Re: [PATCH 1/3] bpf: Factor out nonsleepable arena allocation logic

> commit 434fc482ce1027a7af536690a218094b882f2ad2
> Author: Emil Tsalapatis <[email protected]>
> Date:   Mon Aug 24 04:25:28 2026 -0400
>
>     bpf: Factor out nonsleepable arena allocation logic
>
>     The bpf_arena_alloc_pages() is supposed to transparently choose whether
>     to use _nolock() allocation variants or not depending on the caller's
>     context. This is implemented internally by testing the context a
>     __bpf_alloc_page call is made in. However, the implementation of
>     bpf_arena_alloc_pages() forces the use of the _nolock() variants by
>     calling __bpf_alloc_page after disabling IRQs by taking a spinlock.
>     This in turn causes spurious allocation failures for some workloads
>     like scx schedulers due to ZONE_NORMAL falling below the minimum
>     watermark, even though the caller is sleepable could wait until
>     there is memory available.

This isn't a bug, but could this sentence be reworded (e.g. "even though a
sleepable caller could wait until memory becomes available") so the
motivation reads cleanly?

> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 7b6847200b43..da356989786a 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -661,68 +661,57 @@ static u64 clear_lo32(u64 val)
>  	return val & ~(u64)~0U;
>  }
>
> -/*
> - * Allocate pages and vmap them into kernel vmalloc area.
> - * Later the pages will be mmaped into user space vma.
> - */
> -static long arena_alloc_pages(struct bpf_arena *arena, long uaddr, long page_cnt, int node_id,
> -			      bool sleepable)
> +
> +static int arena_adjust_tree(struct bpf_arena *arena, long uaddr, long page_cnt, long *pgoff)
> +{
> +	int ret;
> +
> +	/* Special case where user is requesting specific range. */
> +	if (uaddr) {
> +		ret = is_range_tree_set(&arena->rt, *pgoff, page_cnt);
> +		if (ret)
> +			return ret;
> +		return range_tree_clear(&arena->rt, *pgoff, page_cnt);
> +	}
> +
> +	ret = range_tree_find(&arena->rt, page_cnt);
> +	if (ret < 0)
> +		return ret;
> +
> +	*pgoff = ret;
> +
> +	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)

This isn't a bug, but would it read better to keep the existing
(uaddr, page_cnt) argument order in arena_alloc_pages_internal() so all
three functions agree, given both parameters are plain long?

arena_adjust_tree() uses (uaddr, page_cnt) and arena_alloc_pages() also
uses (uaddr, page_cnt), but arena_alloc_pages_internal() uses
(page_cnt, uaddr).


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32709208315