Re: [PATCH] mm: might_sleep warning

[Andrew Morton]

On Tue,  6 Mar 2018 14:20:22 -0500 Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:

> Robot reported this issue:
> https://lkml.org/lkml/2018/2/27/851
> 
> That is introduced by:
> mm: initialize pages on demand during boot
> 
> The problem is caused by changing static branch value within spin lock.
> Spin lock disables preemption, and changing static branch value takes
> mutex lock in its path, and thus may sleep.
> 
> The fix is to add another boolean variable to avoid the need to change
> static branch within spinlock.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1579,6 +1579,7 @@ static int __init deferred_init_memmap(void *data)
>   * page_alloc_init_late() soon after smp_init() is complete.
>   */
>  static __initdata DEFINE_SPINLOCK(deferred_zone_grow_lock);
> +static bool deferred_zone_grow __initdata = true;
>  static DEFINE_STATIC_KEY_TRUE(deferred_pages);
>  
>  /*
> @@ -1616,7 +1617,7 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
>  	 * Bail if we raced with another thread that disabled on demand
>  	 * initialization.
>  	 */
> -	if (!static_branch_unlikely(&deferred_pages)) {
> +	if (!static_branch_unlikely(&deferred_pages) || !deferred_zone_grow) {
>  		spin_unlock_irqrestore(&deferred_zone_grow_lock, flags);
>  		return false;
>  	}
> @@ -1683,10 +1684,15 @@ void __init page_alloc_init_late(void)
>  	/*
>  	 * We are about to initialize the rest of deferred pages, permanently
>  	 * disable on-demand struct page initialization.
> +	 *
> +	 * Note: it is prohibited to modify static branches in non-preemptible
> +	 * context. Since, spin_lock() disables preemption, we must use an
> +	 * extra boolean deferred_zone_grow.
>  	 */
>  	spin_lock(&deferred_zone_grow_lock);
> -	static_branch_disable(&deferred_pages);
> +	deferred_zone_grow = false;
>  	spin_unlock(&deferred_zone_grow_lock);
> +	static_branch_disable(&deferred_pages);
>  
>  	/* There will be num_node_state(N_MEMORY) threads */
>  	atomic_set(&pgdat_init_n_undone, num_node_state(N_MEMORY));

Kinda ugly, but I can see the logic behind the decisions.

Can we instead turn deferred_zone_grow_lock into a mutex?