Re: [PATCH bpf-next v2 2/4] bpf: Add memory usage for arena

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Fri Jul 17, 2026 at 7:37 AM EDT, Jiayuan Chen wrote:
> arena is the only map type whose map_mem_usage() still returns 0, so
> "bpftool map show" and fdinfo always showed 0 memlock for an arena no
> matter how many pages it had.
>
> Count the pages that are actually mapped into the arena: bump a counter in
> apply_range_set_cb() when a page goes in and drop it in
> apply_range_clear_cb() when a page goes out, both under the arena spinlock.
> map_mem_usage() then just returns nr_pages << PAGE_SHIFT.
>
> Only real data pages are counted, not the scratch page.
>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

>
> ---
> To sashiko:
> 1. __bpf_alloc_page::can_alloc_pages already check whether we are under lock.
> 2. apply_to_page_range() does not allocate page tables here. arena_map_alloc() already did it.
> ---
>  kernel/bpf/arena.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 8dbc24460890..f046e878f7ae 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -55,8 +55,10 @@ struct bpf_arena {
>  	struct vm_struct *kern_vm;
>  	struct page *scratch_page;
>  	struct range_tree rt;
> -	/* protects rt */
> +	/* protects rt and nr_pages */
>  	rqspinlock_t spinlock;
> +	/* number of pages currently populated in the arena */
> +	u64 nr_pages;
>  	struct list_head vma_list;
>  	/* protects vma_list */
>  	struct mutex lock;
> @@ -196,6 +198,7 @@ static int apply_range_set_cb(pte_t *pte, unsigned long addr, void *data)
>  	set_pte_at(&init_mm, addr, pte, pteval);
>  #endif
>  	d->i++;
> +	WRITE_ONCE(d->arena->nr_pages, d->arena->nr_pages + 1);
>  	return 0;
>  }
>  
> @@ -231,6 +234,7 @@ static int apply_range_clear_cb(pte_t *pte, unsigned long addr, void *data)
>  		return 0;
>  
>  	__llist_add(&page->pcp_llist, d->free_pages);
> +	WRITE_ONCE(d->arena->nr_pages, d->arena->nr_pages - 1);
>  	return 0;
>  }
>  
> @@ -413,7 +417,9 @@ static int arena_map_check_btf(struct bpf_map *map, const struct btf *btf,
>  
>  static u64 arena_map_mem_usage(const struct bpf_map *map)
>  {
> -	return 0;
> +	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
> +
> +	return (u64)READ_ONCE(arena->nr_pages) << PAGE_SHIFT;
>  }
>  
>  struct vma_list {