Re: [PATCH bpf-next v2 1/6] mm, bpf: Introduce __GFP_TRYLOCK for opportunistic page allocation

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 09, 2024 at 06:39:31PM -0800, Alexei Starovoitov wrote:
> +	if (preemptible() && !rcu_preempt_depth())
> +		return alloc_pages_node_noprof(nid,
> +					       GFP_NOWAIT | __GFP_ZERO,
> +					       order);
> +	return alloc_pages_node_noprof(nid,
> +				       __GFP_TRYLOCK | __GFP_NOWARN | __GFP_ZERO,
> +				       order);

[...]

> @@ -4009,7 +4018,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
>  	 * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH).
>  	 */
>  	alloc_flags |= (__force int)
> -		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
> +		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM | __GFP_TRYLOCK));

It's not quite clear to me that we need __GFP_TRYLOCK to implement this.
I was originally wondering if this wasn't a memalloc_nolock_save() /
memalloc_nolock_restore() situation (akin to memalloc_nofs_save/restore),
but I wonder if we can simply do:

	if (!preemptible() || rcu_preempt_depth())
		alloc_flags |= ALLOC_TRYLOCK;