Subject: Re: [PATCH 1/4] powerpc/64s: Add DEBUG_PAGEALLOC for radix

From: Daniel Axtens <[email-masked]>
Date: Fri, 18 Jun 2021 17:28:37 +1000

Jordan Niethe <[email-masked]> writes:

> There is support for DEBUG_PAGEALLOC on hash but not on radix.
> Add support on radix.

Somewhat off-topic but I wonder at what point we can drop the weird !PPC
condition in mm/Kconfig.debug:

config DEBUG_PAGEALLOC
        bool "Debug page memory allocations"
        depends on DEBUG_KERNEL
        depends on !HIBERNATION || ARCH_SUPPORTS_DEBUG_PAGEALLOC && !PPC && !SPARC

I can't figure out from git history why it exists or if it still serves
any function. Given that HIBERNATION isn't much use on Book3S systems it
probably never matters, it just bugs me a bit.

Again, nothing that has to block this series, just maybe something to
follow up at some vague and undefined point in the future!

> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index a666d561b44d..b89482aed82a 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -812,6 +822,15 @@ static inline bool check_pte_access(unsigned long access, unsigned long ptev)
>   * Generic functions with hash/radix callbacks
>   */
>  
> +#ifdef CONFIG_DEBUG_PAGEALLOC
> +static inline void __kernel_map_pages(struct page *page, int numpages, int enable)
> +{
> +	if (radix_enabled())
> +		radix__kernel_map_pages(page, numpages, enable);
> +	hash__kernel_map_pages(page, numpages, enable);

Does this require an else? On radix we will call both radix__ and
hash__kernel_map_pages.

I notice we call both hash__ and radix__ in map_kernel_page under radix,
so maybe this makes sense?

I don't fully understand the mechanism by which memory removal works: it
looks like on radix you mark the page as 'absent', which I think is
enough? Then you fall through to the hash code here:

	for (i = 0; i < numpages; i++, page++) {
		vaddr = (unsigned long)page_address(page);
		lmi = __pa(vaddr) >> PAGE_SHIFT;
		if (lmi >= linear_map_hash_count)
			continue;

I think linear_map_hash_count will be zero unless it gets inited to a
non-zero value in htab_initialize(). I am fairly sure htab_initialize()
doesn't get called for a radix MMU. In that case you'll just `continue;`
out of every iteration of the loop, which would explain why nothing
weird would happen on radix.

Have I missed something here?

The rest of the patch looks good to me.

Kind regards,
Daniel