Re: [PATCH bpf-next 1/2] mm, bpf: Introduce __GFP_TRYLOCK for opportunistic page allocation

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 15, 2024 at 05:48:53PM -0800, Alexei Starovoitov wrote:
> +static inline struct page *try_alloc_page_noprof(int nid)
> +{
> +	/* If spin_locks are not held and interrupts are enabled, use normal path. */
> +	if (preemptible())
> +		return alloc_pages_node_noprof(nid, GFP_NOWAIT | __GFP_ZERO, 0);

This isn't right for PREEMPT_RT, spinlock_t will be preemptible, but you
very much do not want regular allocation calls while inside the
allocator itself for example.

> +	/*
> +	 * Best effort allocation from percpu free list.
> +	 * If it's empty attempt to spin_trylock zone->lock.
> +	 * Do not specify __GFP_KSWAPD_RECLAIM to avoid wakeup_kswapd
> +	 * that may need to grab a lock.
> +	 * Do not specify __GFP_ACCOUNT to avoid local_lock.
> +	 * Do not warn either.
> +	 */
> +	return alloc_pages_node_noprof(nid, __GFP_TRYLOCK | __GFP_NOWARN | __GFP_ZERO, 0);
> +}