Re: [PATCH bpf-next v3 1/3] bpf: Add overwrite mode for BPF ring buffer

[Xu Kuohai]

On 10/28/2025 10:47 AM, Andrii Nakryiko wrote:

On Fri, Oct 17, 2025 at 9:04 PM Xu Kuohai <xukuo...@huaweicloud.com> wrote:

[...]

@@ -72,6 +73,8 @@ struct bpf_ringbuf {
          */
         unsigned long consumer_pos __aligned(PAGE_SIZE);
         unsigned long producer_pos __aligned(PAGE_SIZE);
+       /* points to the record right after the last overwritten one */
+       unsigned long overwrite_pos;

I moved this after pending_pos, as all these fields are actually
exposed to the user space, so didn't want to unnecessarily shift
pending_pos.

OK, that makes sense

         unsigned long pending_pos;
         char data[] __aligned(PAGE_SIZE);
  };
@@ -166,7 +169,7 @@ static void bpf_ringbuf_notify(struct irq_work *work)
   * considering that the maximum value of data_sz is (4GB - 1), there
   * will be no overflow, so just note the size limit in the comments.
   */
-static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node)
+static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node, 
bool overwrite_mode)
  {
         struct bpf_ringbuf *rb;

@@ -183,17 +186,25 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t 
data_sz, int numa_node)
         rb->consumer_pos = 0;
         rb->producer_pos = 0;
         rb->pending_pos = 0;
+       rb->overwrite_mode = overwrite_mode;

         return rb;
  }

  static struct bpf_map *ringbuf_map_alloc(union bpf_attr *attr)
  {
+       bool overwrite_mode = false;
         struct bpf_ringbuf_map *rb_map;

         if (attr->map_flags & ~RINGBUF_CREATE_FLAG_MASK)
                 return ERR_PTR(-EINVAL);

+       if (attr->map_flags & BPF_F_RB_OVERWRITE) {
+               if (attr->map_type == BPF_MAP_TYPE_USER_RINGBUF)

this seemed error prone if we ever add another ringbuf type (unlikely,
but still), so I inverted this all to allow BPF_F_RB_OVERWRITE only
for BPF_MAP_TYPE_RINGBUF. We should try to be as strict as possible by
default.

Got it, thanks.

+                       return ERR_PTR(-EINVAL);
+               overwrite_mode = true;
+       }
+
         if (attr->key_size || attr->value_size ||
             !is_power_of_2(attr->max_entries) ||
             !PAGE_ALIGNED(attr->max_entries))
@@ -205,7 +216,7 @@ static struct bpf_map *ringbuf_map_alloc(union bpf_attr 
*attr)

         bpf_map_init_from_attr(&rb_map->map, attr);

-       rb_map->rb = bpf_ringbuf_alloc(attr->max_entries, 
rb_map->map.numa_node);
+       rb_map->rb = bpf_ringbuf_alloc(attr->max_entries, 
rb_map->map.numa_node, overwrite_mode);
         if (!rb_map->rb) {
                 bpf_map_area_free(rb_map);
                 return ERR_PTR(-ENOMEM);
@@ -293,13 +304,25 @@ static int ringbuf_map_mmap_user(struct bpf_map *map, 
struct vm_area_struct *vma
         return remap_vmalloc_range(vma, rb_map->rb, vma->vm_pgoff + 
RINGBUF_PGOFF);
  }

+/* Return an estimate of the available data in the ring buffer.

Fixed up comment style

[...]

  static u32 ringbuf_total_data_sz(const struct bpf_ringbuf *rb)
@@ -402,11 +425,41 @@ bpf_ringbuf_restore_from_rec(struct bpf_ringbuf_hdr *hdr)
         return (void*)((addr & PAGE_MASK) - off);
  }

+static bool bpf_ringbuf_has_space(const struct bpf_ringbuf *rb,
+                                 unsigned long new_prod_pos,
+                                 unsigned long cons_pos,
+                                 unsigned long pend_pos)
+{
+       /* no space if oldest not yet committed record until the newest
+        * record span more than (ringbuf_size - 1).
+        */

same, keep in mind that we now use kernel-wide comment style with /*
on separate line. Fixed up all other places as well.

Thanks for fixing everything! I think I should update my editor config, it’s
still stuck at an 80-character line width.

+       if (new_prod_pos - pend_pos > rb->mask)
+               return false;
+
+       /* ok, we have space in overwrite mode */
+       if (unlikely(rb->overwrite_mode))
+               return true;
+
+       /* no space if producer position advances more than (ringbuf_size - 1)
+        * ahead of consumer position when not in overwrite mode.
+        */
+       if (new_prod_pos - cons_pos > rb->mask)
+               return false;
+
+       return true;
+}
+

[...]