linux-kernel - Re: [PATCH v1] arm64: mm: Don't sleep in split_kernel_leaf

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Message-ID: <e5fee14a-4569-49c8-9f42-844839e51e85@kernel.org>
Date: Mon, 3 Nov 2025 16:38:02 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>, catalin.marinas@....com,
 will@...nel.org, yang@...amperecomputing.com, ardb@...nel.org,
 dev.jain@....com, scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Guenter Roeck <groeck@...gle.com>
Subject: Re: [PATCH v1] arm64: mm: Don't sleep in split_kernel_leaf_mapping()
 when in atomic context


>   }
> 
> +static inline bool force_pte_mapping(void)
> +{
> +	bool bbml2 = system_capabilities_finalized() ?
> +		system_supports_bbml2_noabort() : cpu_supports_bbml2_noabort();

You are only moving this function. Still, there is some room for improvement I want to point out :)

bbml2 could be a const (or a helper function like bbml2_supported).

> +
> +	return (!bbml2 && (rodata_full || arm64_kfence_can_set_direct_map() ||
> +			   is_realm_world())) ||
> +		debug_pagealloc_enabled();


I suspect this could be made a bit easier to read.

	if (debug_pagealloc_enabled())
		return true;
	if (bbml2)
		return false;
	return rodata_full || arm64_kfence_can_set_direct_map() || is_realm_world();


> +}
> +
>   static DEFINE_MUTEX(pgtable_split_lock);
> 
>   int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
> @@ -723,6 +733,16 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
>   	if (!system_supports_bbml2_noabort())
>   		return 0;
> 
> +	/*
> +	 * If the region is within a pte-mapped area, there is no need to try to
> +	 * split. Additionally, CONFIG_DEBUG_PAGEALLOC and CONFIG_KFENCE may
> +	 * change permissions from softirq context so for those cases (which are
> +	 * always pte-mapped), we must not go any further because taking the
> +	 * mutex below may sleep.
> +	 */
> +	if (force_pte_mapping() || is_kfence_address((void *)start))
> +		return 0;
> +

We're effectively performing two system_supports_bbml2_noabort() checks, similarly in
arch_kfence_init_pool().

I wonder if there is a clean way to avoid that.

I'm not super up-to-date on that code. Nothing else jumped at me.

-- 
Cheers

David