822,9 +822,7 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
> >   	unsigned long size;
> 
> Hi,
> 
> Should size be size_t?

I'm sorry, but I don't have enough knowledge to answer this question.
The "size" variable is used as a return value by struct_size and as
a parameter to the order_base_2() and kzalloc_node() functions.

The size type for the kzalloc_node function is "size_t" but for the
order_base_2() macro it is necessary an unsigned type (since this
is expanded to "__ilog2_u32(u32 n)" or "__ilog2_u64(u64 n)").

So, I don't know if it is correct to change the type to size_t.
Maybe someone can help with this.

> 
> >   	int i, node;
> > -	size = sizeof(struct perf_buffer);
> > -	size += nr_pages * sizeof(void *);
> > -
> > +	size = struct_size(rb, data_pages, nr_pages);
> >   	if (order_base_2(size) > PAGE_SHIFT+MAX_PAGE_ORDER)
> >   		goto fail;
> > @@ -833,6 +831,7 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
> >   	if (!rb)
> >   		goto fail;
> > +	rb->nr_pages = nr_pages;
> >   	rb->user_page = perf_mmap_alloc_page(cpu);
> >   	if (!rb->user_page)
> >   		goto fail_user_page;
> > @@ -843,8 +842,6 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
> >   			goto fail_data_pages;
> >   	}
> > -	rb->nr_pages = nr_pages;
> > -
> >   	ring_buffer_init(rb, watermark, flags);
> >   	return rb;
> > @@ -916,18 +913,15 @@ void rb_free(struct perf_buffer *rb)
> >   struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
> >   {
> >   	struct perf_buffer *rb;
> > -	unsigned long size;
> >   	void *all_buf;
> >   	int node;
> > -	size = sizeof(struct perf_buffer);
> > -	size += sizeof(void *);
> > -
> >   	node = (cpu == -1) ? cpu : cpu_to_node(cpu);
> > -	rb = kzalloc_node(size, GFP_KERNEL, node);
> > +	rb = kzalloc_node(struct_size(rb, data_pages, 1), GFP_KERNEL, node);
> >   	if (!rb)
> >   		goto fail;
> > +	rb->nr_pages = nr_pages;
> 
> I don't think this is correct.

I think you are right. My bad :(

> There is already a logic in place about it a few lines below:
> 
> 	all_buf = vmalloc_user((nr_pages + 1) * PAGE_SIZE);
> 	if (!all_buf)
> 		goto fail_all_buf;
> 
> 	rb->user_page = all_buf;
> 	rb->data_pages[0] = all_buf + PAGE_SIZE;
> 	if (nr_pages) {					<--- here
> 		rb->nr_pages = 1;			<---
> 		rb->page_order = ilog2(nr_pages);
> 	}
> 
> I think that what is needed is to move this block just 2 lines above,
> (before rb->data_pages[0] = ...)
> 
> 
> I'm also wondering what should be done if nr_pages = 0.

Perhaps this is enough since we only allocate memory for one
member of the array.

@@ -916,18 +913,15 @@ void rb_free(struct perf_buffer *rb)
 struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 {
        struct perf_buffer *rb;
-       unsigned long size;
        void *all_buf;
        int node;

-       size = sizeof(struct perf_buffer);
-       size += sizeof(void *);
-
        node = (cpu == -1) ? cpu : cpu_to_node(cpu);
-       rb = kzalloc_node(size, GFP_KERNEL, node);
+       rb = kzalloc_node(struct_size(rb, data_pages, 1), GFP_KERNEL, node);
        if (!rb)
                goto fail;

+       rb->nr_pages = 1;
        INIT_WORK(&rb->work, rb_free_work);

        all_buf = vmalloc_user((nr_pages + 1) * PAGE_SIZE);

I think that we don't need to deal with the "nr_pages = 0" case
since the flex array will always have a length of one.

Kees, can you help us with this?

Regards,
Erick

> CJ
> 
> >   	INIT_WORK(&rb->work, rb_free_work);
> >   	all_buf = vmalloc_user((nr_pages + 1) * PAGE_SIZE);
>