Re: [PATCH resend] kmap_local: don't assume kmap PTEs are linear arrays in memory

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Nov 16 2021 at 10:47, Ard Biesheuvel wrote:
> The kmap_local conversion broke the ARM architecture, because the new
> code assumes that all PTEs used for creating kmaps form a linear array
> in memory, and uses array indexing to look up the kmap PTE belonging to
> a certain kmap index.
>
> On ARM, this cannot work, not only because the PTE pages may be
> non-adjacent in memory, but also because ARM/!LPAE interleaves hardware
> entries and extended entries (carrying software-only bits) in a way that
> is not compatible with array indexing.
>
> Fortunately, this only seems to affect configurations with more than 8
> CPUs, due to the way the per-CPU kmap slots are organized in memory.

Ooops. I completely missed that detail. Sorry for the wreckage.

> Work around this by permitting an architecture to set a Kconfig symbol
> that signifies that the kmap PTEs do not form a lineary array in memory,
> and so the only way to locate the appropriate one is to walk the page
> tables.
> +static pte_t *kmap_get_pte(unsigned long vaddr, int idx)
>  {
> +	if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
> +		/*
> +		 * Set by the arch if __kmap_pte[-idx] does not produce
> +		 * the correct entry.
> +		 */
> +		return virt_to_kpte(vaddr);

Nit. The above is not a one line statement (even if it is from a
compiler perspective).

	/*
	 * Does the architecture have non-linear KMAP pte entries which
	 * cannot be accessed by index?
	 */
	if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
		return virt_to_kpte(vaddr);

or
	if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY)) {
        	/* PTE entries cannot be accessed by index. */
		return virt_to_kpte(vaddr);
        }

are readable but this is really breaking the brain OCR:

	if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
		/*
		 * Set by the arch if __kmap_pte[-idx] does not produce
		 * the correct entry.
		 */
		return virt_to_kpte(vaddr);
  	if (!__kmap_pte)

Other than that.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>