Re: [PATCH 2/3] bpf: Add sleepable arena page allocation path

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Mon Aug 24, 2026 at 1:25 AM PDT, Emil Tsalapatis wrote:
> The bpf_arena_alloc_pages() function currently only allocates pages
> inside a spinlock critical section with IRQs off. This forces the use
> of alloc_pages_nolock() in the BPF allocator, even when the caller is
> a sleepable BPF function. This in turn causes allocation failures even
> in cases where falling into the allocator slow path and possibly
> sleeping would eventually succeed. This can be triggered consistently
> by heavy BPF arena users like scx.
>
> Add a separate arena page allocation path just for sleepable callers.
> The path preallocates the arena memory to be added to the tree before
> taking the critical section.
>
> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
> ---
>  include/linux/bpf.h  |  6 ++++
>  kernel/bpf/arena.c   | 76 +++++++++++++++++++++++++++++++++++++++++---
>  kernel/bpf/syscall.c |  8 +----
>  3 files changed, 79 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index ffa5626411ac..d15ed7a3879b 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -710,6 +710,12 @@ 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);
>  
> +static inline bool is_bpf_alloc_nonsleepable(void)
> +{
> +	return preempt_count() > 0 || irqs_disabled() ||
> +		IS_ENABLED(CONFIG_PREEMPT_RT);
> +}
> +
>  #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);
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index da356989786a..126ddae763f9 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -683,8 +683,73 @@ static int arena_adjust_tree(struct bpf_arena *arena, long uaddr, long page_cnt,
>  	return range_tree_clear(&arena->rt, *pgoff, page_cnt);
>  }
>  
> -static long arena_alloc_pages_internal(struct bpf_arena *arena, long page_cnt,
> -		long uaddr, long pgoff, int node_id, bool sleepable)
> +static long arena_alloc_pages_sleepable(struct bpf_arena *arena, long page_cnt,
> +		long uaddr, long pgoff, int node_id)
> +{
> +	u64 kern_vm_start = bpf_arena_get_kern_vm_start(arena);
> +	struct apply_range_data data;
> +	struct page **pages = NULL;
> +	unsigned long flags;
> +	u32 uaddr32;
> +	int ret, i;
> +
> +	pages = kvcalloc(page_cnt, sizeof(struct page *), GFP_KERNEL_ACCOUNT);
> +	if (!pages)
> +		return 0;
> +
> +	ret = bpf_map_alloc_pages(&arena->map, node_id, page_cnt, pages);
> +	if (ret) {
> +		kvfree(pages);
> +		return 0;
> +	}
> +
> +	data.i = 0;
> +	data.pages = pages;
> +	data.arena = arena;
> +

Why have this split?
Always use an approach of allocating pages outside of the lock?

Considering severity of the issue bpf tree is probably appropriate.

pw-bot: cr