Re: [PATCH v19 3/8] set_memory: allow set_direct_map_*_noflush() for multiple pages

[David Hildenbrand <david@redhat.com>]

On 13.05.21 20:47, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> The underlying implementations of set_direct_map_invalid_noflush() and
> set_direct_map_default_noflush() allow updating multiple contiguous pages
> at once.
> 
> Add numpages parameter to set_direct_map_*_noflush() to expose this
> ability with these APIs.
> 

[...]

Finally doing some in-depth review, sorry for not having a detailed look 
earlier.


>   
> -int set_direct_map_invalid_noflush(struct page *page)
> +int set_direct_map_invalid_noflush(struct page *page, int numpages)
>   {
>   	struct page_change_data data = {
>   		.set_mask = __pgprot(0),
>   		.clear_mask = __pgprot(PTE_VALID),
>   	};
> +	unsigned long size = PAGE_SIZE * numpages;
>   

Nit: I'd have made this const and added an early exit for !numpages. But 
whatever you prefer.

>   	if (!debug_pagealloc_enabled() && !rodata_full)
>   		return 0;
>   
>   	return apply_to_page_range(&init_mm,
>   				   (unsigned long)page_address(page),
> -				   PAGE_SIZE, change_page_range, &data);
> +				   size, change_page_range, &data);
>   }
>   
> -int set_direct_map_default_noflush(struct page *page)
> +int set_direct_map_default_noflush(struct page *page, int numpages)
>   {
>   	struct page_change_data data = {
>   		.set_mask = __pgprot(PTE_VALID | PTE_WRITE),
>   		.clear_mask = __pgprot(PTE_RDONLY),
>   	};
> +	unsigned long size = PAGE_SIZE * numpages;
>   

Nit: dito

>   	if (!debug_pagealloc_enabled() && !rodata_full)
>   		return 0;
>   
>   	return apply_to_page_range(&init_mm,
>   				   (unsigned long)page_address(page),
> -				   PAGE_SIZE, change_page_range, &data);
> +				   size, change_page_range, &data);
>   }
>   


[...]

>   extern int kernel_set_to_readonly;
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 156cd235659f..15a55d6e9cec 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2192,14 +2192,14 @@ static int __set_pages_np(struct page *page, int numpages)
>   	return __change_page_attr_set_clr(&cpa, 0);
>   }
>   
> -int set_direct_map_invalid_noflush(struct page *page)
> +int set_direct_map_invalid_noflush(struct page *page, int numpages)
>   {
> -	return __set_pages_np(page, 1);
> +	return __set_pages_np(page, numpages);
>   }
>   
> -int set_direct_map_default_noflush(struct page *page)
> +int set_direct_map_default_noflush(struct page *page, int numpages)
>   {
> -	return __set_pages_p(page, 1);
> +	return __set_pages_p(page, numpages);
>   }
>   

So, what happens if we succeeded setting 
set_direct_map_invalid_noflush() for some pages but fail when having to 
split a large mapping?

Did I miss something or would the current code not undo what it 
partially did? Or do we simply not care?

I guess to handle this cleanly we would either have to catch all error 
cases first (esp. splitting large mappings) before actually performing 
the set to invalid, or have some recovery code in place if possible.


AFAIKs, your patch #5 right now only calls it with 1 page, do we need 
this change at all? Feels like a leftover from older versions to me 
where we could have had more than a single page.

-- 
Thanks,

David / dhildenb