bpf: arena: make arena kfuncs any context safe

Make arena related kfuncs any context safe by the following changes:

bpf_arena_alloc_pages() and bpf_arena_reserve_pages():
Replace the usage of the mutex with a rqspinlock for range tree and use
kmalloc_nolock() wherever needed. Use free_pages_nolock() to free pages
from any context.
apply_range_set/clear_cb() with apply_to_page_range() has already made
populating the vm_area in bpf_arena_alloc_pages() any context safe.

bpf_arena_free_pages(): defer the main logic to a workqueue if it is
called from a non-sleepable context.

specialize_kfunc() is used to replace the sleepable arena_free_pages()
with bpf_arena_free_pages_non_sleepable() when the verifier detects the
call is from a non-sleepable context.

In the non-sleepable case, arena_free_pages() queues the address and the
page count to be freed to a lock-less list of struct arena_free_spans
and raises an irq_work. The irq_work handler calls schedules_work() as
it is safe to be called from irq context.  arena_free_worker() (the work
queue handler) iterates these spans and clears ptes, flushes tlb, zaps
pages, and calls __free_page().

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20251222195022.431211-4-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index da6a00dd313f..4e7d72dfbcd4 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -673,6 +673,22 @@ void bpf_map_free_internal_structs(struct bpf_map *map, void *obj);
 int bpf_dynptr_from_file_sleepable(struct file *file, u32 flags,
 				   struct bpf_dynptr *ptr__uninit);
 
+#if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
+void *bpf_arena_alloc_pages_non_sleepable(void *p__map, void *addr__ign, u32 page_cnt, int node_id,
+					  u64 flags);
+void bpf_arena_free_pages_non_sleepable(void *p__map, void *ptr__ign, u32 page_cnt);
+#else
+static inline void *bpf_arena_alloc_pages_non_sleepable(void *p__map, void *addr__ign, u32 page_cnt,
+							int node_id, u64 flags)
+{
+	return NULL;
+}
+
+static inline void bpf_arena_free_pages_non_sleepable(void *p__map, void *ptr__ign, u32 page_cnt)
+{
+}
+#endif
+
 extern const struct bpf_map_ops bpf_map_offload_ops;
 
 /* bpf_type_flag contains a set of flags that are applicable to the values of