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

[Dhiraj Shah]

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.
+ *
+ * Return:
+ * * Kernel pointer to the start of the allocated region on success.
+ * * %NULL if @p__map is not an arena, @flags is non-zero, @page_cnt is zero
+ *   or exceeds the arena size, @addr__ign is misaligned or outside the
+ *   arena, @node_id is invalid, or the kernel is out of memory.
+ */
 __bpf_kfunc void *bpf_arena_alloc_pages(void *p__map, void *addr__ign, u32 page_cnt,
 					int node_id, u64 flags)
 {
@@ -893,6 +920,23 @@ void *bpf_arena_alloc_pages_non_sleepable(void *p__map, void *addr__ign, u32 pag
 
 	return (void *)arena_alloc_pages(arena, (long)addr__ign, page_cnt, node_id, false);
 }
+
+/**
+ * bpf_arena_free_pages() - Free a range of pages within a BPF arena.
+ * @p__map: Pointer to a ``BPF_MAP_TYPE_ARENA`` map.
+ * @ptr__ign: User-space virtual address of the first page to free, as used
+ *	      to address the arena from BPF and user space. Typically the
+ *	      same address that was previously returned (in user-space form)
+ *	      by bpf_arena_alloc_pages().
+ * @page_cnt: Number of pages to free.
+ *
+ * Releases the backing pages, unmapping them from the arena's kernel VMA
+ * and from any user-space VMA that previously faulted them in. May sleep,
+ * so the kfunc is callable only from sleepable BPF programs.
+ *
+ * The call is a no-op when @p__map is not an arena, when @page_cnt is zero,
+ * or when @ptr__ign is %NULL.
+ */
 __bpf_kfunc void bpf_arena_free_pages(void *p__map, void *ptr__ign, u32 page_cnt)
 {
 	struct bpf_map *map = p__map;
@@ -913,6 +957,26 @@ void bpf_arena_free_pages_non_sleepable(void *p__map, void *ptr__ign, u32 page_c
 	arena_free_pages(arena, (long)ptr__ign, page_cnt, false);
 }
 
+/**
+ * bpf_arena_reserve_pages() - Reserve a page range within a BPF arena.
+ * @p__map: Pointer to a ``BPF_MAP_TYPE_ARENA`` map.
+ * @ptr__ign: Page-aligned user-space virtual address of the start of the
+ *	      range to reserve.
+ * @page_cnt: Number of pages to reserve. Zero is permitted and is a no-op.
+ *
+ * Marks @page_cnt pages starting at @ptr__ign as reserved so that subsequent
+ * bpf_arena_alloc_pages() calls will not place allocations in that range.
+ * No physical pages are allocated by this kfunc; the range is simply
+ * excluded from the arena's free space.
+ *
+ * Return:
+ * * 0 on success, or when @page_cnt is zero.
+ * * -EINVAL if @p__map is not an arena or the requested range falls outside
+ *   the arena's user VMA.
+ * * -EBUSY if any page in the requested range is already allocated, or if
+ *   contention on the arena's internal spinlock prevents the operation from
+ *   completing.
+ */
 __bpf_kfunc int bpf_arena_reserve_pages(void *p__map, void *ptr__ign, u32 page_cnt)
 {
 	struct bpf_map *map = p__map;
-- 
2.43.0