Re: [PATCH bpf-next v5 2/7] mm, bpf: Introduce free_pages_nolock()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-01-14 18:17:41 [-0800], Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Introduce free_pages_nolock() that can free pages without taking locks.
> It relies on trylock and can be called from any context.
> Since spin_trylock() cannot be used in RT from hard IRQ or NMI
> it uses lockless link list to stash the pages which will be freed
> by subsequent free_pages() from good context.
> 
> Do not use llist unconditionally. BPF maps continuously
> allocate/free, so we cannot unconditionally delay the freeing to
> llist. When the memory becomes free make it available to the
> kernel and BPF users right away if possible, and fallback to
> llist as the last resort.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
…
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 74c2a7af1a77..a9c639e3db91 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1247,13 +1250,44 @@ static void split_large_buddy(struct zone *zone, struct page *page,
…
>  static void free_one_page(struct zone *zone, struct page *page,
>  			  unsigned long pfn, unsigned int order,
>  			  fpi_t fpi_flags)
>  {
> +	struct llist_head *llhead;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&zone->lock, flags);
> +	if (!spin_trylock_irqsave(&zone->lock, flags)) {
> +		if (unlikely(fpi_flags & FPI_TRYLOCK)) {
> +			add_page_to_zone_llist(zone, page, order);
> +			return;
> +		}
> +		spin_lock_irqsave(&zone->lock, flags);
> +	}
> +
> +	/* The lock succeeded. Process deferred pages. */
> +	llhead = &zone->trylock_free_pages;
> +	if (unlikely(!llist_empty(llhead) && !(fpi_flags & FPI_TRYLOCK))) {
Thank you.

> +		struct llist_node *llnode;
> +		struct page *p, *tmp;
> +
> +		llnode = llist_del_all(llhead);
> +		llist_for_each_entry_safe(p, tmp, llnode, pcp_llist) {
> +			unsigned int p_order = p->order;
> +
> +			split_large_buddy(zone, p, page_to_pfn(p), p_order, fpi_flags);
> +			__count_vm_events(PGFREE, 1 << p_order);
> +		}
> +	}

Sebastian