Re: [PATCH bpf-next] bpf: Add kernel-doc for arena page kfuncs

[Alexei Starovoitov]

On Thu, May 21, 2026 at 6:36 AM Dhiraj Shah <find.dhiraj@xxxxxxxxx> wrote:
>
> The page-management kfuncs exposed by BPF arena -
> bpf_arena_alloc_pages(), bpf_arena_free_pages() and
> bpf_arena_reserve_pages() - are part of the BPF kfunc ABI but lack
> rendered documentation. Their contracts (valid argument ranges,
> sleepable-only context, and the set of error returns) are today only
> discoverable by reading kernel/bpf/arena.c.
>
> Add a kernel-doc comment block above each of the three kfuncs and
> render them under a new "BPF arena kfuncs" subsection in
> Documentation/bpf/kfuncs.rst, alongside the existing core kfunc
> subsections.
>
> No functional change.
>
> Signed-off-by: Dhiraj Shah <find.dhiraj@xxxxxxxxx>
> ---
>  Documentation/bpf/kfuncs.rst | 27 +++++++++++++++
>  kernel/bpf/arena.c           | 64 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 91 insertions(+)
>
> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> index 75e6c078e0e7..fe0df1e16453 100644
> --- a/Documentation/bpf/kfuncs.rst
> +++ b/Documentation/bpf/kfuncs.rst
> @@ -732,3 +732,30 @@ the verifier. bpf_cgroup_ancestor() can be used as follows:
>  BPF provides a set of kfuncs that can be used to query, allocate, mutate, and
>  destroy struct cpumask * objects. Please refer to :ref:`cpumasks-header-label`
>  for more details.
> +
> +4.4 BPF arena kfuncs
> +--------------------
> +
> +A BPF arena (``BPF_MAP_TYPE_ARENA``) is a sparsely-populated shared memory
> +region that a BPF program and a user-space process can both address. The
> +following kfuncs allow a sleepable BPF program to allocate, free, and reserve
> +pages within an arena:
> +
> +.. kernel-doc:: kernel/bpf/arena.c
> +   :identifiers: bpf_arena_alloc_pages bpf_arena_free_pages bpf_arena_reserve_pages
> +
> +A typical pattern is to allocate one or more pages, write to them from BPF,
> +and let user space observe the same memory after a page fault populates its
> +VMA:
> +
> +.. code-block:: c
> +
> +       void __arena *page;
> +
> +       page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
> +       if (!page)
> +               return -ENOMEM;
> +
> +       /* ... use the page from BPF; user space sees the same bytes ... */
> +
> +       bpf_arena_free_pages(&arena, page, 1);
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 49a8f7b1beef..b8ec2953dee6 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -870,6 +870,33 @@ static void arena_free_irq(struct irq_work *iw)
>
>  __bpf_kfunc_start_defs();
>
> +/**
> + * bpf_arena_alloc_pages() - Allocate pages within a BPF arena.
> + * @p__map: Pointer to a ``BPF_MAP_TYPE_ARENA`` map.
> + * @addr__ign: Page-aligned user-space address within the arena at which to
> + *            place the allocation, or %NULL to let the kernel choose. When
> + *            non-NULL the address must fall inside the arena's user VMA
> + *            range; otherwise the allocation fails.
> + * @page_cnt: Number of pages to allocate. Must be non-zero and no greater
> + *           than the arena's configured size in pages.
> + * @node_id: NUMA node hint for the backing pages, or %NUMA_NO_NODE.
> + * @flags: Reserved for future use; must be 0.
> + *
> + * Allocates @page_cnt physically-backed pages and inserts them into the
> + * arena's kernel VMA at the offset corresponding to @addr__ign (or at an
> + * arbitrary free offset when @addr__ign is %NULL). A subsequent user-space
> + * page fault on the matching user address populates the user VMA with the
> + * same pages, giving BPF and user space a shared view of the region.
> + *
> + * The underlying allocator may sleep, so this kfunc is only callable from
> + * sleepable BPF programs.

what?

> + * Return:
> + * * Kernel pointer to the start of the allocated region on success.

what?

so much slop nowadays :(

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